Weekly patterns and weekend effects of air pollution in the Moscow megacity

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. Semutni...

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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: