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Weekly patterns and weekend effects of air pollution in the Moscow megacity
Journal Pre-proof Weekly patterns and weekend effects of air pollution in the Moscow megacity N.F. Elansky, A.V. Shilkin, N.A. Ponomarev, E.G. Semutnikova, P.V. Zakharova PII:
S1352-2310(20)30045-5
DOI:
https://doi.org/10.1016/j.atmosenv.2020.117303
Reference:
AEA 117303
To appear in:
Atmospheric Environment
Received Date: 13 June 2019 Revised Date:
24 December 2019
Accepted Date: 19 January 2020
Please cite this article as: Elansky, N.F., Shilkin, A.V., Ponomarev, N.A., Semutnikova, E.G., Zakharova, P.V., Weekly patterns and weekend effects of air pollution in the Moscow megacity, Atmospheric Environment (2020), doi: https://doi.org/10.1016/j.atmosenv.2020.117303. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Ltd.
Credit Author Statement 1. N.F. Elansky – Conceptualization, Methodology, Investigation, Writing Original Draft, Writing - Review & Editing, Supervision, Project administration, Funding acquisition; 2. A.V. Shilkin – Writing - Review & Editing, Visualization, Formal analysis, Software, Validation; 3. N.A. Ponomarev – Writing - Review & Editing, Validation; 4. E.G. Semutnikova – Resources, Data Curation, Project administration, Funding acquisition; 5. P.V. Zakharova – Resources, Data Curation, Formal analysis.
1
Weekly patterns and weekend effects of air pollution in the Moscow megacity
2
N.F. Elansky1, A.V. Shilkin1,2, N.A. Ponomarev1,3, E.G. Semutnikova3,4, P.V. Zakharova3,5
3
1
A.M. Obukhov Institute of Atmospheric Physics, Russian Academy of Sciences, Pyzhevsky pereulok, 3,
4
Moscow, 119017, Russia
5
2
Research and Production Association “Typhoon”, Pobedy st., 4, Obninsk, 249038, Russia
6
3
Lomonosov Moscow State University, Leninskie Gory, 1–2, Moscow, 119234, Russia
7
4
Department for Environmental Management and Protection, Novyi Arbat, 11, Moscow, 119019, Russia
8
5
Budgetary Environmental Protection Institution “MosEcoMonitoring”, Novyi Arbat, 11, Moscow, 119019,
9
Russia
11
Keywords: megacity environment; air quality; pollutant variations; weekend effect; ozone production; anthropogenic emissions.
12
Abstract
10
13
The weekly cycle and weekend effect in the O3, NO, NO2, CO, CH4, SO2, NMHC,
14
and PM10 concentrations were investigated in the Moscow megacity using in-situ
15
measurements from 1 January 2005 to 31 December 2014 at 49 stations of the Moscow
16
Environment Monitoring network. Daily variations in the CO, NOx, NMHC, and PM10
17
concentrations depend mainly on motor transport emissions and the atmospheric
18
boundary layer vertical stratification. The characteristic feature of Moscow is the time
19
coincidence of rush hours and surface temperature inversion during the cold season,
20
which results in pollutant accumulation in the atmospheric surface layer. It was found
21
that the surface concentrations of the pollutants (except ozone and methane) decrease on
22
weekends. Weekday(Tuesday–Friday)-Sunday differences in the daytime (08:00–22:00
23
LT) NO, NO2, CO, SO2, NMHC, and PM10 concentrations relative to those of weekday
24
period averaged for all stations over 2005–2014 amounted to 23.9±5.8, 16.7±2.8,
25
13.6±3.3, 7.6±6.5, 6.3±2.2, and 14.5±5.1 %, respectively. The ozone concentration
26
increased on Sunday by 16.5±4.8 %. The methane concentration on weekends was the
27
same as on weekdays. The weekend effects in all pollutant concentrations were weakened
28
within the greenbelt around Moscow. In different sectors of Moscow the pollutant
29
weekend effects except that in SO2 were approximately the same. The vertical structure of
1
30
the NO, NO2, and CO weekend effects was analyzed based on data obtained from
31
measurements at the TV tower 500 m in height. These weekend effects decreased
32
nonlinearly with height. Estimates obtained for basic criteria of activity of photochemical
33
processes determining the formation of the weekly cycle and weekend effect of ozone
34
(NMHC/NOx ratio, fraction of radical loss via NOx chemistry, concentration of Ox) show
35
that the VOC-limited chemistry is characteristic of Moscow.
36
1. Introduction
37
The atmosphere over Moscow has been the subject of considerable scientific
38
attention in recent years. First, because, in the early 2000s, the up-to-date system of
39
monitoring the composition of surface air started operating and qualitative data became
40
accessible. Second, because due to a rapid change in the urban infrastructure in the 1990s
41
(economic crisis accompanied by a dramatic setback in industrial production), the
42
intensity of pollutant sources decreased and resulted in changes in the urban
43
photochemical system. Due to both rapid population growth and urban-area extension,
44
Moscow has become one of the largest megacities in the world. If, in 2005, the
45
population of the Moscow megacity was 12.1 million residents and its area was about
46
1950 km2, in 2014, the Moscow population was already 13.8 million residents and the
47
area increased up to 2210 km2 (Elansky et al., 2018). Over the same 10-year period, the
48
number of cars increased from 3 to 4.1 million cars, however, the industrial production
49
increased only by 10 %, while remaining at 40 % below the 1991 level. According to the
50
composition and volume of pollutant emissions, Moscow approached the megacities of
51
the United States, European Union, and Japan (Butler et al., 2008; GAW WMO, 2012;
52
Elansky, 2014; Elansky et al., 2018).
53
The weekly cycle of the urban-air quality is the characteristic feature of almost all
54
large cities. As a rule, the largest amounts of pollutants are emitted on working days and
55
their smallest amounts are emitted on weekends. The relation of variations in ozone and
56
other pollutants with the weekly cycle of the operation of industrial enterprises and traffic
57
intensity was illustrated by the example of large cities in the United States (Cleveland et
2
58
al., 1974; Lebron, 1975; Jacobson, 1975). Later studies revealed the relation of the
59
weekend effect with climatic conditions, relief, urban-infrastructure features, and other
60
factors (Bronnimann and Neu, 1997; Paschalidou and Kassomenos, 2004; Murphy et al.,
61
2007; Stephens et al., 2008). In every city, the weekend effect is manifested in both
62
atmospheric photochemical and transport processes that are characteristic of this concrete
63
city (GAW WMO, 2012; Murphy et al., 2007; Stephens et al., 2008; Yoo et al., 2015;
64
Wang et al., 2014). Therefore, the weekend effect is an efficient tool for validating results
65
obtained from simulations of atmospheric-composition variations under different
66
emission scenarios, which is important in forecasting the air quality and extreme
67
ecological situations (Koo et al., 2012).
68
Moscow, the northernmost and coldest megacity, is located in the middle of the
69
East European Plain. Moderate average air temperatures, low solar UV radiation levels,
70
and good ventilation make the accumulation of pollutants emitted into the surface air and
71
the photochemical formation of secondary (often toxic) pollutants (ozone, organochlorine
72
compounds, extra fine aerosol, and others) less intensive (Bezuglaya and Smirnova,
73
2008; Kuznetsova, 2012; Gorchakov et al., 2010;Elansky, 2014). However, an increase in
74
the number of cars, building boom, and the widespread use of new chemically active
75
substances in industry and everyday life begin to manifest themselves in the activation of
76
photochemical processes. Both increasing frequency and duration of blocking
77
anticyclones observed in recent years serve to increase the oxidation ability of the
78
atmosphere (Mokhov et al., 2014; Elansky et al., 2011; Pankratova et al., 2011). The
79
1995–2010 transcontinental observations of the atmospheric composition from a mobile
80
railroad laboratory (TROICA experiments, Elansky et al., 2009; 2016) revealed a
81
noticeable increase in the rate of ozone formation exceeding that over neighboring rural
82
areas during the summer months (Elansky and Lavrova, 2014; Elansky et al., 2010;
83
2015).
84
Previous studies of temporal variations of surface pollutant concentrations showed
85
the presence of a weekend effect in Moscow. However, the volume of data was limited,
3
86
therefore, pollutant concentrations observed at one station during only a few months
87
(Elansky et al., 2011) or concentrations of one (CO) or two (CO and PM10) components
88
observed at a few stations within 1–6 years (Gorchakov et al., 2010; Gorchakov et al.,
89
2014; Sitnov, 2011a) were used for analysis. The weekend effect was also manifested in
90
the atmosphere over the Moscow region according to observational NO2 data obtained at
91
the Zvenigorod Scientific Station (ZSS) located 53 km west of the city center (Gruzdev,
92
2011) and from satellites (Sitnov, 2011b; Gruzdev, 2013).
93
Both daily and weekly cycles and weekend effects of the basic pollutants – O3, CO,
94
CH4, NO, NO2, SO2, NMHC, and PM10 – in the atmospheric surface layer over the
95
Moscow megacity were analyzed in this work. A complete archive of measurement data
96
obtained from 2005 to 2014 at 48 stations entering the Moscow ecological monitoring
97
system (MosEcoMonitoring (MEM)) and at the Ecological Station of the А.М. Obukhov
98
Institute of Atmospheric Physics (ES OIAP) has been used for analysis. The plan of the
99
Moscow megacity with the location of the stations MEM and ES OIAP is presented at the
100
Fig. 1. The obtained parameters of weekly cycles of pollutants concentrations were used
101
to study the intensity of oxidative processes and the balance of radicals in the urban
102
photochemical system.
103
2. Methodology
104
In Moscow, the number of MEM regularly operating stations increased from 25 in
105
2005 to 34 in 2010 and remained unchanged in the following years. In addition, 14
106
stations operated irregularly within periods of 0.5–2 years. From 1 January 2002 to 31
107
December 2014, the concentrations of trace gases and aerosols were also continuously
108
measured at the ES OIAP. This station is located on the Vorob’evy Gory in a green park
109
zone 7 km southwest of the center of Moscow. Data obtained at this station reflect the
110
mean level of Moscow air pollution caused by emissions from different sources (Elansky
111
et al., 2015). The MEM stations and the ES OIAP continuously measured about 20
112
pollutants (in accordance with the World Health Organization recommendations), wind
113
direction and speed, air temperature, and air relative humidity. Surface pollutant
4
114
concentrations and meteorological parameters averaged over 20 min intervals (within
115
each 1 hour: 00–20, 20–40, 40–60 min) were transmitted in the on-line regime to the
116
MEM data analysis center, in which these data were processed, verified and archived.
117
The procedure of data screening was used to eliminate unrealistic values due to, for
118
example, a momentary effect of some car driven up to the station. The number of CO,
119
NO, and NO2 missing hourly values at regularly operating stations does not exceed 10 %;
120
the number of missing SO2, CH4, and PM10 values does not exceed 15 %. At the ES
121
OIAP, the number of omissions does not exceed 5 % for all measured pollutants. The
122
features of time variations in the vertical distribution of CO, NO, and NO2 in the mixing
123
layer over the city were analyzed using the 1 January 2007–31 December 2014
124
observational data obtained at the TV Ostankino tower (7 km north of the city center) at
125
heights of 10, 130, 248, and 348 m.
126 127 128
Fig. 1. The territories of the Moscow megacity and Greater Moscow. Yellow circles indicate the position of regularly working MEM stations at present, red squares – greenbelt MEM stations, the red triangle – Ostankino TV tower, and the red asterisk – ES OIAP.
129
The MEM stations and the ES OIAP are equipped with modern instruments
130
manufactured by TEI (O3, CO, SO2, NOx, CH4, and non-methane hydrocarbons –
5
131
NMHC), Horiba (CH4, NOx, and NMHC), Dasibi (O3), TSI (aerosol), and other leading
132
companies. There is a calibration laboratory at the MEM network. Standard instruments
133
and gas mixtures from the Russian Institute of Metrology, Max Planck Institute for
134
Chemistry (Germany), Earth System Research Laboratory (United States), and other
135
centers were used to calibrate the instruments at the MEM network and the ES OIAP
136
(Bulletin, 2008; Elansky et al., 2009). One of the MEM stations is located in the vicinity
137
of the ES OIAP on the territory of the Moscow State University (MSU). Regular
138
comparisons of data obtained at these two stations showed agreement between pollutant
139
concentrations within measurement errors (Elansky, 2014; Elansky et al., 2015).
140
In order to study both daily and weekly cycles and weekend effects in pollutants,
141
the 2005–2014 data (averaged over 20 min) on O3, CO, CH4, NO, NO2, SO2, NMHC, and
142
aerosol (PM10) surface concentrations measured at 14, 42, 31, 39, 39, 26, 25, and 15
143
MEM stations, respectively, including those irregularly operating, and at the ES OIAP
144
were used (Table 1). Among these stations, 10 of them are located in the vicinity of
145
highways, 14 stations are in residential districts, 15 stations in zones with high business
146
and industrial activities, 2 stations in park zones (urban background conditions), and 6
147
stations
148
http://www.mosecom.ru/stations/). Geographical positions of monitoring stations in the
149
Moscow megacity and surrounding territories are available (see http://mosecom.ru/map)
150
for different chemical species analyzed in this study. Independently of the number of
151
stations measuring each pollutant, their distribution by the type of surrounding territory
152
remained unchanged.
153 154
Table 1 Number of stations measured pollutant concentrations from 1 Jan 2005 to 31 Dec 2014 and total number of data averaged over 20 min.
within
the
greenbelt
(regional
background
conditions)
(URL:
Pollutant
CO
NO
NO2
NOx
O3
Ox
SO2
PM10
NMHC
CH4
Number of stations
42
39
39
38
14
10
26
15
25
31
6.5E6
5.4E6
5.6E6
5.5E6
2.1E6
1.5E6
2.1E6
1.4E6
2.4E6
2.8E6
Total number of measurements
155
The superposed epoch method, in which all Sundays of the time series under
156
analysis served as reference points, was used to determine the weekend effect value. Both
6
157
linear trend and variations with a period of over 8 days were preliminarily eliminated
158
from data on each of pollutant concentrations at each station using the inverse Fourier
159
transform method. The variations obtained are expressed in relative units by their term-
160
by-term division by difference between them and initial data. Weekly cycles were
161
obtained using the superposed epoch method for each pollutant at each station. Weekly
162
cycles averaged over Moscow, which characterize variations in average pollution
163
concentrations for Moscow during a week averaged over the 10-year period, were also
164
obtained. In this study by the weekend effect we mean the Sunday effect (percentage) that
165
was calculated using the following formula: SE = ([weekday]–[Sunday]) / [weekday],
166
where weekday period includes four days from Tuesday to Friday. A large amount of
167
data allows us to study the manifestation of the Sunday effect in the pollutant
168
concentrations separately for day and night time: daytime SE (08:00–22:00 LT), night SE
169
(22:00 Sunday–08:00 Monday), and daily mean SE (08:00 Sun–08:00 Mon). We also
170
used other time intervals for weekend effect calculations to make a comparison with other
171
megacities.
172
3. The daily cycle of pollutants
173
The daily cycles of CO, NO, NO2, O3, CH4, SO2, NMHC, and PM10 surface
174
concentrations averaged for all stations and all days over 2005–2014 are shown in Fig. 2.
175
Due to the centralized heat supply of both residential and industrial sectors from large
176
heat and power plants (HPP) using natural gas, the main source of CO in Moscow is
177
automobile transport (approximately 85–90 % of all emissions) (Bulletin, 2008; Report,
178
2015). The relative contribution of automobile transport to NOx concentration variations
179
is somewhat smaller due to HPP emissions and more significant influence of both
180
photochemical and meteorological processes on NOx concentrations. In the warm season,
181
in addition to automobile transport, fuel-supply stations, industrial enterprises, other
182
urban sources, and vegetation noticeably affect NMHC concentrations.
7
Concentration [ µg⋅m -3 ] Concentration [ µg⋅m -3 ] Concentration [ µg⋅m -3 ] Concentration [ µg⋅m -3 ] Concentration [ µg⋅m -3 ]
183
CO n=37
NO n=33
30
650 600
25
550 20
500 450
15
O3 n=13
45
NO2 n=33
45
40
40
35 35
30 25
30
20
25
CH4 n=25
1350
SO2 n=22
4.8
1340
4.6
1330 4.4
1320 1310
4.2
1300
4
NMHC n=23
230
PM10 n=14
28 26
215
24 22
200
20 185
18
NOx n=32
70
Ox n=9
80 75
60
70 65
50 60 40
55 0
3
6
9 12 15 Local time [ hour ]
Mon
Tue
18
21
Wed
24
0
3
Thu
6
9 12 15 Local time [ hour ]
Fri
Sat
18
21
24
Sun
184 185 186
Fig. 2. Daily cycle of pollutant surface concentrations in Moscow megacity averaged for all stations over 1 Jan 2005–31 Dec 2014. The 95 % confidence intervals for all of the points of O3, NO, NO2, SO2, CO, PM10, NOx (NOx=NO+NO2), and Ox (Ox=O3+NO2) ranged between 2 and 4 %, NMHC between 4 and 7 %, CH4 - about 1 %. (n) is number of stations.
187
In Moscow, the main anthropogenic source of PM10 is the freight and public
188
transport with outdated and defective diesel engines, in which, in addition to fuel,
8
189
machine oil is combusted. The abrasion of roadways and vehicle tires and, in spring, the
190
suspensions of anti-icing chemical reagents and soil particles lifted into the air by winds
191
are also the source of coarse aerosol.
192
On weekdays, the night concentrations of CO, NOx, NMHC, and PM10 decrease
193
and reach their minima at 04:00–04:30. Their concentrations begin to rapidly increase
194
from 05:00 to 08:00–09:00 and reach their morning maxima. The early start of such an
195
increase in the concentrations of these pollutants and a high rate of their increase are
196
caused not only by heavy traffic during this period but also by their accumulation below
197
the surface temperature inversion which is usually begins destroying in the megacity at
198
06:00–07:00 in summer and 08:00–09:00 in winter (Isaev, 2003; Kuznetsova, 2012;
199
Pankratova et al., 2011). Such seasonal variations in temperature stratification may be
200
responsible for the formation of the bimodal structure of CO morning maximum (08:30
201
and 11:30), which is less pronounced also in the NO morning maximum. Such structure
202
is not displayed in daily NO2 and NMHC variations that are less dependent on automobile
203
transport as well as in PM10 variations because the main sources of PM10 (heavy trucks)
204
are concentrated on the outskirts of the city. After the inversion destruction and the start
205
of a rapid growth of the mixing layer, the NO2 and NMHC concentrations begin to
206
decrease and, somewhat later with the end of the morning rush hours (09:00), the CO and
207
NO concentrations also begin to decrease. After reaching their daytime minimum at
208
16:00, both CO and NOx concentrations begin to rapidly increase due to the evening
209
activity of motor transport with simultaneously increased atmospheric boundary layer
210
(ABL) stability accompanied by the formation of surface temperature inversion.
211
The annually mean daily cycle of SO2 depends, to a large extent, on the activity of
212
industrial enterprises located in Moscow and its suburbs (Butler et al., 2008; Elansky,
213
2014; Elansky et al., 2018). Since the number of such enterprises in Moscow is small and
214
the SO2 concentration is low, the Moscow Oil Refinery (MOR) significantly affects the
215
formation of its daily cycle. The MOR and different small industrial and utility
216
enterprises were repeatedly repaired and modernized during the observation period.
9
217
Partially because of this, the SO2 concentration is characterized by a high time variability.
218
The characteristic feature of the daily cycle of SO2 is its accumulation at night when the
219
ABL stability is high. The daily cycle of anthropogenic CH4 in the Moscow megacity is
220
determined by natural-gas escapes in both domestic and industrial sectors and large HPPs
221
and by emissions from treatment facilities, solid-waste landfills, and public transport
222
using natural gas. Therefore, CH4 accumulates in the night stable ABL, but the weekend
223
effect in its surface concentration is not significant. Evening maximum concentrations of
224
all pollutants (except methane and ozone) are observed at 22:00. The daily cycle of O3 is
225
characteristic of large cities (Fig. 2). On weekdays, its predawn minimum concentration
226
is observed at 7:30 and its maximum is observed at 15:00–16:00. On weekend the
227
predawn minimum absent, since at this time reduced road NO emissions attenuate ozone
228
depletion.
229
The described characteristic features of daily variations in surface pollutant
230
concentrations noticeably vary with season. The surface concentrations of CO, NOx, and
231
PM10 are affected by annual variations in traffic activity and meteorological conditions.
232
The centralized heat supply system operating from October through April noticeably
233
affects the anthropogenic emissions of NOx, NMHC, and, to a lesser degree, other
234
pollutants.
235
Fig. 3 shows the daily pollutant variations averaged over the entire data series for
236
winter and summer. The characteristic feature of winter daily variations in the pollutants
237
is a rapid increase in their concentrations in the atmospheric surface layer with the start of
238
morning business activity in the city. The concentrations of NO, PM10, and SO2 reach
239
their maxima by noon (12:00–15:00) and then decrease until 04:00–05:00 next day. The
240
beginning of an afternoon CO decrease is delayed until 18:00 due to the transport of this
241
relatively conservative pollutant from circular roads, highways running from Moscow,
242
and industrial enterprises located at the periphery of the megacity.
10
n=37
n=33
90 135
∆NO [ % ]
∆CO [ % ]
60 30 0 -30
90 45 0 -45
-60
-90
n=13
n=33 90
135
60
∆NO2 [ % ]
∆O3 [ % ]
90 45 0
30 0
-45
-30
-90
-60
n=25
n=22
6 60 4
∆SO2 [ % ]
∆CH4 [ % ]
40 2 0 -2
20 0 -20 -40
-4
-60
-6
n=23
n=14 40
∆PM10 [ % ]
∆NMHC [ % ]
30 15 0 -15
244 245
0 -20 -40
-30
243
20
0 3 6 9 12 15 18 21 24
0 3 6 9 12 15 18 21 24
Local time [ hour ]
Local time [ hour ]
Winter
Summer
Fig. 3. Daily cycle of pollutant surface concentrations in Moscow for winter and summer (the relative deviation (%) of each 20 min concentration from daily mean) averaged for all stations over 1 Jan 2005–31 Dec 2014. (n) is number of stations.
11
246
The daily cycle of NO2 is similar to that of CO. However, the accumulation of NO2
247
during daylight hours is noticeable and caused by its photochemical formation in
248
reactions of NO with O3 and VOCs and by the transport of ozone from higher levels after
249
the destruction of temperature inversion, which is necessary for the NO oxidation
250
(Elansky et al., 2018).
251
The summer profiles of the pollutants (except ozone) are characterized by an
252
afternoon decrease in their surface concentrations due to vertical mixing in the
253
atmospheric urban unstable boundary layer. The relation between the NO, NO2, NMHC,
254
and O3 concentrations is clearly seen in the summer profiles of their daily variations.
255
However, in summer, increased O3 and NMHC concentrations and a high level of UV
256
solar radiation result in a significant increase in the NO2 concentration by the end of the
257
day. In relative value, the late evening maximum of NO2 exceeds that of CO that is
258
associated with rush hours and is independent of photochemical processes. Diurnal
259
changes of CH4 in summer have much more amplitudes then in winter (Fig. 2). This
260
variability of CH4 is determined by the much higher summertime contribution of biogenic
261
sources to the methane balance and their great dependence on weather. In spring and
262
autumn (not shown in Fig. 2), the daily variations in the concentrations of many
263
pollutants are similar in shapes and intermediate in values between their winter and
264
summer values (as shown in Fig. 2). There is a noticeable difference between spring and
265
autumn concentration changes only for NO2. In spring, both morning (14 %) and evening
266
(27 %) maxima of ∆NO2 are significantly higher (in autumn respectively, 4 and 20 %)
267
due to spring maximum and autumn minimum O3 concentrations and spring higher solar-
268
illumination levels in Moscow.
269
4. Weekend effect
270
4.1.
Weekend effect in both weekly and daily cycles
271
The weekend effect is clearly pronounced in weekly pollutant cycles calculated
272
over the entire data series (Fig. 2). The concentrations of CO, NO, NO2, NOx and NMHC
273
from Friday evening and throughout Saturday night exceed their values observed on other
12
274
weekdays during the same time period. Thus, the nightlife of the city at the end of the
275
working week is most active. However, from early Saturday morning, the concentrations
276
of these pollutants and also SO2 and PM10 become significantly lower when compared to
277
those on weekdays. The daytime concentrations of these pollutants maximally decrease
278
on Sunday. In the daily cycle of CO, its characteristic morning maximum vanishes, and,
279
for NO2, NMHC, and PM10, this maximum becomes hardly noticeable. On Saturday and
280
Monday, the daytime decrease in the concentrations of the pollutants amounts to 40–50
281
% and 20–30 % of their Sunday daytime values, respectively. The night from Sunday to
282
Monday is characterized by the lowest concentrations of CO, NO, NO2, PM10, and SO2,
283
which is explained by the minimum activity of traffic and a break in the operation of
284
many industrial enterprises. Such decreased pollutant concentrations remain unchanged
285
until Tuesday early morning.
286
The variations in the surface concentration of O3 during weekend are of opposite
287
character. During Friday evening and Saturday night, the concentration of O3 is lower
288
than during the same time period on weekdays. However, on Saturday afternoon, the O3
289
concentration becomes higher than that on weekday afternoons and, on Sunday, reaches
290
its weekly maximum. On Monday night, the concentration of O3 exceeds its night values
291
for other weekdays. By Monday noon, the O3 weekend effect vanishes. A slight decrease
292
in the concentration of CH4 during weekend is observed in the afternoon and at night;
293
however, its weekday-weekend differences are insignificant. Natural gas is used as a fuel
294
by a few percent of cars, and they did not influence on weekly methane variations. The
295
leakage of CH4 in the urban gas supply system and emissions from other sources slightly
296
depend on the day of the week.
297
The Sunday reduction in pollutant concentrations averaged over the entire
298
measurement period from 2005 to 2014 are given in Table 2 and Fig. 3. Table 2 also
299
gives the 95 % confidence limits for the weekend effect estimates. The 10-year averaged
300
daytime and daily mean SE values for all the pollutants (except ozone) are positive and,
301
except CH4, significant. Maximum daytime SE (23.9 %) and daily mean SE (18.9 %) are
13
302
observed for NO and minimum daytime SE (6.3 %) and daily mean SEs (4.7 %) are
303
observed for NMHC (CH4 excluded). Both daytime SE and daily mean SE take on
304
average values 16.7 and 13.6 % for NO2 and 19.5 and 15.8 % for NOx, respectively. The
305
SO2 concentration also decreases during weekends, however, because of its large
306
variations, the SE values are close to those of the 95 % confidence limit. The SEs for CO
307
and PM10 are almost similar, i.e. approximately 13–15 % in their daytime concentrations
308
and 9–11 % in their daily mean concentrations. On Sunday the concentrations of ozone
309
(unlike other pollutants) exceeds its weekday concentrations by 16.5 % (daytime SE) and
310
13.1 % (daily mean SE). The night SEs averaged over the 10-year period are significant
311
only for NO2, PM10, and O3.
312 313 314 315
Table 2 Weekdays (Tuesday–Friday period) and Sunday differences (%) in the daytime (08:00–22:00 LT), nighttime (22:00–08:00 LT) and daily mean (00:00–24:00 LT) pollutant concentrations averaged for all stations over 1 Jan 2005–31 Dec 2014. The uncertainty of weekend effects is represented by 95 % confidence limit.
Pollutant CO
Weekend effect [ % ] 08:00–22:00
22:00–08:00
00:00–24:00
13.6 ± 3.3
2.2 ± 4.8
9.3 ± 3.2
NO
23.9 ± 5.8
6.4 ± 9.9
18.9 ± 5.6
NO2
16.7 ± 2.8
7.5 ± 3.9
13.6 ± 2.8
NOx
18.8 ± 3.0
7.8 ± 4.5
15.2 ± 3.1
O3
-16.5 ± 4.8
-9.8 ± 8.8
-13.1 ± 4.4
SO2
7.6 ± 6.5
8.1 ± 8.6
7.0 ± 6.9
PM10
14.5 ± 5.1
8.6 ± 7.2
10.9 ± 5.5
NMHC
6.3 ± 2.2
1.4 ± 2.9
4.7 ± 2.1
CH4
0.0 ± 0.2
0.1 ± 0.3
0.0 ± 0.2
316
14
δCO [ % ]
20
Winter
Spring
Summer
Autumn
Year
10 0 -10 -20 -30
δNO [ % ]
20 10 0 -10 -20 -30
δNO2 [ % ]
20 10 0 -10 -20 -30
δO3 [ % ]
20 10 0 -10 -20 -30
δSO2 [ % ]
20 10 0 -10 -20 -30
δPM10 [ % ]
20 10 0 -10 -20 -30
δNMHC [ % ]
20 10 0 -10 -20 -30
Fri
Sun
Tue
Fri
Sun
Tue
Fri
Sun
Tue
Fri
Sun
Tue
Fri
Sun
Tue
Weekday
317 318 319 320 321
Day
Night
Fig. 4. Weekly variations in 08:00–22:00 (day) and 22:00–08:00 (night) surface pollutant concentrations in Moscow for different seasons and year (relative deviation of each day concentration from average for the Tuesday–Friday period, %) averaged for all stations over 1 Jan 2005–31 Dec 2014. The uncertainty in any day point represented by 95 % confidence intervals ranged between 2 and 4 % for O3, CO, NO2, NMHC and between 3 and 6 % for NO, SO2, PM10.
15
322
4.2.
Seasonal variations
323
Sunday effects in 08:00–22:00 (day) and 22:00–08:00 (night) surface pollutant
324
concentrations in Moscow for different seasons averaged for all stations and over 2005–
325
2014 are presented in Tables 3, 4 and Fig. 4. Daytime concentrations (as well as the daily
326
mean concentrations not shown in Fig. 4) of CO, NO, NO2, and NMHC manifest their
327
stable decrease on Sundays throughout the year. On Sunday night, their concentrations
328
are minimum for all seasons, but the night SE values are not significant. The highest SEs
329
are observed for NO. Its value reaches 28 % in winter and 23–25 % in the rest of the
330
seasons. In addition, daytime SEs significantly varies from week to week within each
331
season.
332 333 334
Table 3 Weekday-Sunday differences (%) in the day, night and daily mean CO, NO, NO2, NOx, and O3 concentrations and their corresponding 95 % confidence limit for different seasons averaged for all stations over 1 Jan 2005–31 Dec 2014.
Pollutant
CO
NO
NO2
NOx
O3
Season
Weekend effect [ % ] 08:00–22:00 LT
22:00–08:00 LT
00:00–24:00 LT
Winter
14.8
±
6.3
-2.9
±
21.4
12.5
±
5.7
Spring
12.2
±
5.6
4.4
±
7.7
7.1
±
5.6
Summer
13.8
±
5.3
0.2
±
8.7
8.3
±
5.3
Autumn
14.3
±
7.4
5.7
±
9.1
10.0
±
7.5
Winter
28.0
±
14.7
7.2
±
20.9
26.0
±
13.7
Spring
23.6
±
8.8
9.3
±
14.2
17.5
±
8.6
Summer
22.9
±
10.5
1.8
±
32.7
16.3
±
10.0
Autumn
24.8
±
18.1
8.4
±
36.6
19.9
±
16.0
Winter
15.6
±
6.1
6.0
±
9.5
14.8
±
5.5
Spring
16.1
±
5.7
9.8
±
7.6
11.6
±
7.1
Summer
18.8
±
9.1
4.7
±
8.3
14.0
±
10.2
Autumn
16.7
±
13.8
9.0
±
26.3
14.7
±
13.8
Winter
18.9
±
7.6
6.3
±
11.0
17.4
±
6.9
Spring
17.3
±
5.6
10.3
±
7.7
13.4
±
6.4
Summer
19.6
±
9.0
4.5
±
9.4
14.7
±
10.1
Autumn
20.3
±
13.4
9.7
±
30.4
16.5
±
13.2
Winter
-19.7
±
13.4
-17.7
±
22.3
-13.7
±
9.9
Spring
-12.1
±
7.1
-19.1
±
49.7
-10.2
±
9.6
Summer
-8.1
±
4.8
-9.5
±
17.1
-6.9
±
4.3
Autumn
-17.3
±
11.8
-9.1
±
13.4
-12.6
±
10.9
335
For CO, the daytime SE values are noticeably lower; however, its weekly cycle
336
from season to season is more stable when compared to that for NO. The character of CO
337
seasonal variations is the same: maximum SE values (14.8 %) are observed in winter and,
16
338
in the rest of the seasons, SE values vary from 12.2 to 14.3 %. Unlike NO and CO,
339
maximum daytime SE (18.8 %) for NO2 is observed in summer. In the rest of the seasons,
340
its daytime SEs are almost the same (15.6–16.7 %). The seasonal variability of NO2 is
341
significant only during autumn. Seasonal variations in PM10 concentrations significantly
342
differ from those in CO, NO, and NO2 concentrations. Its maximum daytime SEs (about
343
21.5 %) are recorded in winter and autumn; in summer, the SE amounts to 14.6 %, and,
344
in spring, the SE is insignificant. In winter and spring, there are almost no daytime
345
weekday-weekend differences in SO2 concentrations. They are on the verge of validity in
346
summer and sufficiently significant in autumn.
347 348 349
Table 4 Weekday-Sunday differences (%) in the day, night and daily mean SO2, PM10, NMHC, and CH4 concentrations and their corresponding 95 % confidence limit for different seasons averaged for all stations over 1 Jan 2005–31 Dec 2014.
Pollutant
SO2
PM10
NMHC
CH4
Season
Weekend effect [ % ] 08:00–22:00 LT
22:00–08:00 LT
00:00–24:00 LT
Winter
-0.4
±
19.1
-1.3
±
36.7
-2.5
±
21.3
Spring
2.4
±
15.8
9.2
±
12.2
1.0
±
17.7
Summer
11.5
±
10.3
8.9
±
13.7
12.2
±
9.0
Autumn
15.2
±
11.5
11.3
±
16.5
14.1
±
10.1
Winter
21.6
±
10.5
21.8
±
16.1
17.3
±
12.4
Spring
1.9
±
14.4
-3.5
±
18.8
-2.2
±
15.9
Summer
14.6
±
7.3
7.2
±
14.4
10.0
±
7.0
Autumn
21.2
±
8.9
12.5
±
12.7
19.1
±
9.6
Winter
6.5
±
6.0
0.3
±
6.8
4.9
±
5.6
Spring
4.3
±
4.0
1.0
±
5.8
3.5
±
4.0
Summer
6.9
±
3.7
1.1
±
6.7
4.4
±
3.6
Autumn
7.6
±
5.0
3.3
±
6.3
6.2
±
4.8
Winter
0.2
±
0.6
0.1
±
0.6
0.2
±
0.5
Spring
0.0
±
0.3
0.1
±
0.5
0.0
±
0.3
Summer
-0.2
±
0.4
-0.1
±
0.7
-0.2
±
0.4
Autumn
0.2
±
0.5
0.1
±
0.7
0.1
±
0.5
350
For CH4 (not shown in Fig. 4), both daytime and night SEs are absent throughout
351
the year (Table 4). Weak SEs are observed for NMHC concentrations, because they are
352
strongly affected by vegetation and various anthropogenic sources that do not obey the
353
weekly cycle. The most pronounced SEs for NMHC are observed in summer and autumn.
354
The O3concentrations demonstrates their clearly pronounced weekend effects both by day
17
355
and at night. However, due to high night variations in the concentrations, their average
356
night SEs do not exceed the 95 % confidence limit. The mean seasonal O3 daytime SE
357
values vary from -8.1 % in summer to -17.3 % in autumn and their daily means values
358
vary from -6.9 % in summer to -13.7 % in winter.
359
The combination of both anthropogenic and meteorological factors causes
360
increased weekday-weekend differences in CO, NO, and PM10 in winter and autumn. The
361
centralized heating system most intensively operates in winter. Both morning and
362
evening rush hours most often coincide in time with a high stability of the ABL.
363
According to meteorological data obtained at the MSU Meteorological Observatory
364
(Isaev, 2003) and averaged over a period of 1958–2000, in winter, the night surface
365
temperature inversion is formed, on average, at 17:00–18:00 (LT) (mean inversion
366
recurrence is 40 % at 18:00) and is destructed at 11:00–12:00 (mean inversion recurrence
367
is 27 % at 12:00). In winter, convective boundary layer is usually observed from 12:00 to
368
17:00, but its mean recurrence at 14:00–15:00 does not exceed 20 %. Under these
369
conditions, motor transport emissions result in the accumulation of pollutants in the
370
surface air layer and determine time variations in CO, NO, and PM10 concentrations. In
371
spring and summer, most of Moscow people spend their weekends and holidays away
372
from the city, and, in autumn, when they come back, traffic in Moscow becomes most
373
active. Exchange processes during the transition periods (spring and autumn) have
374
different activity levels. The mean air temperature is lower in autumn (4.8 °С) than in
375
spring (5.8 °С), and the total flux of incoming solar radiation to the land surface is
376
smaller in autumn (497 MJ·m-2) than in spring (1253 MJ·m-2) (Isaev, 2003). Therefore,
377
exchange processes are weaker in autumn than in spring, and the SEs for CO, NO, and
378
PM10 are stronger in autumn. A great number of official holidays in March and May and
379
mass travels of Moscow people from the city to their summer houses on weekends in
380
spring additionally contribute to the weekend effect decrease in spring. These processes
381
are reflected in daytime weekday-weekend differences in other pollutants.
18
382
In spring, the SE in PM10 is completely absent. As was noted by Elansky et al.
383
(2018), the highest PM10 concentrations are observed in March–April, which is associated
384
with the melting of snow and the lifting of a lot of soil fine-dispersed particles and anti-
385
icing reagents accumulated over the winter period into the air. Unlike the CO and NO
386
weekly cycles, the PM10 concentration increases from Tuesday to Friday (Fig. 4), which
387
may be caused by increased heavy truck traffic by the end of working week; this is
388
determined by the logistics system operating in Moscow.
389
Daytime SE in SO2 are maximum in autumn. During the preparation for the winter
390
heating season (September–October), test runs of the heating system are conducted in
391
Moscow and its surroundings using liquid fuel, which results in increased SO2 emissions.
392
Abnormally dangerous SO2 emissions from the MOR more often occur on weekdays in
393
September and October. For example, in 2014, 20 events of such emissions with sharply
394
increased SO2 concentrations were simultaneously recorded at several MEM stations as
395
found in this study. In winter at very low air temperatures, reserve liquid fuel is used at
396
HPPs on any day of the week, which decreases the weekend effect.
397
Night SE in CO, NO, CH4 concentrations are within the 95 % confidence interval.
398
The absence of a significant and regularly observed SE for all pollutants in night time
399
may be due to the good ventilation of the megacity located on the plain and the high
400
variability of the ABL state. In winter and spring, the night NO2 concentrations
401
demonstrate slightly SE. However, the night NO2 maximum weekend effect was
402
observed from Saturday on Sunday nights in winter and from Sunday on Monday nights
403
in spring. This is caused by the features of NO2 formation and destruction in the absence
404
of solar illumination:
405
NO+O3→NO2+O2
(1)
406
NO2+O3→NO3+O2
(2)
407
NO+NO3→NO2+NO2
(3)
19
408
In winter, the daily mean concentrations of NO averaged for all stations over 2005–
409
2014 are maximum and those of O3 are minimum (27 and 9 ppb, respectively) and, quite
410
the reverse, in spring, the daily mean concentrations of NO are minimum and those of O3
411
are maximum (13 and 26 ppb, respectively). Thus, in reaction system (1)–(3), the limiting
412
element is O3 for winter and NO for spring. Since the minimum daily mean concentration
413
of NO was observed on Sundays, in spring this results in decreased concentrations of
414
NO2 and its strongest weekday-weekend differences on Sunday-Monday nights. In
415
winter, due to lower O3 concentrations on Saturdays than on Sundays, minimum
416
concentrations of NO2 and its maximum weekday-weekend differences are observed on
417
Saturday-Sunday nights. In summer, VOC of biogenic origin take part in both NO and
418
NO2 oxidation processes and the weekend effect become indistinct and night SE
419
differences in NO2 become insignificant.
420
4.3.
Sector structure
421
Table 5 shows the daytime SE values averaged over the entire observation period
422
for the central district of the city bounded by the Garden Ring with a diameter of 5 km
423
and each city sector: northeast, northwest, southwest, and southeast (Fig. 1). For
424
comparison, Table 5 also shows the SE values averaged over urban stations remotely
425
located from highways (i.e. city’s background) and over stations located within the
426
greenbelt surrounding Moscow. It follows from Table 5 that, for CO, NOx, and PM10 the
427
SE spatial variations over the Moscow territory are, on the whole, relatively small. This
428
implies that among their sources, the motor transport is dominant. Significant SE in CO,
429
NOx, as well as NMHC concentrations in the city center are characteristic of Moscow.
430
Most of administrative institutions and public offices, which are closed on weekends, are
431
situated here, in the city center. Consequently, the traffic (except the freight transport) –
432
the main anthropogenic source of CO, NOx, and NMHC – is least intensive in the center
433
on weekends. Since these pollutants determine the ozone balance, the largest weekday-
434
weekend negative difference in O3 (-18.3±2.9 %) is also observed in the center of
435
Moscow. In the southwest sector, there are also a lot of administrative and commercial
20
436
offices and a dense network of highways. However, unlike the city center, clear air flows
437
with high O3 concentrations arrive at the southwest sector with prevailing southwesterly
438
winds. Advection smoothes the CO, NMHC, PM10, and O3 weekend effects and promotes
439
faster NO oxidation, which is manifested in decreased NO and increased NO2 SE.
440 441 442
Table 5 Weekday-Sunday differences (%) in 08:00–22:00 pollutant concentrations averaged for different sectors of Moscow, greenbelt, all stations and for stations at any distance from roads. [n] – number of stations. CO City center
NO
NO2
O3
21.0 ± 1.6 [8]
25.1 ± 2.5 [7]
17.8 ± 1.6 [6] -18.3 ± 2.9 [3]
NW
14.6 ± 1.5 [8]
26.5 ± 2.4 [6]
NE
12.2 ± 1.7 [3]
24.8 ± 3.0 [3]
SW
16.1 ± 1.4 [7]
SE All sites City’s background Greenbelt
SO2
PM10
NMHC
8.7 ± 3.2 [3]
14.4 ± 2.3 [4]
9.9 ± 1.1 [4]
16.3 ± 1.7 [7] -16.1 ± 3.8 [2]
3.1 ± 3.4 [4]
15.0 ± 2.5 [2]
9.1 ± 1.0 [5]
17.8 ± 2.0 [3] -17.7 ± 5.1 [2]
18.3 ± 3.7 [2]
21.7 ± 2.3 [7]
19.4 ± 1.8 [7] -16.1 ± 3.4 [3]
9.6 ± 1.9 [4]
12.2 ± 1.6 [11]
23.9 ± 2.5 [10]
14.3 ± 1.5 [10] -16.5 ± 3.0 [3]
13.6 ± 3.3 [37]
23.9 ± 5.8 [33]
16.7 ± 2.8 [33] -16.5 ± 4.8 [13]
12.2 ± 3.1 [29]
23.6 ± 5.7 [27]
3.0 ± 4.3 [5]
14.1 ± 2.5 [6]
[0]
3.1 ± 3.6 [2]
16.7 ± 2.4 [3]
5.9 ± 0.9 [2]
11.1 ± 2.8 [9]
14.4 ± 2.2 [5]
7.2 ± 0.9 [10]
7.6 ± 6.5 [22]
14.5 ± 5.1 [14]
6.3 ± 2.2 [23]
17.1 ± 2.9 [27] -16.0 ± 5.3 [9]
7.0 ± 7.0 [17]
14.3 ± 5.1 [13]
5.6 ± 2.2 [19]
20.1 ± 3.7 [6]
1.2 ± 5.3 [4]
11.3 ± 7.1 [1]
1.3 ± 1.0 [2]
-4.5 ± 2.7 [1]
─────
443
Low values of CO SE are characteristic of the most polluted southeast sector. Here,
444
most of pollutant emissions are associated with both industrial and heat-and-power plants
445
continuously operating. Moreover, in this sector, a significant area is occupied by
446
Moscow treatment facilities and storage and railroad structures, however, the traffic here
447
is not very intensive. Other factors are responsible for minimum amplitudes of the CO
448
and NMHC weekly cycles in the northeast sector. In this sector, most of its territory is
449
occupied by parklands (a part of the Losiny Ostrov National Park and other forestlands).
450
Here, the traffic is least intense on weekdays, which is reflected in very similar minimum
451
SE in CO as in the southeast sector. No significant NMHC SE imply the influence of vast
452
forestlands covering a part of the urban area and extending to the humid flatland to the
453
northeast of Moscow on these pollutants.
454
The cause of maximum SE in SO2 in the northeast sector may be emissions from
455
heavy trucks that transit through Moscow southeast to northwest along highways crossing
456
forestlands. The weekend effect is clearly pronounced against the background of very low
457
SO2 surface concentrations. The SEs of both PM10 and CO concentrations reflects mainly
458
road-traffic conditions. The weekday-Sunday differences in PM10 slightly vary
21
459
throughout Moscow, although the highest values are observed in the southwest sector
460
with heavy traffic.
461
For all the sectors of Moscow, the daytime SE in the basic pollutants are
462
approximately the same for the warm (May–Sep) and cold (Oct–Apr, heating period)
463
seasons. The exception is provided by both the SO2 and O3 SEs. In the center of Moscow,
464
the SE in SO2 amount to 13 % and 6 % in the warm and cold seasons, respectively. The
465
seasonal differences are also observed in other sectors, but they are much smaller. This is
466
probably caused by combustion of liquid fuel in private houses, small boiler-rooms, and
467
at HPP in the suburbs of Moscow and, in this case, SO2 emissions arrive in the center of
468
Moscow independently of wind direction. Ozone is more significantly affected by season
469
changing. In the cold season, the weekday-Sunday differences in O3 1.5–3 times exceed
470
those in the warm season. In this case, its minimum differences are observed in the
471
southwest sector, in which the O3 behavior is determined mainly by transport and
472
unpolluted-air advection, and its maximum differences are observed in the northeast
473
sector, in which polluted air flows more often arrive from small industrial towns located
474
mainly in the east and southeast of the Moscow region. The influence of forestlands is
475
reflected in small (at significance level) SE for NMHC.
476
Within the greenbelt surrounding Moscow, there are six stations located at a
477
distance of 30 to 70 km from the city center in small inhabited localities. Here, the
478
character of weekly variations in surface pollutant concentrations strongly differs from
479
that for the city (Table 5). The CO, CH4, and SO2 weekend effects are absent. The SEs in
480
NMHC and O3 concentrations only slightly exceed the significance level. At the same
481
time, the NO, NO2, and PM10 concentrations significantly decrease on Sundays. In recent
482
decades, within the greenbelt, a great number of cottage settlements, in which people live
483
the whole year round, have been added to many dachas, in which people live during the
484
warm season or occasionally. In the 2000s, on Saturday, Sundays, and holidays, the
485
capacity of highways was regularly exceeded and traffic jams were formed on all roads
486
leading out of the city. In fact, on weekends, the highest transport activity was shifted
22
487
from the central part of Moscow to its suburbs. Taking into account the dense road
488
network and sufficiently long lifetimes of CO, SO2, NOx, and PM10, air flows with their
489
increased concentrations due to heavy traffic on weekends propagate throughout the
490
greenbelt (Elansky et al., 2010). On Sundays, the traffic increased due to Moscow cars
491
within the greenbelt and significant duration of mixing processes decrease the local
492
weekend effects of these pollutants. Only the SE in NO2 exceeds its urban value, which is
493
caused by the active transition of NO into NO2 at significantly higher O3 concentrations
494
within the greenbelt. The effect of mixing during pollution transport is also pronounced
495
over the territory of Moscow, which follows from comparison between the NO2 SE
496
obtained with and without consideration for data recorded at stations located near
497
highways (Table 5). If data obtained at these stations are excluded from consideration, the
498
averaged (over the city) SEs in all the pollutants (except NO2) slightly decrease, however,
499
these changes are still within the limits of confidence intervals.
500
4.4.
Comparison with other megacities
501
The weekend effects in O3, NOx, CO, PM10, and some VOC were studied for
502
Mexico City (Mexico), Sacramento (USA) and Beijing (China) (Stephens et al., 2008;
503
Murphy et al., 2007; Marr and Harley, 2002; Wang et al., 2014). As there was no any
504
standard technology for the WE estimations we recalculated the Moscow weekend effects
505
using those methods that used these authors. The weekend effect for Mexico City was
506
estimated according to three maximum pollutant concentrations on Sundays with respect
507
to weekday concentrations from Monday to Friday (Stephens et al., 2008). On average,
508
over a period of 2005–2007, the Sunday decrease in CO, NOx, and PM10 concentrations
509
amounted to 40, 45, and 24 %, respectively, and the Sunday increase in O3 amounts to 4.7
510
%. Similarly, for Moscow in 2005–2007, the mean three-hour decrease in CO, NOx, and
511
PM10 concentrations amounts to 23.0, 20.9, and 19.5 %, and the O3 increase amounts to
512
10.3 %. For Sacramento, USA (Murphy et al., 2007), the weekend effect was calculated
513
for the summer months of 1998–2002 from 8-h (10:00–18:00) mean concentrations on
514
weekends and weekdays. The weekday-weekend differences in NOx and PM10 amounted
23
515
to 35 and 13 %, respectively. On weekends, the O3 increase averaged over the city was
516
within 3–8 % and reached 12 % at an urban station in the city center. The decrease in
517
VOC concentrations did not exceed 10 %. Marr and Harley (2002) obtained the same
518
weekend effect value (35 %) for NOx in Sacramento. In Moscow, in the summer months,
519
the 8-h daytime NOx and PM10 concentration decrease averaged over Saturday–Sunday is
520
significantly smaller and amounts to 14.8 and 10.4 %, respectively. The NMHC
521
concentration decrease is relatively slight (4.0 %), however, at the same time, the O3
522
concentration decrease on weekends is the same as, on average, in Sacramento (5.4 %).
523
Beijing in its infrastructure and pollutant sources is close to Moscow. Wang et al.
524
(2014) calculated the weekly effect in the form of deviations in daytime (08:00–18:00)
525
pollutant concentrations averaged over Saturday–Sunday–Monday from weekday
526
pollutant concentrations averaged over a period of Wednesday–Friday. The
527
measurements were performed from July 2009 to August 2011 at four stations. In
528
Beijing, the mean decrease in the NO, NO2, and NOx concentrations on weekends
529
amounted to 22.1, 12.0, and 16.5 %, respectively, and the O3 concentration increase
530
amounted to 5.6 %. Weekend effects in NO, NO2, and NOx for Moscow, which were
531
calculated using the methodology described by Wang et al. (2014), yielded the
532
significantly smaller decrease: 10.5, 8.6, and 9.5 %, respectively. Weekend effects in CO,
533
NMHC, SO2, and PM10 (Wang et al. do not provide such data) this methodology of
534
calculation yielded for Moscow 8.8, 5.2, 4.8, and 7.2 %, respectively. However, the O3
535
concentration increase on weekend for Moscow (6.1 %) is almost the same as for Beijing.
536
Minimum pollutant concentrations shifted in Beijing to Monday imply a possibly
537
stronger effect of the advection of pollutants from regional sources on the air quality.
538
This is also supported by high NOx concentrations (62–70 ppb for daily means on
539
weekdays) in Beijing and by daily mean (24 h) weekday-weekend differences in CO,
540
PM10, and UV radiation equal 4.4, 6.0, and 3.3 %, respectively (Wang et al., 2014). These
541
values are close to Moscow daily mean weekday-weekend differences in CO, PM10, and
542
UV radiation (6.2, 7.8, and 4.0 %) calculated by analogy with Beijing. In other words, the
24
543
time interval extended to three days more noticeably decreases the weekend effect for
544
Beijing than for Moscow that may be considered as an isolated city according to a lot of
545
data (Elansky, 2014; Elansky et al., 2018). When compared to Mexico City and Beijing,
546
slight (on the whole) weekday-weekend differences in CO, NOx, and PM10 for Moscow
547
are apparently caused by increased traffic activity on weekends. On Saturdays and, to a
548
lesser degree, Sundays, it is shopping time in Moscow, all shopping and entertainment
549
centers are open on weekend.
550
4.5.
Vertical structure
551
Figure 5 shows the averaged over 1 January 2007–31 December 2014 period
552
deviations of the daytime (08:00–22:00 LT) concentrations of NO, NO2, and CO on
553
Sundays from their means on weekdays according to data obtained at heights of 10, 130,
554
248, and 348 m at the Ostankino TV tower. The variations in the concentrations of these
555
pollutants show their stable decrease at all the heights on Saturdays, Sundays, and
556
Mondays. There are a noticeable weekend effects varying with height. At the lower level,
557
they approximately coincide with means over the Moscow territory. The yearly average
558
SE in NO rapidly decreases with height from 33.3 % at a height of 10 m to 10 % at a
559
height of 348 m (Fig. 5).
25
δCO [ % ]
15
Winter
Spring
Summer
Autumn
Year
0 -15 -30 -45
δNO [ % ]
15 0 -15 -30 -45
δNO2 [ % ]
15 0 -15 -30 -45
Fri
Sun
Tue
Fri
Sun
Tue
Fri
Sun
Tue
Fri
Sun
Tue
Fri
Sun
Tue
Weekday
10 m
130 m
248 m
348 m
560 561 562 563
Fig. 5. Weekly variation in 08:00–22:00 CO, NO, NO2 concentrations (relative deviation of each day concentration from average for Tuesday–Friday) on 10, 130, 248 and 348 m platforms of the Ostankino TV tower averaged over 1 Jan 2007–31 Dec 2014. The 95 % confidence intervals ranged between 3 and 5 % for CO and between 4 and 7 % for NO and NO2.
564
The SE in CO also decreases with height but only slightly from 16.7 to 14.0 %.
565
The NO2 weekend effect is characterized by another vertical distribution. Within the
566
surface air layer (10–130 m), the NO2 SE decreases with height from 18.1 to 13.2 %, then
567
increases, reaches its maximum 24.4 % at a height of 248 m, and again decreases to 19.7
568
% at a height of 348 m.
569
Seasonal variations in the vertical structure of the weekend effect for each of the
570
pollutants have some characteristic features (Fig. 5, Table 6). At the lower level (10 m),
571
the time variations of SEs in NO, NO2, and CO are similar to those obtained from
572
ground-based measurements at MEM stations. During all seasons (except winter), the SE
573
in NO rapidly decreases with height within the lower 130-m layer. In winter, the SEs for
574
10 and 130 m are approximately the same (30 %). It follows from Fig. 5 that, in winter,
575
the concentration of O3 within the surface air layer does not increase during the day
26
576
because of frequent and long temperature inversions that hinder its influx from higher
577
levels. Under these conditions, in the daytime, the SE in NO is characteristic of the whole
578
under inversion layer of ABL. In the warm season, its daytime SEs amount to 10–15 % at
579
130 m and about 10 % at higher levels. Seasonal variations in the ABL vertical
580
stratification also affect the NO2 and CO weekend effects.
581 582 583
Table 6 Maximal weekday-Sunday differences (%) in 08:00–22:00 CO, NO, NO2 concentrations and their height levels for different seasons from measurements at Ostankino TV tower from 1 Jan 2007 to 31 Dec 2014.
Season
NO δNO [ % ]
NO2 Height [ m ]
δNO2 [ % ]
CO Height [ m ]
δCO [ % ]
Height [ m ]
Winter
29.8 ± 9.1
10
22.3 ± 5.7
248
18.2 ± 4.9
130, 248
Spring
28.5 ± 6.9
10
25.8 ± 7.6
248
13.4 ± 3.1
10, 248
Summer
33.4 ± 6.2
10
26.3 ± 8.4
348
15.4 ± 3.8
10, 348
Autumn
41.6 ± 8.8
10
25.5 ± 7.1
248
19.6 ± 4.1
10, 348
584
According to the 1988–1998 sodar data, during winter in Moscow, the recurrence
585
of temperature inversions amounts to 55 % (Isaev, 2003). The depth of temperature-
586
inversion layer increases during both night and morning hours and, by 9:00, amounts to
587
250–270 m. In the winter daytime, ozone accumulated above the inversion reacts with
588
NO coming from below, which results in the formation of NO2. The weekend effect of
589
NO is transmitted to NO2, and as seen in our study, the maximum NO2 SE is noted at a
590
height of 248 m. The same situation is observed in spring and autumn. In these seasons,
591
the recurrence of surface night inversions amounts 39 and 41 %, respectively, and their
592
mean depth during morning hours amounts to 240–260 m. Maximum SE in NO2 is noted
593
also at a height of 248 m. In summer, night inversions are formed not so frequently, their
594
mean depth does not exceed 190 m (Isaev, 2003). The early start (08:00–09:00) of
595
convection favors an active upward NO transport. In summer, maximum daytime SE in
596
NO2 are observed at the highest level (348 m).
597
The vertical structure of the weekend effect in CO differs from that of NO and NO2
598
(Table 6). Its maximum SE is observed at average levels – under and above inversion – in
599
winter and at both the lowest and highest levels in the other seasons. It is evident that the
600
inversion barrier affects the behavior of CO; however, unlike short-lived NO, the
27
601
formation of the CO weekend effect is noticeably affected by advective pollutant
602
transport from regional sources, which is especially intensive above the inversion. Since
603
industrial enterprises (including those continuously operating) are concentrated in small
604
towns located mainly in the east and southeast of the Moscow region, the advection effect
605
extended in time, changes in wind directions, and vertical-mixing intensity significantly
606
decrease the weekend effect of CO when compared to that of NO.
607
Wang et al. (2014) presented data on vertical distributions of NOx and O3 for
608
Beijing (Jul 2009 – Aug 2011). The NOX and O3 concentrations were measured at heights
609
of 8, 47, 120, and 280 m from the Beijing meteorological tower 325 m in height. Unlike
610
Moscow, in Beijing, minimum NOx concentrations occurred on Mondays at each of the
611
four heights due to reduced anthropogenic emissions and lower pollutant accumulation on
612
weekends. High NOx concentrations occurred on Wednesday–Friday. Using the NOx
613
concentration values given by Wang et al. (2014), one can estimate the weekend effect of
614
NOx in the form of its decreased mean concentration on weekends (Saturday–Monday) in
615
relation to its mean concentration on weekdays (Wednesday–Friday). The NOx weekend
616
effects amount to 14.0, 8.2, 9.4 and 15.5 % at heights of 8, 47, 120, and 280 m,
617
respectively. According to the same calculations for Moscow, the weekend effect of NOx
618
(NO and NO2 are converted to ppb) amounts to 15.0, 9.1, 10.8, and 10.6 % at heights of
619
10, 130, 248, and 348 m, respectively. The vertical structures of the weekend effects for
620
Moscow and Beijing almost coincide up to a height of about 200 m. In this case,
621
minimum weekday-weekend differences are noted at heights of 47 m for Beijing and 130
622
m for Moscow. In both megacities, at these heights, NOx is emitted from the pipes of
623
industrial enterprises and HPPs operating also on weekends. Higher than 200 m, the
624
weekend effect slightly increases in Moscow and rapidly increases in Beijing from 9.4 %
625
at 120 m to 15.5 % at 280 m. It is possible that this increase in the weekend effect is
626
caused by pollutants transported at these heights over Beijing area from regional sources,
627
which is also reflected in the shift of minimum pollutant emissions from Sunday to
628
Monday.
28
629
5. Features of the ozone chemistry
630
Several hypotheses of the formation of the ozone weekend effect were proposed
631
(Jacob, 1999; Marr and Harley, 2002; Murphy et al., 2007; Tonse et al., 2008). Ozone
632
increase due to NOx decrease is associated with two chemical mechanisms: (1) decreased
633
ozone loss due to titration reaction and (2) increased ozone production during the
634
oxidation of CO and VOCs by OH and the formation of peroxyradical HO2 and
635
alkylperoxyradical RO2:
636
OH+CO+O2→HO2+CO2
(4)
637
OH+RH+O2→RO2+H2O
(5)
638
Oxidation of NO by HO2 or RO2 results in the formation of NO2 and generation of
639
O3 (Fishman and Crutzen, 1978). Recycling OH during the oxidation of NO and
640
subsequent reactions with RO2 propagate chain reactions for ozone formation. The
641
duration of this process is determined by the elimination of radicals OH and NO2 and the
642
formation of HNO3. Weekday-weekend differences in the reaction products of this cycle
643
depend on NOx variations and the VOC/NOx ratio. An increase in VOC/NOx on
644
weekends shifts the OH chemistry toward increased production of RO2. In turn, increased
645
production of RO2 results in increased production of O3 and other VOC-NOx oxidation
646
products, such as PAN. Fast thermal decomposition of PAN regenerates RO2 and NO2,
647
which provides the continuation of the cycle with ozone formation. The VOC/NOx ratio
648
is an indicator of these processes.
649
Odd oxygen (Ox=O3+NO2) is also an indicator of the relative contribution of
650
titration and photochemical production to the ozone weekly cycle (Murphy et al., 2007;
651
Sadanaga et al., 2008; Tonse et al., 2008). Slight Ox differences between weekdays and
652
weekend or their absence imply titration as a dominant process affecting the ozone cycle.
653
Exceeded Ox concentrations on weekdays, when compared to weekend, imply the
654
dominant role of the O3 photochemical production.
29
655
The fraction of radicals lost via NOx chemistry may serve as an indicator of
656
whether VOC or NOx is more important in the production of O3 (Kleinman, 2005). If the
657
ratio of the radical loss due to NOx chemistry (LN) to the radical production rate (Q) is
658
larger than 0.5, this implies VOC-limited conditions and, if it is smaller than 0.5, this
659
implies NOx-limited conditions. Simplifying the notation of the system of equations for
660
ozone production on the basis of some additional approximations, Stephens et al. (2008)
661
obtained the equation for LN/Q where all terms can be estimated from the weekend
662
effects in different components (δX–SE in Ox, NOx, VOC, JNO2 in percent): (6)
663
664
The composition of VOCs is complex and variable in both time and space and their
665
measurements present difficulties. Stephens et al. (2008) assumed that the weekend effect
666
for VOC reactivity is similar to that for CO. Therefore, they used δCO instead of δVOC.
667
In Moscow, VOC measurements were performed using flask samples taken once a day,
668
and C2–C10 hydrocarbons were analyzed using a gas chromatograph under laboratory
669
conditions (Report, 2014; State of air pollution, 2013). The C2–C10 hydrocarbons,
670
includes basic VOCs, that determine reactivity in urban air masses and are registered by
671
NMHC instrument as well. Therefore, we used SE values in both NMHC and CO as
672
δVOC. In urban air masses there are as well some other O3 active organic compounds.
673
Therefore, the ratio VOC/NOx in Moscow atmosphere could be higher than NMHC/NOx,
674
but not very sufficiently (Isidorov, 2001).
675
Table 7 gives the averaged (over seasons and days of the week) NMHC/NOx ratios
676
calculated for Moscow on the basis of observational data obtained at all stations from
677
2005 to 2014. The NMHC/NOx ratio depends on in what units the NMHC and NOx are
678
given. In Table 7 both NMHC and NOx concentrations are given in µg·m–3. The C2–C10
679
hydrocarbon composition characteristic of Moscow and concentrations of compounds
680
actively participating in ozone formation corrected for Moscow are taken into account
30
681
(Isidorov, 2001; Stockwell et al., 2012). Therefore, we obtained the NMHC/NOx ratio in
682
µg·m–3 corresponds to 1.3 NMHC/NOx in ppbC/ppb. The NMHC/NOx ratio seasonally
683
varies slightly, which implies the same basic urban sources. Its minimum values are noted
684
in winter and maximum in summer, when biogenic emissions are added to anthropogenic
685
ones. The NMHC/NOx ratio slightly varies from Tuesday to Friday within each season.
686
All the NMHC/NOx ratio values for Moscow (from 4.1 to 5.7 in mass units) are smaller
687
than threshold values of 8–10 (Larsen et al., 2003; Murphy et al., 2007). This implies
688
that, on average during a year, VOC-limited chemistry is characteristic of Moscow, when
689
decreased NOx traffic emissions result in increased rate of ozone formation. The ozone
690
and NMHC/NOx weekend effects are most pronounced on Sunday, less pronounced on
691
Saturday, and quite slightly pronounced on Monday. The Sunday increase in the
692
NMHC/NOx ratio amounts from 6.5 % in spring to 10.9 % in autumn (Table 7). Its yearly
693
mean value is 7.9 %.
694 695 696
Table 7 The NMHC/NOx weekly cycles and relative Sunday-weekday differences (%) for different seasons averaged for stations in Moscow city and at region background conditions over 1 Jan 2005–31 Dec 2014. [n] – number of stations. NMHC/NOx
Moscow [ n=22 ]
Weekday
Winter
Spring
Summer
Autumn
Year
Mon
4.26 ± 2.43
4.54 ± 2.83
5.42 ± 3.75
4.34 ± 2.58
4.64 ± 2.98
Tue
4.14 ± 2.34
4.66 ± 2.97
5.35 ± 3.67
4.24 ± 2.57
4.60 ± 2.98
Wed
4.13 ± 2.35
4.53 ± 3.03
5.39 ± 3.60
4.29 ± 2.55
4.59 ± 2.97
Thu
4.10 ± 2.33
4.56 ± 3.00
5.33 ± 3.58
4.28 ± 2.50
4.57 ± 2.94
Fri
4.12 ± 2.34
4.63 ± 3.12
5.28 ± 3.55
4.28 ± 2.50
4.59 ± 2.97
Sat
4.34 ± 2.52
4.79 ± 3.11
5.55 ± 3.87
4.51 ± 2.65
4.80 ± 3.12
Sun
4.48 ± 2.66
4.89 ± 3.11
5.73 ± 3.92
4.74 ± 2.84
4.95 ± 3.19
8.8 ± 5.5
6.5 ± 4.5
7.4 ± 5.4
10.9 ± 7.2
7.9 ± 5.5
Mon
10.42 ± 5.97
10.20 ± 6.00
11.23 ± 6.81
11.49 ± 6.46
10.84 ± 6.31
Tue
10.39 ± 6.01
11.07 ± 6.48
10.03 ± 6.25
11.61 ± 7.81
10.88 ± 6.81
Wed
9.87 ± 5.72
11.16 ± 6.49
10.39 ± 6.08
11.32 ± 7.28
10.74 ± 6.54
Thu
10.07 ± 5.92
10.77 ± 6.46
10.66 ± 6.65
11.19 ± 7.06
10.71 ± 6.58
Fri
10.22 ± 6.17
10.41 ± 6.19
9.80 ± 6.22
11.28 ± 7.35
10.52 ± 6.60
Sat
9.96 ± 5.37
10.01 ± 5.81
10.49 ± 6.59
12.10 ± 7.50
10.73 ± 6.48
Sun
11.01 ± 6.28
11.31 ± 5.87
12.17 ± 7.54
13.35 ± 7.84
11.77 ± 7.04
8.6 ± 5.4
4.2 ± 2.4
19.1 ± 14.1
17.7 ± 12.9
9.8 ± 6.6
Sun-WD
[%]
Region background [ n=2 ]
WD
Sun-WD
[%]
WD
31
697
The NMHC/NOx ratio values for regional background stations in the greenbelt of
698
Moscow (Table 7) were obtained from observations carried out at the Zvenigorod
699
Scientific Station (ZSS) and at a MEM station in the vicinity of the small town of Troitsk.
700
These stations are located at distances of 53 and 35 km to the west and southwest of the
701
center of Moscow, respectively. Both the stations are within the zone of mixed coniferous
702
and deciduous forests and, due to prevailing westerly winds, are within plumes of
703
polluted air from Moscow during no more than 15–20 % of one-year period. The
704
NMHC/NOx ratio value for background conditions is about two times larger than that for
705
Moscow. This is the result of increased concentrations of NMHC of biogenic origin and
706
small amounts of anthropogenic NOx emissions. The NMHC/NOx ratio varies slightly
707
from 9.8 on weekdays in summer to 13.3 on Sundays in autumn. These weekday’s values
708
are about the level of threshold value. This implies transition the VOC-limited to NOx-
709
limited regime which is characteristic of the planetary boundary layer. Lower NMHC
710
concentrations within the greenbelt than in the city imply a smaller contribution of
711
anthropogenic VOCs to the O3 chemistry. Variations in the concentrations of NOx or
712
VOCs may cause a decrease or increase in the rate of O3 production during the transition
713
regime. According to one-dimensional photochemical modeling (Liu et al., 2012), the
714
similar situation is apparently may take place in Beijing (China). However, the VOC-
715
limited chemistry is characteristic of most megacities and urban regions (for example,
716
Mexico, Seoul, Shanghai, Pasadena, and urban areas in Korea) than the NOx-limited
717
chemistry (Murphy et al., 2007; Jin et al., 2012; Pollack et al., 2012; Tie et al., 2013; Yoo
718
et al., 2015).
719
Traffic emissions significantly affect NMHC/NOx ratio variations on weekdays and
720
weekends. For the Moscow megacity, Table 8 gives the NMHC/NOx ratio values
721
calculated for the time period which is characterized by the highest chemical reactivity
722
and by the following features of daily pollutant variations: 07:00 – minimum
723
concentrations of most pollutants, 07:30 – minimum O3 concentrations, 09:30 –
724
maximum concentrations of most pollutants. On weekdays, during the rush hours 07:30–
32
725
09:30, the NMHC/NOx ratio is smaller than in early morning (05:00) and at noon,
726
because NOx dominates over NMHC in road-traffic emissions. On Sundays, the traffic
727
activity is very low during morning hours and gradually increases by the middle of the
728
day. The same regime is noted for the NMHC/NOx ratio – it remains constant until 09:00
729
and increases by 15:00 due to vegetation emissions and evaporation of fuel and other
730
organic materials. By the end of the day the ratio NMHC/NOx decreases simultaneously
731
with the intensity of motor traffic. A relative rapid increase of the Sunday effect in
732
NMHC/NOx ratio occurs between 05:00 and 07:30, and then it slightly increases by 12:00
733
and gradually decreases in the afternoon.
734 735 736 737
Table 8 Temporal variations of NMHC/NOx and LN/Q ratios; O3, NO2 and Ox concentrations (µg·m-3) in the time period 05:00–18:00. Sundayweekday differences in NMHC/NOx and weekday-Sunday differences in Ox (%). Two approximations of Sunday effect values for VOC were used for calculation of the LN/Q ratios: δVOC ≈ δCO and δVOC ≈ δNMHC. Local time
05:00
07:30
09:00
12:00
15:00
18:00
Daily mean 07:30–18:00
WD
3.8 ± 1.7
3.3 ± 1.4
3.3 ± 1.3
3.4 ± 1.5
3.7 ± 1.7
3.7 ± 1.6
3.5 ± 0.2
NMHC Sun
4.0 ± 2.1
3.8 ± 2.0
3.8 ± 1.9
4.1 ± 2.1
4.3 ± 2.3
4.2 ± 2.2
4.1 ± 0.1
5.1 ± 2.3
15.6 ± 7.6
16.0 ± 7.2
19.5 ± 8.0
16.4 ± 8.1
12.9 ± 7.4
16.8 ± 0.7
35.4 ± 15.0
39.2 ± 15.4
39.5 ± 14.9
35.3 ± 15.3
33.5 ± 15.2
36.8 ± 15.2
36.2 ± 2.3
Parameter
NO
x
Sun-WD
[%]
WD
NO2 O3
Ox
WD
[µg·m-3] -3
Sun
[µg·m ]
32.3 ± 16.9
32.1 ± 16.2
31.5 ± 15.2
28.4 ± 15.3
27.7 ± 15.9
31.0 ± 16.1
29.5 ± 1.7
WD
[µg·m-3]
22.1 ± 17.6
20.4 ± 15.6
23.5 ± 13.4
35.1 ± 17.0
41.0 ± 18.7
37.4 ± 18.1
33.1 ± 7.7
-3
23.3 ± 19.7
24.8 ± 17.6
28.6 ± 15.0
39.2 ± 18.8
44.7 ± 21.6
41.2 ± 20.7
37.3 ± 7.2
-3
Sun
[µg·m ]
WD
[µg·m ]
57.3 ± 18.9
59.4 ± 19.4
63.1 ± 18.7
70.9 ± 19.3
75.7 ± 20.0
75.7 ± 19.9
67.0 ± 6.6
Sun
[µg·m-3]
55.1 ± 20.7
56.3 ± 20.0
59.7 ± 19.2
67.7 ± 20.2
73.5 ± 21.5
73.8 ± 20.8
64.3 ± 6.7
3.8 ± 9.5
5.2 ± 3.0
5.3 ± 2.6
4.5 ± 4.6
2.9 ± 7.4
2.5 ± 4.6
4.0 ± 1.8
(δCO)
0.48 ± 1.94
0.93 ± 0.17
0.95 ± 0.14
0.96 ± 0.39
0.87 ± 0.35
0.92 ± 0.17
0.92 ± 0.22
(δNMHC)
0.70 ± 2.99
0.74 ± 0.12
0.74 ± 0.10
0.72 ± 0.27
0.76 ± 0.30
0.79 ± 0.16
0.75 ± 0.17
WD-Sun
[%]
WD
LN Q
738
Early weekend morning, when the intensity of traffic is very low, the ozone
739
concentration grows faster than on weekdays (Table 8; Fig. 2). On Sundays, the excess of
740
the concentration of ozone, when compared to its average concentration on weekdays,
741
amounts to 5.6 % at 05:00 and reaches 21.5 % by 07:30. Maximum weekday-Sunday
33
742
differences in ozone occur earlier than maximum SEs in NOx, NMHC, PM10, and CO
743
(Fig. 2). Early morning on Sundays, when there is no intense traffic, a rapid increase in
744
ozone may be caused by two factors: (1) high reactivity of air masses on Sunday
745
mornings due to a more active city nightlife on weekends and accumulation of NO3,
746
N2O5, and PAN in air masses, which is reflected in low NO and high NO2 concentrations
747
and no decreasing NMHC/NOx level; and (2) mass leaving of inhabitants from the city
748
for their summer houses on Saturdays. Heavy traffic and anthropogenic combustion
749
within the greenbelt are accompanied by significant O3 precursor emissions. NOx and
750
biogenic VOCs transported during the night into the atmosphere over the city result in
751
rapid ozone formation against the background of low VOC, NO2, and CO concentrations.
752
A noticeable contribution made by advection is reflected in high morning (05:00) CO
753
concentrations. High concentrations of ozone in the atmosphere over cities at relatively
754
low concentrations of its precursors (NOx and VOC) during weekends were observed, for
755
example, in California (Marr and Harley, 2002; Qin et al., 2004) and Japan (Sakamoto et
756
al., 2005).
757
The behavior of O3, NO2, and odd oxygen (Ox=O3+NO2) throughout the day also
758
demonstrates the influence of lower weekend NO emissions on ozone production rates
759
(Fig. 2; Table 8). In Moscow, the daily means of Ox amount to 67 µg·m–3 on weekdays
760
and 64.3 µg·m–3 on Sundays. These high values show that NO emissions have sufficient
761
time to be photochemically processed by reaction with O3 and RO2 into Ox. Differences
762
in the behavior between O3 and Ox during a week suggest that the interconversion of O3
763
to NO2 can make a noticeable contribution to the weekend effect. The effect of ozone
764
suppression by NO emissions takes place throughout the day, however, this effect is most
765
pronounced during morning rush hours, when fresh NO emissions activate ozone
766
titration, although the general ozone-chemistry regime in Moscow remains to be VOC-
767
limited.
768
At ZSS and Troitsk regional background stations the mean concentrations of O3
769
and NO2 in 05:00–18:00 averaged over 2005–2014 are equal 28.0 and 23.9 µg·m–3,
34
770
respectively. The mean odd-oxygen concentration is about 52 µg·m–3. The NO2
771
concentration in this area is significantly lower, when compared to ozone, and slighter
772
affects odd-oxygen patterns. In the vicinity of highways and small towns, NO2 may more
773
significantly contribute to odd oxygen. On the whole, the Ox value and its time variability
774
support the above conclusion that the transition VOC-limited to NOx-limited regime
775
dominates within the greenbelt. Therefore, data obtained from ozone observations at
776
individual stations and used for air quality estimations should be extrapolated to the
777
region with consideration for simultaneous NO2 observations.
778
Table 8 gives the calculated daily LN/Q variations averaged over the entire
779
observation period. Since the complete composition of VOCs was not measured, only
780
two versions were considered, when (1) δVOCs were replaced by δNMHCs, which
781
include the main components of NOx-O3-VOC interactions; and (2) δVOCs were replaced
782
by δСO (such a replacement is used, at least, for Mexico and Beijing (Stephens et al.,
783
2008; Wang et al., 2014)).Variations in surface NO2 photolysis frequencies δJNO2 are
784
comparable to those in actinic fluxes at UV wavelengths. Using relations between δPM10
785
and δJ proposed by Wang et al. (2014) on the basis of PM2.5, PM10, and UV radiation
786
measurements in the Beijing region, we obtain that 14.4 % decrease in PM10 on Sundays
787
for Moscow corresponds to δJ equal to 4 %.
788
Early morning (05:00) under the conditions of low illumination and small NO
789
emissions from traffic, the rate of photochemical production of O3 is very low. The LN/Q
790
ratio (δNMHC is used as δVOC) amounts to 0.70. After 07:30 with the start of morning
791
rush hours, the LN/Q ratio grows a little and keeps between 0.72–0.79. This implies that
792
about 74 % of total radical loss is removed via NOx chemistry. Calculations of LN/Q for
793
the all Moscow sectors yield similar results, LN/Q ratios vary from 0.69±0.15 in SE sector
794
to 0.8±0.17 in the city center, which suggests that the VOC-limited regime is
795
characteristic of the entire Moscow megacity. Similar results were obtained by Stephens
796
et al. (2008) for Mexico City, however, δCO instead of VOC was used in calculations of
797
LN/Q. The LN/Q ratios (for Mexico City) calculated from differences in three-hour δX
35
798
values between weekdays (Monday–Friday) and Sunday (2004–2007 measurements)
799
amount to 0.90–0.95 and coincide with the LN/Q ratio values for Moscow only in the
800
case, when δCO is used instead of δVOC. The discrepancies between LN/Q (obtained
801
with replacement of δVOC by δCO and δNMHC, table 3) imply a high sensitivity of
802
photochemical-reactivity estimates to the composition and concentration of VOCs.
803
6. Conclusions
804
Moscow is a fast-developing megacity within the administrative boundaries that
805
remained unchanged for a long time. Therefore Moscow has become one of the most
806
densely populated cities in the world. A large scatter in estimates of pollutant emissions
807
from urban sources, which were obtained using both inventory and observational data
808
(Elansky et al., 2018), hampers the use of chemical-transport models in simulating air
809
quality and taking steps to improve it. The obtained features of the pollutant weekend
810
effect for Moscow and its greenbelt may help in diagnosing the pollutant regime and
811
suggest an effective action plan. For the Moscow megacity, the daytime (08:00–22:00)
812
weekday-Sunday differences averaged over the 10-year measurement period at 49
813
stations show that the Sunday effects in NO, NO2, CO, NMHC, SO2, and PM10
814
concentrations amounts to 23.9, 16.7, 13.6, 6.3, 7.6, and 14.5 %, respectively. The daily
815
mean (24 h) weekday-Sunday differences in these pollutants are also significant. The
816
concentration of CH4 decreases on Sundays, but its decrease is not reliable. On Sundays,
817
ozone increases by 16.5 %. For comparison with other cities (Mexico City, Sacramento,
818
Beijing, urban areas of Korea) the weekend effect for Moscow was recalculated using the
819
methodologies excepted by authors of corresponding studies. The weekend decrease in
820
the basic pollutants in Moscow is significantly smaller than that in cities located at
821
southern latitudes then Moscow, however, the ozone increase is approximately the same.
822
Combinations of both anthropogenic and meteorological factors affect weekly
823
patterns and weekend effects of air pollution in the Moscow. Among anthropogenic
824
factors, the main factors are the motor transport, the operation of the central heating
825
system in Moscow and its suburbs (October–April), and mass travels of inhabitants from
36
826
the city to their country houses on weekends. Meteorological processes are manifested in
827
the form of annual variations in the recurrence, duration, and intensity of temperature
828
inversions, air mass advection and convective mixing. In connection with this, pollutant
829
emissions and the NO, CO, and PM10 weekend effects are most significant in winter.
830
Maximum weekend effects in NO2 are noted in summer and in O3 are noted in winter and
831
autumn. There are no differences in PM10 in spring because of the lifting of a large
832
amount of aerosols into the air after snow melting. The weekend effect of SO2, which is,
833
to a great extent, associated with emissions from heat and power plants, including those
834
located outside Moscow, is significant only in summer and autumn, when the effect of
835
their emissions is minimum as compared to traffic emissions. Unlike other pollutants, the
836
daytime weekend effect of SO2 is noticeably different for all the sectors of Moscow.
837
The weekly pollutant cycles within the greenbelt differ from those within the city.
838
There are no weekday-weekend reliable differences in CO, CH4, SO2 concentrations, in
839
NO, NO2, PM10 weekend effects are significantly weakened, in NMHC and O3 effects are
840
at the reliable level. Regular travels of inhabitants from the city to their country houses
841
densely built up by the early 2000s shift, on weekends, the highest road-traffic activity
842
towards the suburbs of the city and decrease the weekend effect within the greenbelt.
843
Data obtained from measurements at the Ostankino TV tower show a stable
844
decrease in the concentrations of NO, NO2, and CO on weekends at all height levels up to
845
348 m. The weekend effects in NOx and CO concentrations are decreased on weekends at
846
a height of 130 m. The comparison of the Moscow and Beijing vertical structure of
847
weekend effects shows that the influence of transport of pollutants from regional sources
848
on their weekly cycles is weaker in Moscow than in Beijing. This result support the
849
conclusion made up earlier by Elansky et al. (2018) that Moscow may be considered as
850
an isolated city.
851
Photochemical processes determining the formation of the ozone weekly cycle in
852
Moscow are similar to those observed in other megacities. The obtained estimates of the
853
basic criteria of the activity of such processes (time variability of the OX concentration
37
854
and the ratios of NMHC/NOx and LN/Q) give an idea of the ozone weekly-cycle
855
formation characteristic of Moscow. Seasonal, weekly and daily changes in these criteria
856
imply the VOC-limited O3 chemistry. Under these conditions, even a slight decrease in
857
VOC concentrations is accompanied by a noticeable decrease in ozone concentrations. At
858
regional background stations remotely located from Moscow, these criteria implies the
859
transition VOC-limited to NOX-limited regime. The obtained characteristics of the spatial
860
and temporal variability of the pollutant weekly patterns and weekend effects are
861
important for adapting chemistry-transport and climate models to the conditions of
862
Moscow and simulation possible changes in air quality and regional climate. The results
863
are useful for developing measures to improve the state of urban infrastructure.
864
Acknowledgements. The authors thank the management of the Moscow Department of
865
Natural Resources and Environmental Protection and the staff of the Moscow Ecological
866
Monitoring network for observational data provided. The collection and analysis of
867
longterm observation data were carried out with the support of the Russian Science
868
Foundation (project No.16-17-10275). The methodology for calculating weekly cycle
869
parameters was developed in the framework of the Russian Foundation for Basic
870
Research project No.17-29-05102 , the balance of radicals in the atmosphere and the
871
aerosol content in the urban atmosphere were estimated according to the RFBR project
872
No. 19-05-00352.
873
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45
Highlights Daily and weekly cycles in O3, CO, CH4, NO, NO2, SO2, NMHC, and PM10were analyzed Measurements at 49 sites in 2005-2014 were used for species weekend effect studies Weekend effect in pollutants depends on PBL stratification, city sectors and height The VOC-limited chemistry of O3 formation is characteristic of Moscow atmosphere
Declaration of interests ☑The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
S1352-2310(20)30045-5
DOI:
https://doi.org/10.1016/j.atmosenv.2020.117303
Reference:
AEA 117303
To appear in:
Atmospheric Environment
Received Date: 13 June 2019 Revised Date:
24 December 2019
Accepted Date: 19 January 2020
Please cite this article as: Elansky, N.F., Shilkin, A.V., Ponomarev, N.A., Semutnikova, E.G., Zakharova, P.V., Weekly patterns and weekend effects of air pollution in the Moscow megacity, Atmospheric Environment (2020), doi: https://doi.org/10.1016/j.atmosenv.2020.117303. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Ltd.
Credit Author Statement 1. N.F. Elansky – Conceptualization, Methodology, Investigation, Writing Original Draft, Writing - Review & Editing, Supervision, Project administration, Funding acquisition; 2. A.V. Shilkin – Writing - Review & Editing, Visualization, Formal analysis, Software, Validation; 3. N.A. Ponomarev – Writing - Review & Editing, Validation; 4. E.G. Semutnikova – Resources, Data Curation, Project administration, Funding acquisition; 5. P.V. Zakharova – Resources, Data Curation, Formal analysis.
1
Weekly patterns and weekend effects of air pollution in the Moscow megacity
2
N.F. Elansky1, A.V. Shilkin1,2, N.A. Ponomarev1,3, E.G. Semutnikova3,4, P.V. Zakharova3,5
3
1
A.M. Obukhov Institute of Atmospheric Physics, Russian Academy of Sciences, Pyzhevsky pereulok, 3,
4
Moscow, 119017, Russia
5
2
Research and Production Association “Typhoon”, Pobedy st., 4, Obninsk, 249038, Russia
6
3
Lomonosov Moscow State University, Leninskie Gory, 1–2, Moscow, 119234, Russia
7
4
Department for Environmental Management and Protection, Novyi Arbat, 11, Moscow, 119019, Russia
8
5
Budgetary Environmental Protection Institution “MosEcoMonitoring”, Novyi Arbat, 11, Moscow, 119019,
9
Russia
11
Keywords: megacity environment; air quality; pollutant variations; weekend effect; ozone production; anthropogenic emissions.
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Abstract
10
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The weekly cycle and weekend effect in the O3, NO, NO2, CO, CH4, SO2, NMHC,
14
and PM10 concentrations were investigated in the Moscow megacity using in-situ
15
measurements from 1 January 2005 to 31 December 2014 at 49 stations of the Moscow
16
Environment Monitoring network. Daily variations in the CO, NOx, NMHC, and PM10
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concentrations depend mainly on motor transport emissions and the atmospheric
18
boundary layer vertical stratification. The characteristic feature of Moscow is the time
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coincidence of rush hours and surface temperature inversion during the cold season,
20
which results in pollutant accumulation in the atmospheric surface layer. It was found
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that the surface concentrations of the pollutants (except ozone and methane) decrease on
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weekends. Weekday(Tuesday–Friday)-Sunday differences in the daytime (08:00–22:00
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LT) NO, NO2, CO, SO2, NMHC, and PM10 concentrations relative to those of weekday
24
period averaged for all stations over 2005–2014 amounted to 23.9±5.8, 16.7±2.8,
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13.6±3.3, 7.6±6.5, 6.3±2.2, and 14.5±5.1 %, respectively. The ozone concentration
26
increased on Sunday by 16.5±4.8 %. The methane concentration on weekends was the
27
same as on weekdays. The weekend effects in all pollutant concentrations were weakened
28
within the greenbelt around Moscow. In different sectors of Moscow the pollutant
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weekend effects except that in SO2 were approximately the same. The vertical structure of
1
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the NO, NO2, and CO weekend effects was analyzed based on data obtained from
31
measurements at the TV tower 500 m in height. These weekend effects decreased
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nonlinearly with height. Estimates obtained for basic criteria of activity of photochemical
33
processes determining the formation of the weekly cycle and weekend effect of ozone
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(NMHC/NOx ratio, fraction of radical loss via NOx chemistry, concentration of Ox) show
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that the VOC-limited chemistry is characteristic of Moscow.
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1. Introduction
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The atmosphere over Moscow has been the subject of considerable scientific
38
attention in recent years. First, because, in the early 2000s, the up-to-date system of
39
monitoring the composition of surface air started operating and qualitative data became
40
accessible. Second, because due to a rapid change in the urban infrastructure in the 1990s
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(economic crisis accompanied by a dramatic setback in industrial production), the
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intensity of pollutant sources decreased and resulted in changes in the urban
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photochemical system. Due to both rapid population growth and urban-area extension,
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Moscow has become one of the largest megacities in the world. If, in 2005, the
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population of the Moscow megacity was 12.1 million residents and its area was about
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1950 km2, in 2014, the Moscow population was already 13.8 million residents and the
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area increased up to 2210 km2 (Elansky et al., 2018). Over the same 10-year period, the
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number of cars increased from 3 to 4.1 million cars, however, the industrial production
49
increased only by 10 %, while remaining at 40 % below the 1991 level. According to the
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composition and volume of pollutant emissions, Moscow approached the megacities of
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the United States, European Union, and Japan (Butler et al., 2008; GAW WMO, 2012;
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Elansky, 2014; Elansky et al., 2018).
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The weekly cycle of the urban-air quality is the characteristic feature of almost all
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large cities. As a rule, the largest amounts of pollutants are emitted on working days and
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their smallest amounts are emitted on weekends. The relation of variations in ozone and
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other pollutants with the weekly cycle of the operation of industrial enterprises and traffic
57
intensity was illustrated by the example of large cities in the United States (Cleveland et
2
58
al., 1974; Lebron, 1975; Jacobson, 1975). Later studies revealed the relation of the
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weekend effect with climatic conditions, relief, urban-infrastructure features, and other
60
factors (Bronnimann and Neu, 1997; Paschalidou and Kassomenos, 2004; Murphy et al.,
61
2007; Stephens et al., 2008). In every city, the weekend effect is manifested in both
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atmospheric photochemical and transport processes that are characteristic of this concrete
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city (GAW WMO, 2012; Murphy et al., 2007; Stephens et al., 2008; Yoo et al., 2015;
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Wang et al., 2014). Therefore, the weekend effect is an efficient tool for validating results
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obtained from simulations of atmospheric-composition variations under different
66
emission scenarios, which is important in forecasting the air quality and extreme
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ecological situations (Koo et al., 2012).
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Moscow, the northernmost and coldest megacity, is located in the middle of the
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East European Plain. Moderate average air temperatures, low solar UV radiation levels,
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and good ventilation make the accumulation of pollutants emitted into the surface air and
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the photochemical formation of secondary (often toxic) pollutants (ozone, organochlorine
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compounds, extra fine aerosol, and others) less intensive (Bezuglaya and Smirnova,
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2008; Kuznetsova, 2012; Gorchakov et al., 2010;Elansky, 2014). However, an increase in
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the number of cars, building boom, and the widespread use of new chemically active
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substances in industry and everyday life begin to manifest themselves in the activation of
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photochemical processes. Both increasing frequency and duration of blocking
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anticyclones observed in recent years serve to increase the oxidation ability of the
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atmosphere (Mokhov et al., 2014; Elansky et al., 2011; Pankratova et al., 2011). The
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1995–2010 transcontinental observations of the atmospheric composition from a mobile
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railroad laboratory (TROICA experiments, Elansky et al., 2009; 2016) revealed a
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noticeable increase in the rate of ozone formation exceeding that over neighboring rural
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areas during the summer months (Elansky and Lavrova, 2014; Elansky et al., 2010;
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2015).
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Previous studies of temporal variations of surface pollutant concentrations showed
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the presence of a weekend effect in Moscow. However, the volume of data was limited,
3
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therefore, pollutant concentrations observed at one station during only a few months
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(Elansky et al., 2011) or concentrations of one (CO) or two (CO and PM10) components
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observed at a few stations within 1–6 years (Gorchakov et al., 2010; Gorchakov et al.,
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2014; Sitnov, 2011a) were used for analysis. The weekend effect was also manifested in
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the atmosphere over the Moscow region according to observational NO2 data obtained at
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the Zvenigorod Scientific Station (ZSS) located 53 km west of the city center (Gruzdev,
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2011) and from satellites (Sitnov, 2011b; Gruzdev, 2013).
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Both daily and weekly cycles and weekend effects of the basic pollutants – O3, CO,
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CH4, NO, NO2, SO2, NMHC, and PM10 – in the atmospheric surface layer over the
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Moscow megacity were analyzed in this work. A complete archive of measurement data
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obtained from 2005 to 2014 at 48 stations entering the Moscow ecological monitoring
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system (MosEcoMonitoring (MEM)) and at the Ecological Station of the А.М. Obukhov
98
Institute of Atmospheric Physics (ES OIAP) has been used for analysis. The plan of the
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Moscow megacity with the location of the stations MEM and ES OIAP is presented at the
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Fig. 1. The obtained parameters of weekly cycles of pollutants concentrations were used
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to study the intensity of oxidative processes and the balance of radicals in the urban
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photochemical system.
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2. Methodology
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In Moscow, the number of MEM regularly operating stations increased from 25 in
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2005 to 34 in 2010 and remained unchanged in the following years. In addition, 14
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stations operated irregularly within periods of 0.5–2 years. From 1 January 2002 to 31
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December 2014, the concentrations of trace gases and aerosols were also continuously
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measured at the ES OIAP. This station is located on the Vorob’evy Gory in a green park
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zone 7 km southwest of the center of Moscow. Data obtained at this station reflect the
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mean level of Moscow air pollution caused by emissions from different sources (Elansky
111
et al., 2015). The MEM stations and the ES OIAP continuously measured about 20
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pollutants (in accordance with the World Health Organization recommendations), wind
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direction and speed, air temperature, and air relative humidity. Surface pollutant
4
114
concentrations and meteorological parameters averaged over 20 min intervals (within
115
each 1 hour: 00–20, 20–40, 40–60 min) were transmitted in the on-line regime to the
116
MEM data analysis center, in which these data were processed, verified and archived.
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The procedure of data screening was used to eliminate unrealistic values due to, for
118
example, a momentary effect of some car driven up to the station. The number of CO,
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NO, and NO2 missing hourly values at regularly operating stations does not exceed 10 %;
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the number of missing SO2, CH4, and PM10 values does not exceed 15 %. At the ES
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OIAP, the number of omissions does not exceed 5 % for all measured pollutants. The
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features of time variations in the vertical distribution of CO, NO, and NO2 in the mixing
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layer over the city were analyzed using the 1 January 2007–31 December 2014
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observational data obtained at the TV Ostankino tower (7 km north of the city center) at
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heights of 10, 130, 248, and 348 m.
126 127 128
Fig. 1. The territories of the Moscow megacity and Greater Moscow. Yellow circles indicate the position of regularly working MEM stations at present, red squares – greenbelt MEM stations, the red triangle – Ostankino TV tower, and the red asterisk – ES OIAP.
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The MEM stations and the ES OIAP are equipped with modern instruments
130
manufactured by TEI (O3, CO, SO2, NOx, CH4, and non-methane hydrocarbons –
5
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NMHC), Horiba (CH4, NOx, and NMHC), Dasibi (O3), TSI (aerosol), and other leading
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companies. There is a calibration laboratory at the MEM network. Standard instruments
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and gas mixtures from the Russian Institute of Metrology, Max Planck Institute for
134
Chemistry (Germany), Earth System Research Laboratory (United States), and other
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centers were used to calibrate the instruments at the MEM network and the ES OIAP
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(Bulletin, 2008; Elansky et al., 2009). One of the MEM stations is located in the vicinity
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of the ES OIAP on the territory of the Moscow State University (MSU). Regular
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comparisons of data obtained at these two stations showed agreement between pollutant
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concentrations within measurement errors (Elansky, 2014; Elansky et al., 2015).
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In order to study both daily and weekly cycles and weekend effects in pollutants,
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the 2005–2014 data (averaged over 20 min) on O3, CO, CH4, NO, NO2, SO2, NMHC, and
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aerosol (PM10) surface concentrations measured at 14, 42, 31, 39, 39, 26, 25, and 15
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MEM stations, respectively, including those irregularly operating, and at the ES OIAP
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were used (Table 1). Among these stations, 10 of them are located in the vicinity of
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highways, 14 stations are in residential districts, 15 stations in zones with high business
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and industrial activities, 2 stations in park zones (urban background conditions), and 6
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stations
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http://www.mosecom.ru/stations/). Geographical positions of monitoring stations in the
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Moscow megacity and surrounding territories are available (see http://mosecom.ru/map)
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for different chemical species analyzed in this study. Independently of the number of
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stations measuring each pollutant, their distribution by the type of surrounding territory
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remained unchanged.
153 154
Table 1 Number of stations measured pollutant concentrations from 1 Jan 2005 to 31 Dec 2014 and total number of data averaged over 20 min.
within
the
greenbelt
(regional
background
conditions)
(URL:
Pollutant
CO
NO
NO2
NOx
O3
Ox
SO2
PM10
NMHC
CH4
Number of stations
42
39
39
38
14
10
26
15
25
31
6.5E6
5.4E6
5.6E6
5.5E6
2.1E6
1.5E6
2.1E6
1.4E6
2.4E6
2.8E6
Total number of measurements
155
The superposed epoch method, in which all Sundays of the time series under
156
analysis served as reference points, was used to determine the weekend effect value. Both
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linear trend and variations with a period of over 8 days were preliminarily eliminated
158
from data on each of pollutant concentrations at each station using the inverse Fourier
159
transform method. The variations obtained are expressed in relative units by their term-
160
by-term division by difference between them and initial data. Weekly cycles were
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obtained using the superposed epoch method for each pollutant at each station. Weekly
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cycles averaged over Moscow, which characterize variations in average pollution
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concentrations for Moscow during a week averaged over the 10-year period, were also
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obtained. In this study by the weekend effect we mean the Sunday effect (percentage) that
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was calculated using the following formula: SE = ([weekday]–[Sunday]) / [weekday],
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where weekday period includes four days from Tuesday to Friday. A large amount of
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data allows us to study the manifestation of the Sunday effect in the pollutant
168
concentrations separately for day and night time: daytime SE (08:00–22:00 LT), night SE
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(22:00 Sunday–08:00 Monday), and daily mean SE (08:00 Sun–08:00 Mon). We also
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used other time intervals for weekend effect calculations to make a comparison with other
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megacities.
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3. The daily cycle of pollutants
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The daily cycles of CO, NO, NO2, O3, CH4, SO2, NMHC, and PM10 surface
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concentrations averaged for all stations and all days over 2005–2014 are shown in Fig. 2.
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Due to the centralized heat supply of both residential and industrial sectors from large
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heat and power plants (HPP) using natural gas, the main source of CO in Moscow is
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automobile transport (approximately 85–90 % of all emissions) (Bulletin, 2008; Report,
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2015). The relative contribution of automobile transport to NOx concentration variations
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is somewhat smaller due to HPP emissions and more significant influence of both
180
photochemical and meteorological processes on NOx concentrations. In the warm season,
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in addition to automobile transport, fuel-supply stations, industrial enterprises, other
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urban sources, and vegetation noticeably affect NMHC concentrations.
7
Concentration [ µg⋅m -3 ] Concentration [ µg⋅m -3 ] Concentration [ µg⋅m -3 ] Concentration [ µg⋅m -3 ] Concentration [ µg⋅m -3 ]
183
CO n=37
NO n=33
30
650 600
25
550 20
500 450
15
O3 n=13
45
NO2 n=33
45
40
40
35 35
30 25
30
20
25
CH4 n=25
1350
SO2 n=22
4.8
1340
4.6
1330 4.4
1320 1310
4.2
1300
4
NMHC n=23
230
PM10 n=14
28 26
215
24 22
200
20 185
18
NOx n=32
70
Ox n=9
80 75
60
70 65
50 60 40
55 0
3
6
9 12 15 Local time [ hour ]
Mon
Tue
18
21
Wed
24
0
3
Thu
6
9 12 15 Local time [ hour ]
Fri
Sat
18
21
24
Sun
184 185 186
Fig. 2. Daily cycle of pollutant surface concentrations in Moscow megacity averaged for all stations over 1 Jan 2005–31 Dec 2014. The 95 % confidence intervals for all of the points of O3, NO, NO2, SO2, CO, PM10, NOx (NOx=NO+NO2), and Ox (Ox=O3+NO2) ranged between 2 and 4 %, NMHC between 4 and 7 %, CH4 - about 1 %. (n) is number of stations.
187
In Moscow, the main anthropogenic source of PM10 is the freight and public
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transport with outdated and defective diesel engines, in which, in addition to fuel,
8
189
machine oil is combusted. The abrasion of roadways and vehicle tires and, in spring, the
190
suspensions of anti-icing chemical reagents and soil particles lifted into the air by winds
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are also the source of coarse aerosol.
192
On weekdays, the night concentrations of CO, NOx, NMHC, and PM10 decrease
193
and reach their minima at 04:00–04:30. Their concentrations begin to rapidly increase
194
from 05:00 to 08:00–09:00 and reach their morning maxima. The early start of such an
195
increase in the concentrations of these pollutants and a high rate of their increase are
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caused not only by heavy traffic during this period but also by their accumulation below
197
the surface temperature inversion which is usually begins destroying in the megacity at
198
06:00–07:00 in summer and 08:00–09:00 in winter (Isaev, 2003; Kuznetsova, 2012;
199
Pankratova et al., 2011). Such seasonal variations in temperature stratification may be
200
responsible for the formation of the bimodal structure of CO morning maximum (08:30
201
and 11:30), which is less pronounced also in the NO morning maximum. Such structure
202
is not displayed in daily NO2 and NMHC variations that are less dependent on automobile
203
transport as well as in PM10 variations because the main sources of PM10 (heavy trucks)
204
are concentrated on the outskirts of the city. After the inversion destruction and the start
205
of a rapid growth of the mixing layer, the NO2 and NMHC concentrations begin to
206
decrease and, somewhat later with the end of the morning rush hours (09:00), the CO and
207
NO concentrations also begin to decrease. After reaching their daytime minimum at
208
16:00, both CO and NOx concentrations begin to rapidly increase due to the evening
209
activity of motor transport with simultaneously increased atmospheric boundary layer
210
(ABL) stability accompanied by the formation of surface temperature inversion.
211
The annually mean daily cycle of SO2 depends, to a large extent, on the activity of
212
industrial enterprises located in Moscow and its suburbs (Butler et al., 2008; Elansky,
213
2014; Elansky et al., 2018). Since the number of such enterprises in Moscow is small and
214
the SO2 concentration is low, the Moscow Oil Refinery (MOR) significantly affects the
215
formation of its daily cycle. The MOR and different small industrial and utility
216
enterprises were repeatedly repaired and modernized during the observation period.
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217
Partially because of this, the SO2 concentration is characterized by a high time variability.
218
The characteristic feature of the daily cycle of SO2 is its accumulation at night when the
219
ABL stability is high. The daily cycle of anthropogenic CH4 in the Moscow megacity is
220
determined by natural-gas escapes in both domestic and industrial sectors and large HPPs
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and by emissions from treatment facilities, solid-waste landfills, and public transport
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using natural gas. Therefore, CH4 accumulates in the night stable ABL, but the weekend
223
effect in its surface concentration is not significant. Evening maximum concentrations of
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all pollutants (except methane and ozone) are observed at 22:00. The daily cycle of O3 is
225
characteristic of large cities (Fig. 2). On weekdays, its predawn minimum concentration
226
is observed at 7:30 and its maximum is observed at 15:00–16:00. On weekend the
227
predawn minimum absent, since at this time reduced road NO emissions attenuate ozone
228
depletion.
229
The described characteristic features of daily variations in surface pollutant
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concentrations noticeably vary with season. The surface concentrations of CO, NOx, and
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PM10 are affected by annual variations in traffic activity and meteorological conditions.
232
The centralized heat supply system operating from October through April noticeably
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affects the anthropogenic emissions of NOx, NMHC, and, to a lesser degree, other
234
pollutants.
235
Fig. 3 shows the daily pollutant variations averaged over the entire data series for
236
winter and summer. The characteristic feature of winter daily variations in the pollutants
237
is a rapid increase in their concentrations in the atmospheric surface layer with the start of
238
morning business activity in the city. The concentrations of NO, PM10, and SO2 reach
239
their maxima by noon (12:00–15:00) and then decrease until 04:00–05:00 next day. The
240
beginning of an afternoon CO decrease is delayed until 18:00 due to the transport of this
241
relatively conservative pollutant from circular roads, highways running from Moscow,
242
and industrial enterprises located at the periphery of the megacity.
10
n=37
n=33
90 135
∆NO [ % ]
∆CO [ % ]
60 30 0 -30
90 45 0 -45
-60
-90
n=13
n=33 90
135
60
∆NO2 [ % ]
∆O3 [ % ]
90 45 0
30 0
-45
-30
-90
-60
n=25
n=22
6 60 4
∆SO2 [ % ]
∆CH4 [ % ]
40 2 0 -2
20 0 -20 -40
-4
-60
-6
n=23
n=14 40
∆PM10 [ % ]
∆NMHC [ % ]
30 15 0 -15
244 245
0 -20 -40
-30
243
20
0 3 6 9 12 15 18 21 24
0 3 6 9 12 15 18 21 24
Local time [ hour ]
Local time [ hour ]
Winter
Summer
Fig. 3. Daily cycle of pollutant surface concentrations in Moscow for winter and summer (the relative deviation (%) of each 20 min concentration from daily mean) averaged for all stations over 1 Jan 2005–31 Dec 2014. (n) is number of stations.
11
246
The daily cycle of NO2 is similar to that of CO. However, the accumulation of NO2
247
during daylight hours is noticeable and caused by its photochemical formation in
248
reactions of NO with O3 and VOCs and by the transport of ozone from higher levels after
249
the destruction of temperature inversion, which is necessary for the NO oxidation
250
(Elansky et al., 2018).
251
The summer profiles of the pollutants (except ozone) are characterized by an
252
afternoon decrease in their surface concentrations due to vertical mixing in the
253
atmospheric urban unstable boundary layer. The relation between the NO, NO2, NMHC,
254
and O3 concentrations is clearly seen in the summer profiles of their daily variations.
255
However, in summer, increased O3 and NMHC concentrations and a high level of UV
256
solar radiation result in a significant increase in the NO2 concentration by the end of the
257
day. In relative value, the late evening maximum of NO2 exceeds that of CO that is
258
associated with rush hours and is independent of photochemical processes. Diurnal
259
changes of CH4 in summer have much more amplitudes then in winter (Fig. 2). This
260
variability of CH4 is determined by the much higher summertime contribution of biogenic
261
sources to the methane balance and their great dependence on weather. In spring and
262
autumn (not shown in Fig. 2), the daily variations in the concentrations of many
263
pollutants are similar in shapes and intermediate in values between their winter and
264
summer values (as shown in Fig. 2). There is a noticeable difference between spring and
265
autumn concentration changes only for NO2. In spring, both morning (14 %) and evening
266
(27 %) maxima of ∆NO2 are significantly higher (in autumn respectively, 4 and 20 %)
267
due to spring maximum and autumn minimum O3 concentrations and spring higher solar-
268
illumination levels in Moscow.
269
4. Weekend effect
270
4.1.
Weekend effect in both weekly and daily cycles
271
The weekend effect is clearly pronounced in weekly pollutant cycles calculated
272
over the entire data series (Fig. 2). The concentrations of CO, NO, NO2, NOx and NMHC
273
from Friday evening and throughout Saturday night exceed their values observed on other
12
274
weekdays during the same time period. Thus, the nightlife of the city at the end of the
275
working week is most active. However, from early Saturday morning, the concentrations
276
of these pollutants and also SO2 and PM10 become significantly lower when compared to
277
those on weekdays. The daytime concentrations of these pollutants maximally decrease
278
on Sunday. In the daily cycle of CO, its characteristic morning maximum vanishes, and,
279
for NO2, NMHC, and PM10, this maximum becomes hardly noticeable. On Saturday and
280
Monday, the daytime decrease in the concentrations of the pollutants amounts to 40–50
281
% and 20–30 % of their Sunday daytime values, respectively. The night from Sunday to
282
Monday is characterized by the lowest concentrations of CO, NO, NO2, PM10, and SO2,
283
which is explained by the minimum activity of traffic and a break in the operation of
284
many industrial enterprises. Such decreased pollutant concentrations remain unchanged
285
until Tuesday early morning.
286
The variations in the surface concentration of O3 during weekend are of opposite
287
character. During Friday evening and Saturday night, the concentration of O3 is lower
288
than during the same time period on weekdays. However, on Saturday afternoon, the O3
289
concentration becomes higher than that on weekday afternoons and, on Sunday, reaches
290
its weekly maximum. On Monday night, the concentration of O3 exceeds its night values
291
for other weekdays. By Monday noon, the O3 weekend effect vanishes. A slight decrease
292
in the concentration of CH4 during weekend is observed in the afternoon and at night;
293
however, its weekday-weekend differences are insignificant. Natural gas is used as a fuel
294
by a few percent of cars, and they did not influence on weekly methane variations. The
295
leakage of CH4 in the urban gas supply system and emissions from other sources slightly
296
depend on the day of the week.
297
The Sunday reduction in pollutant concentrations averaged over the entire
298
measurement period from 2005 to 2014 are given in Table 2 and Fig. 3. Table 2 also
299
gives the 95 % confidence limits for the weekend effect estimates. The 10-year averaged
300
daytime and daily mean SE values for all the pollutants (except ozone) are positive and,
301
except CH4, significant. Maximum daytime SE (23.9 %) and daily mean SE (18.9 %) are
13
302
observed for NO and minimum daytime SE (6.3 %) and daily mean SEs (4.7 %) are
303
observed for NMHC (CH4 excluded). Both daytime SE and daily mean SE take on
304
average values 16.7 and 13.6 % for NO2 and 19.5 and 15.8 % for NOx, respectively. The
305
SO2 concentration also decreases during weekends, however, because of its large
306
variations, the SE values are close to those of the 95 % confidence limit. The SEs for CO
307
and PM10 are almost similar, i.e. approximately 13–15 % in their daytime concentrations
308
and 9–11 % in their daily mean concentrations. On Sunday the concentrations of ozone
309
(unlike other pollutants) exceeds its weekday concentrations by 16.5 % (daytime SE) and
310
13.1 % (daily mean SE). The night SEs averaged over the 10-year period are significant
311
only for NO2, PM10, and O3.
312 313 314 315
Table 2 Weekdays (Tuesday–Friday period) and Sunday differences (%) in the daytime (08:00–22:00 LT), nighttime (22:00–08:00 LT) and daily mean (00:00–24:00 LT) pollutant concentrations averaged for all stations over 1 Jan 2005–31 Dec 2014. The uncertainty of weekend effects is represented by 95 % confidence limit.
Pollutant CO
Weekend effect [ % ] 08:00–22:00
22:00–08:00
00:00–24:00
13.6 ± 3.3
2.2 ± 4.8
9.3 ± 3.2
NO
23.9 ± 5.8
6.4 ± 9.9
18.9 ± 5.6
NO2
16.7 ± 2.8
7.5 ± 3.9
13.6 ± 2.8
NOx
18.8 ± 3.0
7.8 ± 4.5
15.2 ± 3.1
O3
-16.5 ± 4.8
-9.8 ± 8.8
-13.1 ± 4.4
SO2
7.6 ± 6.5
8.1 ± 8.6
7.0 ± 6.9
PM10
14.5 ± 5.1
8.6 ± 7.2
10.9 ± 5.5
NMHC
6.3 ± 2.2
1.4 ± 2.9
4.7 ± 2.1
CH4
0.0 ± 0.2
0.1 ± 0.3
0.0 ± 0.2
316
14
δCO [ % ]
20
Winter
Spring
Summer
Autumn
Year
10 0 -10 -20 -30
δNO [ % ]
20 10 0 -10 -20 -30
δNO2 [ % ]
20 10 0 -10 -20 -30
δO3 [ % ]
20 10 0 -10 -20 -30
δSO2 [ % ]
20 10 0 -10 -20 -30
δPM10 [ % ]
20 10 0 -10 -20 -30
δNMHC [ % ]
20 10 0 -10 -20 -30
Fri
Sun
Tue
Fri
Sun
Tue
Fri
Sun
Tue
Fri
Sun
Tue
Fri
Sun
Tue
Weekday
317 318 319 320 321
Day
Night
Fig. 4. Weekly variations in 08:00–22:00 (day) and 22:00–08:00 (night) surface pollutant concentrations in Moscow for different seasons and year (relative deviation of each day concentration from average for the Tuesday–Friday period, %) averaged for all stations over 1 Jan 2005–31 Dec 2014. The uncertainty in any day point represented by 95 % confidence intervals ranged between 2 and 4 % for O3, CO, NO2, NMHC and between 3 and 6 % for NO, SO2, PM10.
15
322
4.2.
Seasonal variations
323
Sunday effects in 08:00–22:00 (day) and 22:00–08:00 (night) surface pollutant
324
concentrations in Moscow for different seasons averaged for all stations and over 2005–
325
2014 are presented in Tables 3, 4 and Fig. 4. Daytime concentrations (as well as the daily
326
mean concentrations not shown in Fig. 4) of CO, NO, NO2, and NMHC manifest their
327
stable decrease on Sundays throughout the year. On Sunday night, their concentrations
328
are minimum for all seasons, but the night SE values are not significant. The highest SEs
329
are observed for NO. Its value reaches 28 % in winter and 23–25 % in the rest of the
330
seasons. In addition, daytime SEs significantly varies from week to week within each
331
season.
332 333 334
Table 3 Weekday-Sunday differences (%) in the day, night and daily mean CO, NO, NO2, NOx, and O3 concentrations and their corresponding 95 % confidence limit for different seasons averaged for all stations over 1 Jan 2005–31 Dec 2014.
Pollutant
CO
NO
NO2
NOx
O3
Season
Weekend effect [ % ] 08:00–22:00 LT
22:00–08:00 LT
00:00–24:00 LT
Winter
14.8
±
6.3
-2.9
±
21.4
12.5
±
5.7
Spring
12.2
±
5.6
4.4
±
7.7
7.1
±
5.6
Summer
13.8
±
5.3
0.2
±
8.7
8.3
±
5.3
Autumn
14.3
±
7.4
5.7
±
9.1
10.0
±
7.5
Winter
28.0
±
14.7
7.2
±
20.9
26.0
±
13.7
Spring
23.6
±
8.8
9.3
±
14.2
17.5
±
8.6
Summer
22.9
±
10.5
1.8
±
32.7
16.3
±
10.0
Autumn
24.8
±
18.1
8.4
±
36.6
19.9
±
16.0
Winter
15.6
±
6.1
6.0
±
9.5
14.8
±
5.5
Spring
16.1
±
5.7
9.8
±
7.6
11.6
±
7.1
Summer
18.8
±
9.1
4.7
±
8.3
14.0
±
10.2
Autumn
16.7
±
13.8
9.0
±
26.3
14.7
±
13.8
Winter
18.9
±
7.6
6.3
±
11.0
17.4
±
6.9
Spring
17.3
±
5.6
10.3
±
7.7
13.4
±
6.4
Summer
19.6
±
9.0
4.5
±
9.4
14.7
±
10.1
Autumn
20.3
±
13.4
9.7
±
30.4
16.5
±
13.2
Winter
-19.7
±
13.4
-17.7
±
22.3
-13.7
±
9.9
Spring
-12.1
±
7.1
-19.1
±
49.7
-10.2
±
9.6
Summer
-8.1
±
4.8
-9.5
±
17.1
-6.9
±
4.3
Autumn
-17.3
±
11.8
-9.1
±
13.4
-12.6
±
10.9
335
For CO, the daytime SE values are noticeably lower; however, its weekly cycle
336
from season to season is more stable when compared to that for NO. The character of CO
337
seasonal variations is the same: maximum SE values (14.8 %) are observed in winter and,
16
338
in the rest of the seasons, SE values vary from 12.2 to 14.3 %. Unlike NO and CO,
339
maximum daytime SE (18.8 %) for NO2 is observed in summer. In the rest of the seasons,
340
its daytime SEs are almost the same (15.6–16.7 %). The seasonal variability of NO2 is
341
significant only during autumn. Seasonal variations in PM10 concentrations significantly
342
differ from those in CO, NO, and NO2 concentrations. Its maximum daytime SEs (about
343
21.5 %) are recorded in winter and autumn; in summer, the SE amounts to 14.6 %, and,
344
in spring, the SE is insignificant. In winter and spring, there are almost no daytime
345
weekday-weekend differences in SO2 concentrations. They are on the verge of validity in
346
summer and sufficiently significant in autumn.
347 348 349
Table 4 Weekday-Sunday differences (%) in the day, night and daily mean SO2, PM10, NMHC, and CH4 concentrations and their corresponding 95 % confidence limit for different seasons averaged for all stations over 1 Jan 2005–31 Dec 2014.
Pollutant
SO2
PM10
NMHC
CH4
Season
Weekend effect [ % ] 08:00–22:00 LT
22:00–08:00 LT
00:00–24:00 LT
Winter
-0.4
±
19.1
-1.3
±
36.7
-2.5
±
21.3
Spring
2.4
±
15.8
9.2
±
12.2
1.0
±
17.7
Summer
11.5
±
10.3
8.9
±
13.7
12.2
±
9.0
Autumn
15.2
±
11.5
11.3
±
16.5
14.1
±
10.1
Winter
21.6
±
10.5
21.8
±
16.1
17.3
±
12.4
Spring
1.9
±
14.4
-3.5
±
18.8
-2.2
±
15.9
Summer
14.6
±
7.3
7.2
±
14.4
10.0
±
7.0
Autumn
21.2
±
8.9
12.5
±
12.7
19.1
±
9.6
Winter
6.5
±
6.0
0.3
±
6.8
4.9
±
5.6
Spring
4.3
±
4.0
1.0
±
5.8
3.5
±
4.0
Summer
6.9
±
3.7
1.1
±
6.7
4.4
±
3.6
Autumn
7.6
±
5.0
3.3
±
6.3
6.2
±
4.8
Winter
0.2
±
0.6
0.1
±
0.6
0.2
±
0.5
Spring
0.0
±
0.3
0.1
±
0.5
0.0
±
0.3
Summer
-0.2
±
0.4
-0.1
±
0.7
-0.2
±
0.4
Autumn
0.2
±
0.5
0.1
±
0.7
0.1
±
0.5
350
For CH4 (not shown in Fig. 4), both daytime and night SEs are absent throughout
351
the year (Table 4). Weak SEs are observed for NMHC concentrations, because they are
352
strongly affected by vegetation and various anthropogenic sources that do not obey the
353
weekly cycle. The most pronounced SEs for NMHC are observed in summer and autumn.
354
The O3concentrations demonstrates their clearly pronounced weekend effects both by day
17
355
and at night. However, due to high night variations in the concentrations, their average
356
night SEs do not exceed the 95 % confidence limit. The mean seasonal O3 daytime SE
357
values vary from -8.1 % in summer to -17.3 % in autumn and their daily means values
358
vary from -6.9 % in summer to -13.7 % in winter.
359
The combination of both anthropogenic and meteorological factors causes
360
increased weekday-weekend differences in CO, NO, and PM10 in winter and autumn. The
361
centralized heating system most intensively operates in winter. Both morning and
362
evening rush hours most often coincide in time with a high stability of the ABL.
363
According to meteorological data obtained at the MSU Meteorological Observatory
364
(Isaev, 2003) and averaged over a period of 1958–2000, in winter, the night surface
365
temperature inversion is formed, on average, at 17:00–18:00 (LT) (mean inversion
366
recurrence is 40 % at 18:00) and is destructed at 11:00–12:00 (mean inversion recurrence
367
is 27 % at 12:00). In winter, convective boundary layer is usually observed from 12:00 to
368
17:00, but its mean recurrence at 14:00–15:00 does not exceed 20 %. Under these
369
conditions, motor transport emissions result in the accumulation of pollutants in the
370
surface air layer and determine time variations in CO, NO, and PM10 concentrations. In
371
spring and summer, most of Moscow people spend their weekends and holidays away
372
from the city, and, in autumn, when they come back, traffic in Moscow becomes most
373
active. Exchange processes during the transition periods (spring and autumn) have
374
different activity levels. The mean air temperature is lower in autumn (4.8 °С) than in
375
spring (5.8 °С), and the total flux of incoming solar radiation to the land surface is
376
smaller in autumn (497 MJ·m-2) than in spring (1253 MJ·m-2) (Isaev, 2003). Therefore,
377
exchange processes are weaker in autumn than in spring, and the SEs for CO, NO, and
378
PM10 are stronger in autumn. A great number of official holidays in March and May and
379
mass travels of Moscow people from the city to their summer houses on weekends in
380
spring additionally contribute to the weekend effect decrease in spring. These processes
381
are reflected in daytime weekday-weekend differences in other pollutants.
18
382
In spring, the SE in PM10 is completely absent. As was noted by Elansky et al.
383
(2018), the highest PM10 concentrations are observed in March–April, which is associated
384
with the melting of snow and the lifting of a lot of soil fine-dispersed particles and anti-
385
icing reagents accumulated over the winter period into the air. Unlike the CO and NO
386
weekly cycles, the PM10 concentration increases from Tuesday to Friday (Fig. 4), which
387
may be caused by increased heavy truck traffic by the end of working week; this is
388
determined by the logistics system operating in Moscow.
389
Daytime SE in SO2 are maximum in autumn. During the preparation for the winter
390
heating season (September–October), test runs of the heating system are conducted in
391
Moscow and its surroundings using liquid fuel, which results in increased SO2 emissions.
392
Abnormally dangerous SO2 emissions from the MOR more often occur on weekdays in
393
September and October. For example, in 2014, 20 events of such emissions with sharply
394
increased SO2 concentrations were simultaneously recorded at several MEM stations as
395
found in this study. In winter at very low air temperatures, reserve liquid fuel is used at
396
HPPs on any day of the week, which decreases the weekend effect.
397
Night SE in CO, NO, CH4 concentrations are within the 95 % confidence interval.
398
The absence of a significant and regularly observed SE for all pollutants in night time
399
may be due to the good ventilation of the megacity located on the plain and the high
400
variability of the ABL state. In winter and spring, the night NO2 concentrations
401
demonstrate slightly SE. However, the night NO2 maximum weekend effect was
402
observed from Saturday on Sunday nights in winter and from Sunday on Monday nights
403
in spring. This is caused by the features of NO2 formation and destruction in the absence
404
of solar illumination:
405
NO+O3→NO2+O2
(1)
406
NO2+O3→NO3+O2
(2)
407
NO+NO3→NO2+NO2
(3)
19
408
In winter, the daily mean concentrations of NO averaged for all stations over 2005–
409
2014 are maximum and those of O3 are minimum (27 and 9 ppb, respectively) and, quite
410
the reverse, in spring, the daily mean concentrations of NO are minimum and those of O3
411
are maximum (13 and 26 ppb, respectively). Thus, in reaction system (1)–(3), the limiting
412
element is O3 for winter and NO for spring. Since the minimum daily mean concentration
413
of NO was observed on Sundays, in spring this results in decreased concentrations of
414
NO2 and its strongest weekday-weekend differences on Sunday-Monday nights. In
415
winter, due to lower O3 concentrations on Saturdays than on Sundays, minimum
416
concentrations of NO2 and its maximum weekday-weekend differences are observed on
417
Saturday-Sunday nights. In summer, VOC of biogenic origin take part in both NO and
418
NO2 oxidation processes and the weekend effect become indistinct and night SE
419
differences in NO2 become insignificant.
420
4.3.
Sector structure
421
Table 5 shows the daytime SE values averaged over the entire observation period
422
for the central district of the city bounded by the Garden Ring with a diameter of 5 km
423
and each city sector: northeast, northwest, southwest, and southeast (Fig. 1). For
424
comparison, Table 5 also shows the SE values averaged over urban stations remotely
425
located from highways (i.e. city’s background) and over stations located within the
426
greenbelt surrounding Moscow. It follows from Table 5 that, for CO, NOx, and PM10 the
427
SE spatial variations over the Moscow territory are, on the whole, relatively small. This
428
implies that among their sources, the motor transport is dominant. Significant SE in CO,
429
NOx, as well as NMHC concentrations in the city center are characteristic of Moscow.
430
Most of administrative institutions and public offices, which are closed on weekends, are
431
situated here, in the city center. Consequently, the traffic (except the freight transport) –
432
the main anthropogenic source of CO, NOx, and NMHC – is least intensive in the center
433
on weekends. Since these pollutants determine the ozone balance, the largest weekday-
434
weekend negative difference in O3 (-18.3±2.9 %) is also observed in the center of
435
Moscow. In the southwest sector, there are also a lot of administrative and commercial
20
436
offices and a dense network of highways. However, unlike the city center, clear air flows
437
with high O3 concentrations arrive at the southwest sector with prevailing southwesterly
438
winds. Advection smoothes the CO, NMHC, PM10, and O3 weekend effects and promotes
439
faster NO oxidation, which is manifested in decreased NO and increased NO2 SE.
440 441 442
Table 5 Weekday-Sunday differences (%) in 08:00–22:00 pollutant concentrations averaged for different sectors of Moscow, greenbelt, all stations and for stations at any distance from roads. [n] – number of stations. CO City center
NO
NO2
O3
21.0 ± 1.6 [8]
25.1 ± 2.5 [7]
17.8 ± 1.6 [6] -18.3 ± 2.9 [3]
NW
14.6 ± 1.5 [8]
26.5 ± 2.4 [6]
NE
12.2 ± 1.7 [3]
24.8 ± 3.0 [3]
SW
16.1 ± 1.4 [7]
SE All sites City’s background Greenbelt
SO2
PM10
NMHC
8.7 ± 3.2 [3]
14.4 ± 2.3 [4]
9.9 ± 1.1 [4]
16.3 ± 1.7 [7] -16.1 ± 3.8 [2]
3.1 ± 3.4 [4]
15.0 ± 2.5 [2]
9.1 ± 1.0 [5]
17.8 ± 2.0 [3] -17.7 ± 5.1 [2]
18.3 ± 3.7 [2]
21.7 ± 2.3 [7]
19.4 ± 1.8 [7] -16.1 ± 3.4 [3]
9.6 ± 1.9 [4]
12.2 ± 1.6 [11]
23.9 ± 2.5 [10]
14.3 ± 1.5 [10] -16.5 ± 3.0 [3]
13.6 ± 3.3 [37]
23.9 ± 5.8 [33]
16.7 ± 2.8 [33] -16.5 ± 4.8 [13]
12.2 ± 3.1 [29]
23.6 ± 5.7 [27]
3.0 ± 4.3 [5]
14.1 ± 2.5 [6]
[0]
3.1 ± 3.6 [2]
16.7 ± 2.4 [3]
5.9 ± 0.9 [2]
11.1 ± 2.8 [9]
14.4 ± 2.2 [5]
7.2 ± 0.9 [10]
7.6 ± 6.5 [22]
14.5 ± 5.1 [14]
6.3 ± 2.2 [23]
17.1 ± 2.9 [27] -16.0 ± 5.3 [9]
7.0 ± 7.0 [17]
14.3 ± 5.1 [13]
5.6 ± 2.2 [19]
20.1 ± 3.7 [6]
1.2 ± 5.3 [4]
11.3 ± 7.1 [1]
1.3 ± 1.0 [2]
-4.5 ± 2.7 [1]
─────
443
Low values of CO SE are characteristic of the most polluted southeast sector. Here,
444
most of pollutant emissions are associated with both industrial and heat-and-power plants
445
continuously operating. Moreover, in this sector, a significant area is occupied by
446
Moscow treatment facilities and storage and railroad structures, however, the traffic here
447
is not very intensive. Other factors are responsible for minimum amplitudes of the CO
448
and NMHC weekly cycles in the northeast sector. In this sector, most of its territory is
449
occupied by parklands (a part of the Losiny Ostrov National Park and other forestlands).
450
Here, the traffic is least intense on weekdays, which is reflected in very similar minimum
451
SE in CO as in the southeast sector. No significant NMHC SE imply the influence of vast
452
forestlands covering a part of the urban area and extending to the humid flatland to the
453
northeast of Moscow on these pollutants.
454
The cause of maximum SE in SO2 in the northeast sector may be emissions from
455
heavy trucks that transit through Moscow southeast to northwest along highways crossing
456
forestlands. The weekend effect is clearly pronounced against the background of very low
457
SO2 surface concentrations. The SEs of both PM10 and CO concentrations reflects mainly
458
road-traffic conditions. The weekday-Sunday differences in PM10 slightly vary
21
459
throughout Moscow, although the highest values are observed in the southwest sector
460
with heavy traffic.
461
For all the sectors of Moscow, the daytime SE in the basic pollutants are
462
approximately the same for the warm (May–Sep) and cold (Oct–Apr, heating period)
463
seasons. The exception is provided by both the SO2 and O3 SEs. In the center of Moscow,
464
the SE in SO2 amount to 13 % and 6 % in the warm and cold seasons, respectively. The
465
seasonal differences are also observed in other sectors, but they are much smaller. This is
466
probably caused by combustion of liquid fuel in private houses, small boiler-rooms, and
467
at HPP in the suburbs of Moscow and, in this case, SO2 emissions arrive in the center of
468
Moscow independently of wind direction. Ozone is more significantly affected by season
469
changing. In the cold season, the weekday-Sunday differences in O3 1.5–3 times exceed
470
those in the warm season. In this case, its minimum differences are observed in the
471
southwest sector, in which the O3 behavior is determined mainly by transport and
472
unpolluted-air advection, and its maximum differences are observed in the northeast
473
sector, in which polluted air flows more often arrive from small industrial towns located
474
mainly in the east and southeast of the Moscow region. The influence of forestlands is
475
reflected in small (at significance level) SE for NMHC.
476
Within the greenbelt surrounding Moscow, there are six stations located at a
477
distance of 30 to 70 km from the city center in small inhabited localities. Here, the
478
character of weekly variations in surface pollutant concentrations strongly differs from
479
that for the city (Table 5). The CO, CH4, and SO2 weekend effects are absent. The SEs in
480
NMHC and O3 concentrations only slightly exceed the significance level. At the same
481
time, the NO, NO2, and PM10 concentrations significantly decrease on Sundays. In recent
482
decades, within the greenbelt, a great number of cottage settlements, in which people live
483
the whole year round, have been added to many dachas, in which people live during the
484
warm season or occasionally. In the 2000s, on Saturday, Sundays, and holidays, the
485
capacity of highways was regularly exceeded and traffic jams were formed on all roads
486
leading out of the city. In fact, on weekends, the highest transport activity was shifted
22
487
from the central part of Moscow to its suburbs. Taking into account the dense road
488
network and sufficiently long lifetimes of CO, SO2, NOx, and PM10, air flows with their
489
increased concentrations due to heavy traffic on weekends propagate throughout the
490
greenbelt (Elansky et al., 2010). On Sundays, the traffic increased due to Moscow cars
491
within the greenbelt and significant duration of mixing processes decrease the local
492
weekend effects of these pollutants. Only the SE in NO2 exceeds its urban value, which is
493
caused by the active transition of NO into NO2 at significantly higher O3 concentrations
494
within the greenbelt. The effect of mixing during pollution transport is also pronounced
495
over the territory of Moscow, which follows from comparison between the NO2 SE
496
obtained with and without consideration for data recorded at stations located near
497
highways (Table 5). If data obtained at these stations are excluded from consideration, the
498
averaged (over the city) SEs in all the pollutants (except NO2) slightly decrease, however,
499
these changes are still within the limits of confidence intervals.
500
4.4.
Comparison with other megacities
501
The weekend effects in O3, NOx, CO, PM10, and some VOC were studied for
502
Mexico City (Mexico), Sacramento (USA) and Beijing (China) (Stephens et al., 2008;
503
Murphy et al., 2007; Marr and Harley, 2002; Wang et al., 2014). As there was no any
504
standard technology for the WE estimations we recalculated the Moscow weekend effects
505
using those methods that used these authors. The weekend effect for Mexico City was
506
estimated according to three maximum pollutant concentrations on Sundays with respect
507
to weekday concentrations from Monday to Friday (Stephens et al., 2008). On average,
508
over a period of 2005–2007, the Sunday decrease in CO, NOx, and PM10 concentrations
509
amounted to 40, 45, and 24 %, respectively, and the Sunday increase in O3 amounts to 4.7
510
%. Similarly, for Moscow in 2005–2007, the mean three-hour decrease in CO, NOx, and
511
PM10 concentrations amounts to 23.0, 20.9, and 19.5 %, and the O3 increase amounts to
512
10.3 %. For Sacramento, USA (Murphy et al., 2007), the weekend effect was calculated
513
for the summer months of 1998–2002 from 8-h (10:00–18:00) mean concentrations on
514
weekends and weekdays. The weekday-weekend differences in NOx and PM10 amounted
23
515
to 35 and 13 %, respectively. On weekends, the O3 increase averaged over the city was
516
within 3–8 % and reached 12 % at an urban station in the city center. The decrease in
517
VOC concentrations did not exceed 10 %. Marr and Harley (2002) obtained the same
518
weekend effect value (35 %) for NOx in Sacramento. In Moscow, in the summer months,
519
the 8-h daytime NOx and PM10 concentration decrease averaged over Saturday–Sunday is
520
significantly smaller and amounts to 14.8 and 10.4 %, respectively. The NMHC
521
concentration decrease is relatively slight (4.0 %), however, at the same time, the O3
522
concentration decrease on weekends is the same as, on average, in Sacramento (5.4 %).
523
Beijing in its infrastructure and pollutant sources is close to Moscow. Wang et al.
524
(2014) calculated the weekly effect in the form of deviations in daytime (08:00–18:00)
525
pollutant concentrations averaged over Saturday–Sunday–Monday from weekday
526
pollutant concentrations averaged over a period of Wednesday–Friday. The
527
measurements were performed from July 2009 to August 2011 at four stations. In
528
Beijing, the mean decrease in the NO, NO2, and NOx concentrations on weekends
529
amounted to 22.1, 12.0, and 16.5 %, respectively, and the O3 concentration increase
530
amounted to 5.6 %. Weekend effects in NO, NO2, and NOx for Moscow, which were
531
calculated using the methodology described by Wang et al. (2014), yielded the
532
significantly smaller decrease: 10.5, 8.6, and 9.5 %, respectively. Weekend effects in CO,
533
NMHC, SO2, and PM10 (Wang et al. do not provide such data) this methodology of
534
calculation yielded for Moscow 8.8, 5.2, 4.8, and 7.2 %, respectively. However, the O3
535
concentration increase on weekend for Moscow (6.1 %) is almost the same as for Beijing.
536
Minimum pollutant concentrations shifted in Beijing to Monday imply a possibly
537
stronger effect of the advection of pollutants from regional sources on the air quality.
538
This is also supported by high NOx concentrations (62–70 ppb for daily means on
539
weekdays) in Beijing and by daily mean (24 h) weekday-weekend differences in CO,
540
PM10, and UV radiation equal 4.4, 6.0, and 3.3 %, respectively (Wang et al., 2014). These
541
values are close to Moscow daily mean weekday-weekend differences in CO, PM10, and
542
UV radiation (6.2, 7.8, and 4.0 %) calculated by analogy with Beijing. In other words, the
24
543
time interval extended to three days more noticeably decreases the weekend effect for
544
Beijing than for Moscow that may be considered as an isolated city according to a lot of
545
data (Elansky, 2014; Elansky et al., 2018). When compared to Mexico City and Beijing,
546
slight (on the whole) weekday-weekend differences in CO, NOx, and PM10 for Moscow
547
are apparently caused by increased traffic activity on weekends. On Saturdays and, to a
548
lesser degree, Sundays, it is shopping time in Moscow, all shopping and entertainment
549
centers are open on weekend.
550
4.5.
Vertical structure
551
Figure 5 shows the averaged over 1 January 2007–31 December 2014 period
552
deviations of the daytime (08:00–22:00 LT) concentrations of NO, NO2, and CO on
553
Sundays from their means on weekdays according to data obtained at heights of 10, 130,
554
248, and 348 m at the Ostankino TV tower. The variations in the concentrations of these
555
pollutants show their stable decrease at all the heights on Saturdays, Sundays, and
556
Mondays. There are a noticeable weekend effects varying with height. At the lower level,
557
they approximately coincide with means over the Moscow territory. The yearly average
558
SE in NO rapidly decreases with height from 33.3 % at a height of 10 m to 10 % at a
559
height of 348 m (Fig. 5).
25
δCO [ % ]
15
Winter
Spring
Summer
Autumn
Year
0 -15 -30 -45
δNO [ % ]
15 0 -15 -30 -45
δNO2 [ % ]
15 0 -15 -30 -45
Fri
Sun
Tue
Fri
Sun
Tue
Fri
Sun
Tue
Fri
Sun
Tue
Fri
Sun
Tue
Weekday
10 m
130 m
248 m
348 m
560 561 562 563
Fig. 5. Weekly variation in 08:00–22:00 CO, NO, NO2 concentrations (relative deviation of each day concentration from average for Tuesday–Friday) on 10, 130, 248 and 348 m platforms of the Ostankino TV tower averaged over 1 Jan 2007–31 Dec 2014. The 95 % confidence intervals ranged between 3 and 5 % for CO and between 4 and 7 % for NO and NO2.
564
The SE in CO also decreases with height but only slightly from 16.7 to 14.0 %.
565
The NO2 weekend effect is characterized by another vertical distribution. Within the
566
surface air layer (10–130 m), the NO2 SE decreases with height from 18.1 to 13.2 %, then
567
increases, reaches its maximum 24.4 % at a height of 248 m, and again decreases to 19.7
568
% at a height of 348 m.
569
Seasonal variations in the vertical structure of the weekend effect for each of the
570
pollutants have some characteristic features (Fig. 5, Table 6). At the lower level (10 m),
571
the time variations of SEs in NO, NO2, and CO are similar to those obtained from
572
ground-based measurements at MEM stations. During all seasons (except winter), the SE
573
in NO rapidly decreases with height within the lower 130-m layer. In winter, the SEs for
574
10 and 130 m are approximately the same (30 %). It follows from Fig. 5 that, in winter,
575
the concentration of O3 within the surface air layer does not increase during the day
26
576
because of frequent and long temperature inversions that hinder its influx from higher
577
levels. Under these conditions, in the daytime, the SE in NO is characteristic of the whole
578
under inversion layer of ABL. In the warm season, its daytime SEs amount to 10–15 % at
579
130 m and about 10 % at higher levels. Seasonal variations in the ABL vertical
580
stratification also affect the NO2 and CO weekend effects.
581 582 583
Table 6 Maximal weekday-Sunday differences (%) in 08:00–22:00 CO, NO, NO2 concentrations and their height levels for different seasons from measurements at Ostankino TV tower from 1 Jan 2007 to 31 Dec 2014.
Season
NO δNO [ % ]
NO2 Height [ m ]
δNO2 [ % ]
CO Height [ m ]
δCO [ % ]
Height [ m ]
Winter
29.8 ± 9.1
10
22.3 ± 5.7
248
18.2 ± 4.9
130, 248
Spring
28.5 ± 6.9
10
25.8 ± 7.6
248
13.4 ± 3.1
10, 248
Summer
33.4 ± 6.2
10
26.3 ± 8.4
348
15.4 ± 3.8
10, 348
Autumn
41.6 ± 8.8
10
25.5 ± 7.1
248
19.6 ± 4.1
10, 348
584
According to the 1988–1998 sodar data, during winter in Moscow, the recurrence
585
of temperature inversions amounts to 55 % (Isaev, 2003). The depth of temperature-
586
inversion layer increases during both night and morning hours and, by 9:00, amounts to
587
250–270 m. In the winter daytime, ozone accumulated above the inversion reacts with
588
NO coming from below, which results in the formation of NO2. The weekend effect of
589
NO is transmitted to NO2, and as seen in our study, the maximum NO2 SE is noted at a
590
height of 248 m. The same situation is observed in spring and autumn. In these seasons,
591
the recurrence of surface night inversions amounts 39 and 41 %, respectively, and their
592
mean depth during morning hours amounts to 240–260 m. Maximum SE in NO2 is noted
593
also at a height of 248 m. In summer, night inversions are formed not so frequently, their
594
mean depth does not exceed 190 m (Isaev, 2003). The early start (08:00–09:00) of
595
convection favors an active upward NO transport. In summer, maximum daytime SE in
596
NO2 are observed at the highest level (348 m).
597
The vertical structure of the weekend effect in CO differs from that of NO and NO2
598
(Table 6). Its maximum SE is observed at average levels – under and above inversion – in
599
winter and at both the lowest and highest levels in the other seasons. It is evident that the
600
inversion barrier affects the behavior of CO; however, unlike short-lived NO, the
27
601
formation of the CO weekend effect is noticeably affected by advective pollutant
602
transport from regional sources, which is especially intensive above the inversion. Since
603
industrial enterprises (including those continuously operating) are concentrated in small
604
towns located mainly in the east and southeast of the Moscow region, the advection effect
605
extended in time, changes in wind directions, and vertical-mixing intensity significantly
606
decrease the weekend effect of CO when compared to that of NO.
607
Wang et al. (2014) presented data on vertical distributions of NOx and O3 for
608
Beijing (Jul 2009 – Aug 2011). The NOX and O3 concentrations were measured at heights
609
of 8, 47, 120, and 280 m from the Beijing meteorological tower 325 m in height. Unlike
610
Moscow, in Beijing, minimum NOx concentrations occurred on Mondays at each of the
611
four heights due to reduced anthropogenic emissions and lower pollutant accumulation on
612
weekends. High NOx concentrations occurred on Wednesday–Friday. Using the NOx
613
concentration values given by Wang et al. (2014), one can estimate the weekend effect of
614
NOx in the form of its decreased mean concentration on weekends (Saturday–Monday) in
615
relation to its mean concentration on weekdays (Wednesday–Friday). The NOx weekend
616
effects amount to 14.0, 8.2, 9.4 and 15.5 % at heights of 8, 47, 120, and 280 m,
617
respectively. According to the same calculations for Moscow, the weekend effect of NOx
618
(NO and NO2 are converted to ppb) amounts to 15.0, 9.1, 10.8, and 10.6 % at heights of
619
10, 130, 248, and 348 m, respectively. The vertical structures of the weekend effects for
620
Moscow and Beijing almost coincide up to a height of about 200 m. In this case,
621
minimum weekday-weekend differences are noted at heights of 47 m for Beijing and 130
622
m for Moscow. In both megacities, at these heights, NOx is emitted from the pipes of
623
industrial enterprises and HPPs operating also on weekends. Higher than 200 m, the
624
weekend effect slightly increases in Moscow and rapidly increases in Beijing from 9.4 %
625
at 120 m to 15.5 % at 280 m. It is possible that this increase in the weekend effect is
626
caused by pollutants transported at these heights over Beijing area from regional sources,
627
which is also reflected in the shift of minimum pollutant emissions from Sunday to
628
Monday.
28
629
5. Features of the ozone chemistry
630
Several hypotheses of the formation of the ozone weekend effect were proposed
631
(Jacob, 1999; Marr and Harley, 2002; Murphy et al., 2007; Tonse et al., 2008). Ozone
632
increase due to NOx decrease is associated with two chemical mechanisms: (1) decreased
633
ozone loss due to titration reaction and (2) increased ozone production during the
634
oxidation of CO and VOCs by OH and the formation of peroxyradical HO2 and
635
alkylperoxyradical RO2:
636
OH+CO+O2→HO2+CO2
(4)
637
OH+RH+O2→RO2+H2O
(5)
638
Oxidation of NO by HO2 or RO2 results in the formation of NO2 and generation of
639
O3 (Fishman and Crutzen, 1978). Recycling OH during the oxidation of NO and
640
subsequent reactions with RO2 propagate chain reactions for ozone formation. The
641
duration of this process is determined by the elimination of radicals OH and NO2 and the
642
formation of HNO3. Weekday-weekend differences in the reaction products of this cycle
643
depend on NOx variations and the VOC/NOx ratio. An increase in VOC/NOx on
644
weekends shifts the OH chemistry toward increased production of RO2. In turn, increased
645
production of RO2 results in increased production of O3 and other VOC-NOx oxidation
646
products, such as PAN. Fast thermal decomposition of PAN regenerates RO2 and NO2,
647
which provides the continuation of the cycle with ozone formation. The VOC/NOx ratio
648
is an indicator of these processes.
649
Odd oxygen (Ox=O3+NO2) is also an indicator of the relative contribution of
650
titration and photochemical production to the ozone weekly cycle (Murphy et al., 2007;
651
Sadanaga et al., 2008; Tonse et al., 2008). Slight Ox differences between weekdays and
652
weekend or their absence imply titration as a dominant process affecting the ozone cycle.
653
Exceeded Ox concentrations on weekdays, when compared to weekend, imply the
654
dominant role of the O3 photochemical production.
29
655
The fraction of radicals lost via NOx chemistry may serve as an indicator of
656
whether VOC or NOx is more important in the production of O3 (Kleinman, 2005). If the
657
ratio of the radical loss due to NOx chemistry (LN) to the radical production rate (Q) is
658
larger than 0.5, this implies VOC-limited conditions and, if it is smaller than 0.5, this
659
implies NOx-limited conditions. Simplifying the notation of the system of equations for
660
ozone production on the basis of some additional approximations, Stephens et al. (2008)
661
obtained the equation for LN/Q where all terms can be estimated from the weekend
662
effects in different components (δX–SE in Ox, NOx, VOC, JNO2 in percent): (6)
663
664
The composition of VOCs is complex and variable in both time and space and their
665
measurements present difficulties. Stephens et al. (2008) assumed that the weekend effect
666
for VOC reactivity is similar to that for CO. Therefore, they used δCO instead of δVOC.
667
In Moscow, VOC measurements were performed using flask samples taken once a day,
668
and C2–C10 hydrocarbons were analyzed using a gas chromatograph under laboratory
669
conditions (Report, 2014; State of air pollution, 2013). The C2–C10 hydrocarbons,
670
includes basic VOCs, that determine reactivity in urban air masses and are registered by
671
NMHC instrument as well. Therefore, we used SE values in both NMHC and CO as
672
δVOC. In urban air masses there are as well some other O3 active organic compounds.
673
Therefore, the ratio VOC/NOx in Moscow atmosphere could be higher than NMHC/NOx,
674
but not very sufficiently (Isidorov, 2001).
675
Table 7 gives the averaged (over seasons and days of the week) NMHC/NOx ratios
676
calculated for Moscow on the basis of observational data obtained at all stations from
677
2005 to 2014. The NMHC/NOx ratio depends on in what units the NMHC and NOx are
678
given. In Table 7 both NMHC and NOx concentrations are given in µg·m–3. The C2–C10
679
hydrocarbon composition characteristic of Moscow and concentrations of compounds
680
actively participating in ozone formation corrected for Moscow are taken into account
30
681
(Isidorov, 2001; Stockwell et al., 2012). Therefore, we obtained the NMHC/NOx ratio in
682
µg·m–3 corresponds to 1.3 NMHC/NOx in ppbC/ppb. The NMHC/NOx ratio seasonally
683
varies slightly, which implies the same basic urban sources. Its minimum values are noted
684
in winter and maximum in summer, when biogenic emissions are added to anthropogenic
685
ones. The NMHC/NOx ratio slightly varies from Tuesday to Friday within each season.
686
All the NMHC/NOx ratio values for Moscow (from 4.1 to 5.7 in mass units) are smaller
687
than threshold values of 8–10 (Larsen et al., 2003; Murphy et al., 2007). This implies
688
that, on average during a year, VOC-limited chemistry is characteristic of Moscow, when
689
decreased NOx traffic emissions result in increased rate of ozone formation. The ozone
690
and NMHC/NOx weekend effects are most pronounced on Sunday, less pronounced on
691
Saturday, and quite slightly pronounced on Monday. The Sunday increase in the
692
NMHC/NOx ratio amounts from 6.5 % in spring to 10.9 % in autumn (Table 7). Its yearly
693
mean value is 7.9 %.
694 695 696
Table 7 The NMHC/NOx weekly cycles and relative Sunday-weekday differences (%) for different seasons averaged for stations in Moscow city and at region background conditions over 1 Jan 2005–31 Dec 2014. [n] – number of stations. NMHC/NOx
Moscow [ n=22 ]
Weekday
Winter
Spring
Summer
Autumn
Year
Mon
4.26 ± 2.43
4.54 ± 2.83
5.42 ± 3.75
4.34 ± 2.58
4.64 ± 2.98
Tue
4.14 ± 2.34
4.66 ± 2.97
5.35 ± 3.67
4.24 ± 2.57
4.60 ± 2.98
Wed
4.13 ± 2.35
4.53 ± 3.03
5.39 ± 3.60
4.29 ± 2.55
4.59 ± 2.97
Thu
4.10 ± 2.33
4.56 ± 3.00
5.33 ± 3.58
4.28 ± 2.50
4.57 ± 2.94
Fri
4.12 ± 2.34
4.63 ± 3.12
5.28 ± 3.55
4.28 ± 2.50
4.59 ± 2.97
Sat
4.34 ± 2.52
4.79 ± 3.11
5.55 ± 3.87
4.51 ± 2.65
4.80 ± 3.12
Sun
4.48 ± 2.66
4.89 ± 3.11
5.73 ± 3.92
4.74 ± 2.84
4.95 ± 3.19
8.8 ± 5.5
6.5 ± 4.5
7.4 ± 5.4
10.9 ± 7.2
7.9 ± 5.5
Mon
10.42 ± 5.97
10.20 ± 6.00
11.23 ± 6.81
11.49 ± 6.46
10.84 ± 6.31
Tue
10.39 ± 6.01
11.07 ± 6.48
10.03 ± 6.25
11.61 ± 7.81
10.88 ± 6.81
Wed
9.87 ± 5.72
11.16 ± 6.49
10.39 ± 6.08
11.32 ± 7.28
10.74 ± 6.54
Thu
10.07 ± 5.92
10.77 ± 6.46
10.66 ± 6.65
11.19 ± 7.06
10.71 ± 6.58
Fri
10.22 ± 6.17
10.41 ± 6.19
9.80 ± 6.22
11.28 ± 7.35
10.52 ± 6.60
Sat
9.96 ± 5.37
10.01 ± 5.81
10.49 ± 6.59
12.10 ± 7.50
10.73 ± 6.48
Sun
11.01 ± 6.28
11.31 ± 5.87
12.17 ± 7.54
13.35 ± 7.84
11.77 ± 7.04
8.6 ± 5.4
4.2 ± 2.4
19.1 ± 14.1
17.7 ± 12.9
9.8 ± 6.6
Sun-WD
[%]
Region background [ n=2 ]
WD
Sun-WD
[%]
WD
31
697
The NMHC/NOx ratio values for regional background stations in the greenbelt of
698
Moscow (Table 7) were obtained from observations carried out at the Zvenigorod
699
Scientific Station (ZSS) and at a MEM station in the vicinity of the small town of Troitsk.
700
These stations are located at distances of 53 and 35 km to the west and southwest of the
701
center of Moscow, respectively. Both the stations are within the zone of mixed coniferous
702
and deciduous forests and, due to prevailing westerly winds, are within plumes of
703
polluted air from Moscow during no more than 15–20 % of one-year period. The
704
NMHC/NOx ratio value for background conditions is about two times larger than that for
705
Moscow. This is the result of increased concentrations of NMHC of biogenic origin and
706
small amounts of anthropogenic NOx emissions. The NMHC/NOx ratio varies slightly
707
from 9.8 on weekdays in summer to 13.3 on Sundays in autumn. These weekday’s values
708
are about the level of threshold value. This implies transition the VOC-limited to NOx-
709
limited regime which is characteristic of the planetary boundary layer. Lower NMHC
710
concentrations within the greenbelt than in the city imply a smaller contribution of
711
anthropogenic VOCs to the O3 chemistry. Variations in the concentrations of NOx or
712
VOCs may cause a decrease or increase in the rate of O3 production during the transition
713
regime. According to one-dimensional photochemical modeling (Liu et al., 2012), the
714
similar situation is apparently may take place in Beijing (China). However, the VOC-
715
limited chemistry is characteristic of most megacities and urban regions (for example,
716
Mexico, Seoul, Shanghai, Pasadena, and urban areas in Korea) than the NOx-limited
717
chemistry (Murphy et al., 2007; Jin et al., 2012; Pollack et al., 2012; Tie et al., 2013; Yoo
718
et al., 2015).
719
Traffic emissions significantly affect NMHC/NOx ratio variations on weekdays and
720
weekends. For the Moscow megacity, Table 8 gives the NMHC/NOx ratio values
721
calculated for the time period which is characterized by the highest chemical reactivity
722
and by the following features of daily pollutant variations: 07:00 – minimum
723
concentrations of most pollutants, 07:30 – minimum O3 concentrations, 09:30 –
724
maximum concentrations of most pollutants. On weekdays, during the rush hours 07:30–
32
725
09:30, the NMHC/NOx ratio is smaller than in early morning (05:00) and at noon,
726
because NOx dominates over NMHC in road-traffic emissions. On Sundays, the traffic
727
activity is very low during morning hours and gradually increases by the middle of the
728
day. The same regime is noted for the NMHC/NOx ratio – it remains constant until 09:00
729
and increases by 15:00 due to vegetation emissions and evaporation of fuel and other
730
organic materials. By the end of the day the ratio NMHC/NOx decreases simultaneously
731
with the intensity of motor traffic. A relative rapid increase of the Sunday effect in
732
NMHC/NOx ratio occurs between 05:00 and 07:30, and then it slightly increases by 12:00
733
and gradually decreases in the afternoon.
734 735 736 737
Table 8 Temporal variations of NMHC/NOx and LN/Q ratios; O3, NO2 and Ox concentrations (µg·m-3) in the time period 05:00–18:00. Sundayweekday differences in NMHC/NOx and weekday-Sunday differences in Ox (%). Two approximations of Sunday effect values for VOC were used for calculation of the LN/Q ratios: δVOC ≈ δCO and δVOC ≈ δNMHC. Local time
05:00
07:30
09:00
12:00
15:00
18:00
Daily mean 07:30–18:00
WD
3.8 ± 1.7
3.3 ± 1.4
3.3 ± 1.3
3.4 ± 1.5
3.7 ± 1.7
3.7 ± 1.6
3.5 ± 0.2
NMHC Sun
4.0 ± 2.1
3.8 ± 2.0
3.8 ± 1.9
4.1 ± 2.1
4.3 ± 2.3
4.2 ± 2.2
4.1 ± 0.1
5.1 ± 2.3
15.6 ± 7.6
16.0 ± 7.2
19.5 ± 8.0
16.4 ± 8.1
12.9 ± 7.4
16.8 ± 0.7
35.4 ± 15.0
39.2 ± 15.4
39.5 ± 14.9
35.3 ± 15.3
33.5 ± 15.2
36.8 ± 15.2
36.2 ± 2.3
Parameter
NO
x
Sun-WD
[%]
WD
NO2 O3
Ox
WD
[µg·m-3] -3
Sun
[µg·m ]
32.3 ± 16.9
32.1 ± 16.2
31.5 ± 15.2
28.4 ± 15.3
27.7 ± 15.9
31.0 ± 16.1
29.5 ± 1.7
WD
[µg·m-3]
22.1 ± 17.6
20.4 ± 15.6
23.5 ± 13.4
35.1 ± 17.0
41.0 ± 18.7
37.4 ± 18.1
33.1 ± 7.7
-3
23.3 ± 19.7
24.8 ± 17.6
28.6 ± 15.0
39.2 ± 18.8
44.7 ± 21.6
41.2 ± 20.7
37.3 ± 7.2
-3
Sun
[µg·m ]
WD
[µg·m ]
57.3 ± 18.9
59.4 ± 19.4
63.1 ± 18.7
70.9 ± 19.3
75.7 ± 20.0
75.7 ± 19.9
67.0 ± 6.6
Sun
[µg·m-3]
55.1 ± 20.7
56.3 ± 20.0
59.7 ± 19.2
67.7 ± 20.2
73.5 ± 21.5
73.8 ± 20.8
64.3 ± 6.7
3.8 ± 9.5
5.2 ± 3.0
5.3 ± 2.6
4.5 ± 4.6
2.9 ± 7.4
2.5 ± 4.6
4.0 ± 1.8
(δCO)
0.48 ± 1.94
0.93 ± 0.17
0.95 ± 0.14
0.96 ± 0.39
0.87 ± 0.35
0.92 ± 0.17
0.92 ± 0.22
(δNMHC)
0.70 ± 2.99
0.74 ± 0.12
0.74 ± 0.10
0.72 ± 0.27
0.76 ± 0.30
0.79 ± 0.16
0.75 ± 0.17
WD-Sun
[%]
WD
LN Q
738
Early weekend morning, when the intensity of traffic is very low, the ozone
739
concentration grows faster than on weekdays (Table 8; Fig. 2). On Sundays, the excess of
740
the concentration of ozone, when compared to its average concentration on weekdays,
741
amounts to 5.6 % at 05:00 and reaches 21.5 % by 07:30. Maximum weekday-Sunday
33
742
differences in ozone occur earlier than maximum SEs in NOx, NMHC, PM10, and CO
743
(Fig. 2). Early morning on Sundays, when there is no intense traffic, a rapid increase in
744
ozone may be caused by two factors: (1) high reactivity of air masses on Sunday
745
mornings due to a more active city nightlife on weekends and accumulation of NO3,
746
N2O5, and PAN in air masses, which is reflected in low NO and high NO2 concentrations
747
and no decreasing NMHC/NOx level; and (2) mass leaving of inhabitants from the city
748
for their summer houses on Saturdays. Heavy traffic and anthropogenic combustion
749
within the greenbelt are accompanied by significant O3 precursor emissions. NOx and
750
biogenic VOCs transported during the night into the atmosphere over the city result in
751
rapid ozone formation against the background of low VOC, NO2, and CO concentrations.
752
A noticeable contribution made by advection is reflected in high morning (05:00) CO
753
concentrations. High concentrations of ozone in the atmosphere over cities at relatively
754
low concentrations of its precursors (NOx and VOC) during weekends were observed, for
755
example, in California (Marr and Harley, 2002; Qin et al., 2004) and Japan (Sakamoto et
756
al., 2005).
757
The behavior of O3, NO2, and odd oxygen (Ox=O3+NO2) throughout the day also
758
demonstrates the influence of lower weekend NO emissions on ozone production rates
759
(Fig. 2; Table 8). In Moscow, the daily means of Ox amount to 67 µg·m–3 on weekdays
760
and 64.3 µg·m–3 on Sundays. These high values show that NO emissions have sufficient
761
time to be photochemically processed by reaction with O3 and RO2 into Ox. Differences
762
in the behavior between O3 and Ox during a week suggest that the interconversion of O3
763
to NO2 can make a noticeable contribution to the weekend effect. The effect of ozone
764
suppression by NO emissions takes place throughout the day, however, this effect is most
765
pronounced during morning rush hours, when fresh NO emissions activate ozone
766
titration, although the general ozone-chemistry regime in Moscow remains to be VOC-
767
limited.
768
At ZSS and Troitsk regional background stations the mean concentrations of O3
769
and NO2 in 05:00–18:00 averaged over 2005–2014 are equal 28.0 and 23.9 µg·m–3,
34
770
respectively. The mean odd-oxygen concentration is about 52 µg·m–3. The NO2
771
concentration in this area is significantly lower, when compared to ozone, and slighter
772
affects odd-oxygen patterns. In the vicinity of highways and small towns, NO2 may more
773
significantly contribute to odd oxygen. On the whole, the Ox value and its time variability
774
support the above conclusion that the transition VOC-limited to NOx-limited regime
775
dominates within the greenbelt. Therefore, data obtained from ozone observations at
776
individual stations and used for air quality estimations should be extrapolated to the
777
region with consideration for simultaneous NO2 observations.
778
Table 8 gives the calculated daily LN/Q variations averaged over the entire
779
observation period. Since the complete composition of VOCs was not measured, only
780
two versions were considered, when (1) δVOCs were replaced by δNMHCs, which
781
include the main components of NOx-O3-VOC interactions; and (2) δVOCs were replaced
782
by δСO (such a replacement is used, at least, for Mexico and Beijing (Stephens et al.,
783
2008; Wang et al., 2014)).Variations in surface NO2 photolysis frequencies δJNO2 are
784
comparable to those in actinic fluxes at UV wavelengths. Using relations between δPM10
785
and δJ proposed by Wang et al. (2014) on the basis of PM2.5, PM10, and UV radiation
786
measurements in the Beijing region, we obtain that 14.4 % decrease in PM10 on Sundays
787
for Moscow corresponds to δJ equal to 4 %.
788
Early morning (05:00) under the conditions of low illumination and small NO
789
emissions from traffic, the rate of photochemical production of O3 is very low. The LN/Q
790
ratio (δNMHC is used as δVOC) amounts to 0.70. After 07:30 with the start of morning
791
rush hours, the LN/Q ratio grows a little and keeps between 0.72–0.79. This implies that
792
about 74 % of total radical loss is removed via NOx chemistry. Calculations of LN/Q for
793
the all Moscow sectors yield similar results, LN/Q ratios vary from 0.69±0.15 in SE sector
794
to 0.8±0.17 in the city center, which suggests that the VOC-limited regime is
795
characteristic of the entire Moscow megacity. Similar results were obtained by Stephens
796
et al. (2008) for Mexico City, however, δCO instead of VOC was used in calculations of
797
LN/Q. The LN/Q ratios (for Mexico City) calculated from differences in three-hour δX
35
798
values between weekdays (Monday–Friday) and Sunday (2004–2007 measurements)
799
amount to 0.90–0.95 and coincide with the LN/Q ratio values for Moscow only in the
800
case, when δCO is used instead of δVOC. The discrepancies between LN/Q (obtained
801
with replacement of δVOC by δCO and δNMHC, table 3) imply a high sensitivity of
802
photochemical-reactivity estimates to the composition and concentration of VOCs.
803
6. Conclusions
804
Moscow is a fast-developing megacity within the administrative boundaries that
805
remained unchanged for a long time. Therefore Moscow has become one of the most
806
densely populated cities in the world. A large scatter in estimates of pollutant emissions
807
from urban sources, which were obtained using both inventory and observational data
808
(Elansky et al., 2018), hampers the use of chemical-transport models in simulating air
809
quality and taking steps to improve it. The obtained features of the pollutant weekend
810
effect for Moscow and its greenbelt may help in diagnosing the pollutant regime and
811
suggest an effective action plan. For the Moscow megacity, the daytime (08:00–22:00)
812
weekday-Sunday differences averaged over the 10-year measurement period at 49
813
stations show that the Sunday effects in NO, NO2, CO, NMHC, SO2, and PM10
814
concentrations amounts to 23.9, 16.7, 13.6, 6.3, 7.6, and 14.5 %, respectively. The daily
815
mean (24 h) weekday-Sunday differences in these pollutants are also significant. The
816
concentration of CH4 decreases on Sundays, but its decrease is not reliable. On Sundays,
817
ozone increases by 16.5 %. For comparison with other cities (Mexico City, Sacramento,
818
Beijing, urban areas of Korea) the weekend effect for Moscow was recalculated using the
819
methodologies excepted by authors of corresponding studies. The weekend decrease in
820
the basic pollutants in Moscow is significantly smaller than that in cities located at
821
southern latitudes then Moscow, however, the ozone increase is approximately the same.
822
Combinations of both anthropogenic and meteorological factors affect weekly
823
patterns and weekend effects of air pollution in the Moscow. Among anthropogenic
824
factors, the main factors are the motor transport, the operation of the central heating
825
system in Moscow and its suburbs (October–April), and mass travels of inhabitants from
36
826
the city to their country houses on weekends. Meteorological processes are manifested in
827
the form of annual variations in the recurrence, duration, and intensity of temperature
828
inversions, air mass advection and convective mixing. In connection with this, pollutant
829
emissions and the NO, CO, and PM10 weekend effects are most significant in winter.
830
Maximum weekend effects in NO2 are noted in summer and in O3 are noted in winter and
831
autumn. There are no differences in PM10 in spring because of the lifting of a large
832
amount of aerosols into the air after snow melting. The weekend effect of SO2, which is,
833
to a great extent, associated with emissions from heat and power plants, including those
834
located outside Moscow, is significant only in summer and autumn, when the effect of
835
their emissions is minimum as compared to traffic emissions. Unlike other pollutants, the
836
daytime weekend effect of SO2 is noticeably different for all the sectors of Moscow.
837
The weekly pollutant cycles within the greenbelt differ from those within the city.
838
There are no weekday-weekend reliable differences in CO, CH4, SO2 concentrations, in
839
NO, NO2, PM10 weekend effects are significantly weakened, in NMHC and O3 effects are
840
at the reliable level. Regular travels of inhabitants from the city to their country houses
841
densely built up by the early 2000s shift, on weekends, the highest road-traffic activity
842
towards the suburbs of the city and decrease the weekend effect within the greenbelt.
843
Data obtained from measurements at the Ostankino TV tower show a stable
844
decrease in the concentrations of NO, NO2, and CO on weekends at all height levels up to
845
348 m. The weekend effects in NOx and CO concentrations are decreased on weekends at
846
a height of 130 m. The comparison of the Moscow and Beijing vertical structure of
847
weekend effects shows that the influence of transport of pollutants from regional sources
848
on their weekly cycles is weaker in Moscow than in Beijing. This result support the
849
conclusion made up earlier by Elansky et al. (2018) that Moscow may be considered as
850
an isolated city.
851
Photochemical processes determining the formation of the ozone weekly cycle in
852
Moscow are similar to those observed in other megacities. The obtained estimates of the
853
basic criteria of the activity of such processes (time variability of the OX concentration
37
854
and the ratios of NMHC/NOx and LN/Q) give an idea of the ozone weekly-cycle
855
formation characteristic of Moscow. Seasonal, weekly and daily changes in these criteria
856
imply the VOC-limited O3 chemistry. Under these conditions, even a slight decrease in
857
VOC concentrations is accompanied by a noticeable decrease in ozone concentrations. At
858
regional background stations remotely located from Moscow, these criteria implies the
859
transition VOC-limited to NOX-limited regime. The obtained characteristics of the spatial
860
and temporal variability of the pollutant weekly patterns and weekend effects are
861
important for adapting chemistry-transport and climate models to the conditions of
862
Moscow and simulation possible changes in air quality and regional climate. The results
863
are useful for developing measures to improve the state of urban infrastructure.
864
Acknowledgements. The authors thank the management of the Moscow Department of
865
Natural Resources and Environmental Protection and the staff of the Moscow Ecological
866
Monitoring network for observational data provided. The collection and analysis of
867
longterm observation data were carried out with the support of the Russian Science
868
Foundation (project No.16-17-10275). The methodology for calculating weekly cycle
869
parameters was developed in the framework of the Russian Foundation for Basic
870
Research project No.17-29-05102 , the balance of radicals in the atmosphere and the
871
aerosol content in the urban atmosphere were estimated according to the RFBR project
872
No. 19-05-00352.
873
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Highlights Daily and weekly cycles in O3, CO, CH4, NO, NO2, SO2, NMHC, and PM10were analyzed Measurements at 49 sites in 2005-2014 were used for species weekend effect studies Weekend effect in pollutants depends on PBL stratification, city sectors and height The VOC-limited chemistry of O3 formation is characteristic of Moscow atmosphere
Declaration of interests ☑The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: