- Email: [email protected]
Analysis of wintertime NO2 pollution in the Tokyo metropolitan area
Pergamon
Atmospheric
Environment Vol. 30, No. 5, pp. 703-713, 1996 Copyright 0 1996 Eleevier Science Ltd Prmted in Great Britain. All rights reserved 135% 2310/96 %lS.tXl + 0.00
1352-2310(95)00177-8 ANALYSIS
OF
WINTERTIME
TOKYO ITSUSHI
UNO**t,
NO2
METROPOLITAN
TOSHIMASA
OHARAS
POLLUTION
IN
THE
AREA
and SHINJI WAKAMATSUt
tNationa1 Institute for Environmental Studies, Onogawa 16-2, Tsukuba 305, Japan; SInstitute for Behaviour Sciences, Honmurcho 2-9, Ichigaya, Shinjuku, Tokyo 162, Japan
Abstract-The high concentration of NO, in the Tokyo Metropolitan area was investigated by means of air quality monitoring station data analysis and two numerical models: a photochemical box model and a photochemical grid model. Based on monitoring data, the typical variation with time of NO, NO, and 0, for days with high NO2 concentrations is presented. The application of the photochemical box and photochemical grid models successfully reproduced these variations when using typical emission intensities for NO, and NMHC from the Tokyo downtown area. Model results revealed that the high concentrations of NO2 in winter (exceeding the Japan environmental quality standard) mainly result from photochemical oxidation of NO to NOZ. Key word index: N02, photochemical air pollution, air quality monitoring, photochemical box model, prognostic meteorological model.
INTRODUCTION
High wintertime concentrations of NO, and suspended particulate matter (SPM) are major environmental concerns in Japan, especially in the Tokyo metropolitan area (TMA). Figure 1 shows the annual average concentration of NO and NO2 measured at monitoring stations which have been in continuous operation since 1970 (Japan Environmental Agency, 1990). Data are from both automobile exhaust monitoring stations (road side) and general air quality monitoring stations (far from road side). As can be seen in Fig. la, the annual average concentration of NO decreased from 0.034 to 0.023 ppm in the period 1971-1989. This was due to significant controls on the emission of air pollutants. However, NO2 concentrations are almost unchanged or slightly increasing (Fig. lb). They frequently exceed the Japanese environmental quality standard (EQS). The EQS for NO2 states that: “the daily average of hourly values shall be within or less than a range 0.04-0.06ppm” (Japan Environmental Agency, 1990). The EQS was exceeded at 4.1% of the general air pollution monitoring stations and 31.8% of the automobile exhaust monitoring stations in 1988. (Exceedence was especially frequent at the stations in Tokyo, Yokohama and Osaka). Figure l(c) shows the observed NO;1 concentration in relation to its EQS. Direct emissions of NO2 from all sources amounts to only 10% of total NO, emitted, however, the per-
*Author to whom correspondence should be addressed. 703
cent of NO2 in the atmosphere is much higher. The increase is due to oxidation of NO in the atmosphere. Thus, knowledge of atmospheric oxidation processes is important to understand the formation of NO2 better and to formulate control strategies better. In this paper, high wintertime concentrations of NOz are analyzed by using air quality monitoring station data in the TMA in conjunction with a photochemical box model (PBM) and a photochemical grid model. First, based on analysis of monitoring data, the typical variation with time of NO, NO2 and OJ observed on days with high NO2 concentrations will be presented. Then a PBM is applied to the Tokyo downtown area using typical emission patterns for NO, and non-methane hydrocarbons (NMHC) and characteristic meteorological conditions (of temperature, humidity and mixed layer height). Finally, a sensitivity analysis based on a photochemical grid model and tendency analysis of NO2 production based on the photochemical grid model, are described.
WINTERTIME LOCAL WINDS IN THE TMA
The complex topography of the Kanto Plain (which contains the TMA) is shown in Fig. 2. Because of the complex topography relatively complex local wind circulation patterns (such as sea/land breezes, mountain/valley winds, and combinations thereof) often develop even in winter. Fujibe (1985) and Ohara et al. (1989, 1990) showed that the sea/land breeze plays an
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704
a)
O.12~ NO (ppm) (Automobile exhaust monitoring stations)
4 b 2
0.06 0.05
=2 0.04
2
(General environment air monitoring swtions)
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ol
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air monitoring stations)
environment ’
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'70 '71 '72 '73 '74 '75 '76 '77 '78 '79 '80 '81 '82 '83 '84 '85 '86 '87 '88
General environmental air
monitoring stations (Readings at left)
1984(I.3091 1985
(1.302l Fig. 1.
’
(Automobile exhaust monitoring srations)
W
r\
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73 74 75 76 77 78 79 80
1986 r1.3241 1987 l9EB Il.3211
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Automobilt exhaust moni!oringstations(stations, (readingsat rwht)
\904
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(282)
1282)
(yc~r,
(2991
Yearly variation of average annual values of (a) NO, (b) NO,; and (c) the relation with the environmental quality standard for NO2 (Japan Environmental Agency, 1990).
Wintertime NO2 pollution in Tokyo
705
TMA. Another cause is the topographical blocking effect of the central mountainous area in Japan which occurs in early winter (Mizuno and Kondo, 1992).
ANALYSIS OF MONlTORING
50 km
I
Fig. 2. Topography of the Kanto District. Elevation contours are 200, 500, 1000,2000 m. important role in the formation of high air pollutant concentrations in the TMA. Figure 3 shows a wintertime wind pattern over the Kanto Plain observed on 27 January 1983. The variation with time of surface winds in the southern Kanto district (plotted at four hour intervals starting from 1700 JST on 27 January 1983) clearly shows the sea/land breeze change. At 1700 JST sea breeze penetration from Sagami Bay and Tokyo Bay prevailed; at 2100 JST the sea breeze weakened on the west coast of Tokyo Bay, while on the east coast SE winds developed (land breeze and/or down slope winds); and by 0100 JST on 28 January land breeze covered the entire Kanto district. Figure 4 shows air trajectories for the same day as plotted in Fig. 3, and Fig. 5 shows the variation with time of NO, NO, and NO2 along the trajectory paths. Both figures clearly reveal the influence of local wind circulation on the transport of pollutants. Closed wind circulation patterns such as those just described are not the only causes of high pollution levels in the
STATION DATA
Days with a high concentration of NO2 (October-March) were analyzed using observational data from the general air quality monitoring stations in Tokyo, Kawasaki and Yokohama. A day with “high concentration” is defined as a day when 80% or more of the monitoring stations record hourly NO2 concentrations of 60 ppb or more, and hourly wind speeds of 2 m s- 1 or less. A total of 72 d were selected within the time period 1983-1989 (70% of the selected days occurred in November and December). Based on the data recorded during these 72 d from 23 monitoring stations located in the TMA, the average variation with time of NO, NO2 and 0~ was calculated and was assumed to be representative of TMA concentration patterns (Uno and Wakamatsu, 1992). The set of 72 d was divided into two categories of days, fine and cloudy. Analysis of the monitoring data was conducted for the fine days only. Figure 6a shows the variation with time of NO, NO2 and 03 for the fine weather days (53 out of the 72 days). To indicate seasonal differences, Fig. 6b shows a typical summertime pattern for the TMA. In winter, NO exhibits peaks in the morning (110 ppb between 0700 and 0800 JST) and at night (175 ppb between 2000 and 2200 JST). The NO minimum in the afternoon (26 ppb) is reached at 1400 JST. NO2 concentration increases from 36 ppb to a high of 50 ppb at 0800 JST (a l-h delay compared to NO), and then remains more or less constant thereafter until 1500 JST, except for a small dip (120@-1400 JST). From 1600 to 1900 PST, the NOz concentration increases rapidly, reaching a maximum of 82 ppb at 1900 JST. The increase of NOz concentration in the afternoon starts earlier (by
1983 OlOOJST
: 28 Jan,
,' .
C
< .
Sagami Bay@
. I)\: \..
3
.
’
d
Fig. 3. Surface wind vectors in the southern Kanto district at 4-h intervals starting from 1700 JST on 27 January 1983. Dashed lines are borders of prefectures (Ohara et al., 1989).
I. UN0 et al.
706
-
-c-
Surface Wind Surface Wind X 1.5
\
APPLICATION
\
OF A PHOTOCHEMICAL
k.1 ‘i 1 ‘. ’ ‘1.
BOX MODEL
(PBM)
I ‘:
To investigate the basic behavior of pollutants in winter, the photochemical box model (PBM) developed by Schere and Demerjian (1984) was applied to the Tokyo area. The PBM is a single-box model in which the vertical depth is that of the mixed layer height. Figure 7 shows a schematic of the PBM. The basic equation is
ac. -_1= at
us+aziacj+Qi
ax
+
at aZ
zi
..jcn)
Rj(Cl,C2,.
where Cl is the jth species concentration, U is volume average wind speed, Zi is the mixed layer height, Qj is jth species emission intensity, and Rj is jth species production rate by chemical reaction. The carbon bond mechanism, version 4 (CBM-4) set of chemical reactions (Gery et al., 1989) was used in the model. Removal by dry deposition is not included in the PBM. Typical NO, and NMHC emission patterns observed in the central part of Tokyo area and characteristic meteorological conditions, were used in the model calculations. A modified version of the source fingerprints of NMHC developed by Wadden et al. (1984) was used to specify the CBM-4’s HC categories. Figure 8 shows the observed variation with time of the mixed layer height (squares) using data from Matsui (1990), and the resulting calculated variation (solid line) which was used in the PBM. (Mie scattering laser-radar was used to measure the time variation of Z,.) The days shown in the figure were chosen from those used in Fig. 6a with a wintertime high concentration of NO*. Initial concentrations of the pollutant species were taken from the average time variations shown in Fig. 6a. Based on aircraft measurements, the 03 concentration above the mixed layer height (i.e. boundary condition at the top of the box) was set to
Fig. 4. Trajectories passing through site A at 2200 JST. Numbers denote the time (JST). S1 and Sz are the starting points of wind trajectories (Ohara et al., 1990).
about 2 h) compared to NO. An 0~ maximum (20 ppb) is observed at 1400 JST. In summer, NO, NO* and 03 show time variations which are typical for photochemically dominant reactions (Fig. 6b). Because of the high emissions of NO, in Tokyo, 03 has a maximum of only 74 ppb. The highest concentrations of 03 are typically observed over the inland part of the Kanto plain (Wakamatsu et al., 1983; Uno et al., 1984). The wintertime variation shown in Fig. 6a is typical for the TMA. Its dynamics are basically controlled by (1) mixed layer height, (2) time variation of emissions, (3) chemical production/removal processes and (4) local wind systems (e.g. sea/land breeze).
1””
275-
.__--. ,’ *.___.
250225zooz*
175-
,Q
150-
”
i25-
0
5
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7
6
9
a
10
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11
a
12
*
13
(1)
3
14
”
15
“I”’
16
Time (JST)
17
16
19
20
2!
22
c
’
23
24
i
Fig. 5. Time variation of NO, NO* and NO concentrations on trajectory Sz to A in Fig. 4 (Ohara et al., 1990).
Wintertime NO2 pollution in Tokyo 2OOL ,
I
I
I
I
I
1
I
707 I
I
I
I
a) Winter (Fine Day) ......... 0 150 'j: 0
---
N:,
(ppb) (ppb)
-
NO
(ppb)
3 e
S.R. S.S. _
4
6
6
10
12 14 Time (JST)
16
16
20
22
24
8
10
12 14 Time (JST)
16
18
20
22
24
b) Summer 60 -
.........4 ---
(wb)
NO* (ppb) NO (ppb)
60-
2
4
6
Fig. 6. Average variation with time of NO, NO2 and O3 for (a) winter and (b) summer (calculated from monitoring station data, see text). SR and SS in figure indicate the range of sunrise and sunset for the selected days. 60 ppb. Furthermore, it was assumed that the volume average wind speed was zero. This is because only wintertime high concentration days with very light winds were selected for the model calculations. Figure 9 shows the results of the PBM calculations
for a typical winter day. The “obs” symbols in the figure represent observational data points (typical averaged time series of observed concentrations) taken from the graphs in Fig. 6a. Model calculations agree well with observations during the daytime, however, agreement is poor after 1700 JST, especially for NO. This may result from the fact that the PBM assumes a well-mixed box. Though this is a good approximation during the daytime, it is not a good approximation when the surface inversion layer becomes important during the night-time. Reproduction of NO2 concentrations, including the observed midafternoon concentration dip, is very good. It appears that the PBM is able to simulate the various processes which result in high NO2 concentrations. A sensitivity analysis consisting of a set of special case simulations was subsequently performed. The cases simulated include: (1) a case eliminating reaction
chemistry (no-reaction case), (2) a case without any photochemical reactions but including the dark reaction (dark-reaction-only case) and (3) a control-run case (control run). Figure 10 shows the results from the sensitivity analysis runs. The results of the control run and no-reaction cases clearly indicate that the photochemical reactions (oxidation of NO to NO4 and the transformation of NO, to PAN and HNOJ are important, even in winter (these graphs are not shown here). The reactions decrease the mid-afternoon concentration of NO2 (the dips shown in Figs 9 and 10). These dips are not reproduced in the case without photolysis reactions (dark reaction only). The detailed kinetics of NO2 production are discussed further in the following section.
APPLICATION
OF A PROGNOSTIC
AND PHOTOCHEMfCAL
METEOROLOGICAL
GRID MODEL
The PBM is useful in estimating the time variatilon of volume averaged concentrations. However, knowledge of the vertical concentration profiles is impor-
708
I. UN0 et al. RISINGMIXED HEIGHT
_A ONS
Fig. 7. Schematic illustration of the photochemical box model (PBM) (Schere and Demerjian, 1984).
1400
-
1200 E
Ziused
:
inPBM
Dec. 27, 1986 Dec. 7. 1988 Dec. 4, 1988
.
:..
1000 E
....
EOO-
w EOO-
.O@4
I 6
I 8
I
I
10
12
I 14
I
I
.-. . I
I
16
18
20
22
A 24
Time (JST)
Fig. 8. Variation with time of observed mixed layer height Zi. Squares indicate observational data (Matsui, 1990), and solid line indicates the average used in the PBM application.
tant to understand detailed transport mechanisms better. A prognostic
and dispersion meteorological
model coupled to a transport/reaction/removal model was applied with a 2D configuration to examine the importance of chemical reactions for wintertime NO2 production. As a prognostic mesoscale meteorological model, the Colorado State University Mesoscale Model
(CSUMM) was used to generate a flow field (Pielke, 1974; Ulrickson and Mass, 1990). The model is hydrostatic and consists of the equations of motion, moisture and continuity within a 3D, terrain-following coordinate system. It includes a thermodynamic equation, a diagnostic equation for pressure, and a surface heat budget. The output of CSUMM was used as input to the transport/reaction/removal
Wintertime NO2 pollution in Tokyo
709
Wink; 0
NO
0
(Ohs)
0
n ??
F
-NO
(PBM)
0
&
12
14 Time (JST)
16
18
20
22
Fig. 9. Average variation with time of NO, NO2 and O3 for winter days. Observational data and PBM calculations are plotted using symbols and lines, respectively.
12OL
100 -
80 -
I
I A - - -.---
I
I
I
I
I
I
I
-’
Observation Dark Reaction only No Reaction Control Run
60 -
A-
8
10
12
14
16
10
20
22
Time (JST)
Fig. 10. Winter day sensitivity analysis for NO2 by PBM.
model (photochemical grid model). The basic model equation (with z* coordinate system) is as follows:
where S is the model top height, Kh is the eddy diffusivity for pollutants, and Ri and Qi are the same as in the basic PBM equation (1). The surface boundary condition is given by Xi = Qi(z = 0) - ud,/Ci Kh dz. SurfWe
(3)
where u&i is a dry deposition velocity. The CBM-4 chemical mechanism was used as the reaction module. The top boundary concentration was kept constant during the calculations.
The main purpose of this model application was to understand the wintertime NO2 production mechanisms quantitatively. Figure 11 shows a 2D calculation domain and the location of an urban area (the emission source area). A relatively small mountainous area was placed at the center of the calculation domain. The urban area was located on the east side of the mountain from x = 300 km to x = 330 km. The sea was placed on both sides of the region. The roughness length (~0) was set at 0.15 m over the land. A horizontal grid size of Ax = 3 km, with 23 vertical levels (with fine resolution near the surface, and coarser resolution near the top), was used. The same grid resolution was used in both the CSUMM and photochemical grid model. The sea surface temperature was kept constant (T.,. = 13°C) during the calculations. The initial condition for the wind profile was set to
I. UN0 et al.
710
aj
0
20
40 Horixgal
MestitNumbf;O 140
120 sooot
n
'
m
'
o
'
e
'
m
'
'
'
'
'
2j
b)
280
290
300
3;o
320
340
3;o
350
x (km) Fig. 11. (a) Topography of modeling region for use in prognostic meteorological model and photochemical grid model. Also shown is the location of an urban area (emission area), and (b) the detailed topography
near urban area.
calm, the gradient of potential temperature was 0.04 K/100 m, and the relative humidity was assumed to be 65%. Average diurnal emission intensities of NO, NO2 and hydrocarbons (NMHC) were taken from the TMA emission inventories, the same as used in the PBM application. Initial concentrations for all species except 0s were set to 0 ppb (and 03 to 35 ppb). Dry deposition velocities were taken from Chang et al. (1989). The numerical calculation was started on 1 December and run for 3 d. The third simulation day’s result were used for analysis. Figure 12 shows the time-height cross-section of NO, NO2 and 03 over the city (Point A in Fig. 11). The typical wind direction reversal of the sea/land breeze and the diurnal development of the stable and the mixed layer were well simulated (not shown). The NO2 isopleth of 30-40 ppb agrees well with the development of the mixed layer height from 1000 to 1600 JST. NO2 concentration displays an almost uniform concentration profile within the daytime mixed layer, which validates the PBM assumption. This uniform vertical distribution results from the nonlinear interaction among vertical diffusion, emission intensity, deposition, and complex chemical reactions. This high concentration region aloft is maintained until the next morning. Concentration levels near the surface decrease because deposition becomes important after 1900 JST. The behavior of NO is different from that of
NO*. O3 shows an inverse relation with NO. The surface level maximum concentration of 03 was 25 ppb at 1400 JST and almost zero at night. The simulated concentration levels are slightly different from the observed ones shown in Fig. 6a, however, the characteristic variation of the pollutants is qualitatively well simulated. The identification of the relative contribution of meteorological effects and chemical reactions is important to know for air pollution control strategy. The tendency equation of NOz change is
&
J3ADVZ
&
Overall
ADVX
-i-c
Y
DIFF( + DEPO)
+
QNO,
+
RNO,
*L1-’ SOURCE
(4)
REACT
where Overall, ADVX, ADVZ, DIFF, SOURCE and REACT indicate the overall time variation rate of N02, and the changes by horizontal advection, by vertical advection, by vertical diffusion (first vertical grid level includes dry deposition), by emissions, and
Wintertime NO, pollution in Tokyo
.
a)NO(ppb)
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b) NO, (PP~)
.
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14
_ _ _ _ _ . .
1200
_ .
16
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20
22
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22
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.
-
1000 g N
800 600
-
-
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_
.
,
.
400
.
.
.
200 0 0
2
4
6
8
10
12
14
16
18
20
1400 1200 1000
z
00°
N
600
. -
. .
. .
. .
, .
. .
_ _ .
. .
. .
. _.
400 200 0
0
2
4
6
8
10
12
Time (JST)
14
16
It3
20
22
24
10
Fig. 12. Time-height cross-section (over the city; x = 306 km) of (a) NO, (b) NOz and (c) 0s calculated by the 2D version of photochemical grid model. The data represent OGOO JST on 3rd simulation day (3 December). Arrows in figure are wind vectors (horizontal and vertical direction). (Note that the vertical wind scaling is multiplied by factor 30.)
I. UN0 et al.
712
radical reaction). The first, second and third terms of equation (5) are the major contributors to the changes in NO2 concentrations. Figure 13 shows the time variation of the terms in equations (4) and (5) at the surface (z = 10 m) for x = 306 km (Point A). Figure 13a shows the meteorological and source emission terms, and Met.-total in the figure indicates the sum of ADVX, ADVZ, DIFF and SOURCE. SOURCE shows an almost constant value from 900 to 1900 JST. ADVX and ADVZ show a good balance. The Overall value is almost zero from 0400 to 0800 JST. After 0900 JST, the DIFF term becomes dominant, and Met.-total becomes
by chemical reactions (ppbv min- I), respectively. The chemical reaction term RNO~can be expressed by, R NO2
=
-
kiCN021+ kdNWW
v
Net-Photo +
RIHO,]
-t
R[othcr radiacP,l
(5)
where the first term indicates the change of NO2 photolysis, the second term the change by NO + 03 reaction, the third term the change by NO + HO2 reaction, and the last term the sum of the other NO1 production/destruction reactions. The sum of the first and second terms (Net-Photo) is the net tendency of NO2 change by the NO-NO*-03 reaction (without
0
c)
5
1”“l”’ 0 .4 - ....‘..‘-.
10 15 Time (JST) ’
El
I
Net-Photo
”
”
I
20
”
“I”’
20
ii.‘..*_v
* P
-0.2 _““‘... --.....-----.....---...........-...................!.~..-.......................-...;~...........-...........-......-..-........--......-.-.. : ‘. : ,/’ 3 -0.4 -............. l..,,?rI .._....._............................................ 5 J 0
I I I I I I I I I I , I a I I I I I , 5
10
15
I
I
,
I
-_20
20
Time (JST)
Fig. 13. Time variation of the terms in the NO* budget equation at x = 306 km calculated at z = 10 m. (a) Meteorological terms for NO2 budget equation, (b) Overall, REACT, Met.-total and NO2 concentration, (c) major chemical reaction temw.
Wintertime
NO,
- 0.2 ppbvmin-’ from 1000 to 1800 JST. This indicates that meteorological processes tend to diminish the NO2 concentration level. Figure 13b shows the terms Overall, REACT, Met.-total and NO* concentration. Overall shows two peaks at 1000 and 1700 JST because REACT overcomes the Met.-total during the daytime. The time variation of Overall accounts for that of NOz, and indicates that REACT plays a significant role in the rapid increase of NO2 concentration in the morning (090&l 100 JST) and the evening (1600-1700 JST). Figure 13c shows the time variation of the terms in REACT ( = RNo2 in equation (5)). This figure indicates that the reaction of HO1 + NO plays a dominant role in the morning hours and the reaction of NO + O3 is important in the evening (because the Net-Photo becomes dominant in the REACT term). This detailed examination of NO2 time variation clearly indicates that the photochemical reactions plays an important role in NO2 production, and that the nonlinear interaction between meteorological processes (e.g. advection and diffusion) and chemical reactions should be carefully considered when planning controls strategies for NO1 in winter.
CONCLUDING
REMARKS
The high wintertime concentration of NO2 is an important environmental issue in Japan. In this paper the dynamics of the high NO2 concentrations were explored through (1) analysis of air quality monitoring station data, (2) a photochemical box model (PBM), and (3) a photochemical grid model. The typical variation with time of NO, NO2 and 03 for days with high concentrations (determined from the air quality monitoring data) was presented. The application of a PBM and 2D versions of a photochemical grid model successfully reproduced the pattern of observational data using typical emission intensities for NO, and NMHC from the Tokyo downtown area. It was found that photochemical reactions are important in determining NO2 concentrations. The high concentrations of NO2 in winter (exceeding the Japan environmental quality standard) mainly result from photochemical oxidation of NO to NO*.
Acknowledgements-The thanks to Mr Kenneth
authors want to express their L. Schere and Mr James M.
pollution
in Tokyo
713
Godowitch of U.S. EPA (Atmospheric Research and Exposure Assessment Laboratory) for their kind permission to use the photochemical box model (PBM) and the prognostic meteorological model (CSUMM).
REFERENCES
Chang Y. S., Carmichael G. R., Kurita H. and Ueda H. (1989) The transport and formation of photochemical oxidants in central Japan. Atmospheric Environment 23, 363-393. Fujibe F. (1985) Air pollution in the surface layer accompanying a local front at the onset of the land breeze. J. Met. Sot. Japan 53, 226-237. Grey M. W., Whitlen G. Z., Killus J. P. and Dodge M. C. (1989) A photochemical kinetics mechanism for urban and regional scale computer modeling. J. geophys. Rex 94, 12,925-12,956. Japan Environmental Agency (1990) Quality ofthe Enuirontnent in Japan 1990. Matsui I. (1990) Observation of the lower atmospheric structures by Mie scattering laser radar in urban area (in Japanese). Kogaku 19,438&446. Mizuno T. and Kondo H. (1992) Generation of a local front and high levels of air pollution on the Kanto Plain in early winter. Atmospheric Environment 26A, 137-143. Ohara T., Uno I. and Wakamatsu S. (1989) Observed structure of a land breeze head in the Tokyo metropolitan area. J. appl. Met. 28, 693-704. Ohara T., Uno I. and Wakamatsu S. (1990) Observational study of high concentration of NO, accompanied by the passage of land breeze front (in Japanese). J. Japan Air Pollut. Ass. 2S, 66-76. Pielke R. A. (1974) A three-dimensional numerical model of the sea breeze over south Florida. Mon. Weath. Rev. 102, 115-134. Pielke R. A. (1984) Mesoscale Meteorological Model. Academic Press, New York. Schere K. L. and Demerjian K. L. (1984) User’s Guide&r the Phorochemical Box Model (PBM). EPA-600/8-84-022a. Ulrickson B. L. and Mass C. F. (1990) Numerical investigation of mesoscale circulations over the Los Angles basin. Part I. A verification study. Mon. Weath. Reo. 118, 2138-2161. Uno I. and Wakamatsu S. (1992) Analysis of wintertime high concentration of NO* using a photochemical box model (in Japanese). J. Japan Air Pollut. Ass. 27, 179-195. Uno I., Wakamatsu S., Suzuki M. and Ogawa Y. (1984) Three-dimensional behavior of photochemical pollutant over the Tokyo metropolitan area. Atmospheric Environment 18, 751-761. Wadden R. A., Uno I. and Wakamatsu S. (1986) Source discrimination of short-term hydrocarbon samples measured aloft. Enoir. Sci. Technol. 20, 473-483 Wakamatsu S.; Ogawa Y., Murano K., Goi K. and Aburamoto Y. (1983) Aircraft survey of the secondary photochemical pollutants covering the Tokyo metropolitan area. Atmospheric Environment 17, 827-835.
Atmospheric
Environment Vol. 30, No. 5, pp. 703-713, 1996 Copyright 0 1996 Eleevier Science Ltd Prmted in Great Britain. All rights reserved 135% 2310/96 %lS.tXl + 0.00
1352-2310(95)00177-8 ANALYSIS
OF
WINTERTIME
TOKYO ITSUSHI
UNO**t,
NO2
METROPOLITAN
TOSHIMASA
OHARAS
POLLUTION
IN
THE
AREA
and SHINJI WAKAMATSUt
tNationa1 Institute for Environmental Studies, Onogawa 16-2, Tsukuba 305, Japan; SInstitute for Behaviour Sciences, Honmurcho 2-9, Ichigaya, Shinjuku, Tokyo 162, Japan
Abstract-The high concentration of NO, in the Tokyo Metropolitan area was investigated by means of air quality monitoring station data analysis and two numerical models: a photochemical box model and a photochemical grid model. Based on monitoring data, the typical variation with time of NO, NO, and 0, for days with high NO2 concentrations is presented. The application of the photochemical box and photochemical grid models successfully reproduced these variations when using typical emission intensities for NO, and NMHC from the Tokyo downtown area. Model results revealed that the high concentrations of NO2 in winter (exceeding the Japan environmental quality standard) mainly result from photochemical oxidation of NO to NOZ. Key word index: N02, photochemical air pollution, air quality monitoring, photochemical box model, prognostic meteorological model.
INTRODUCTION
High wintertime concentrations of NO, and suspended particulate matter (SPM) are major environmental concerns in Japan, especially in the Tokyo metropolitan area (TMA). Figure 1 shows the annual average concentration of NO and NO2 measured at monitoring stations which have been in continuous operation since 1970 (Japan Environmental Agency, 1990). Data are from both automobile exhaust monitoring stations (road side) and general air quality monitoring stations (far from road side). As can be seen in Fig. la, the annual average concentration of NO decreased from 0.034 to 0.023 ppm in the period 1971-1989. This was due to significant controls on the emission of air pollutants. However, NO2 concentrations are almost unchanged or slightly increasing (Fig. lb). They frequently exceed the Japanese environmental quality standard (EQS). The EQS for NO2 states that: “the daily average of hourly values shall be within or less than a range 0.04-0.06ppm” (Japan Environmental Agency, 1990). The EQS was exceeded at 4.1% of the general air pollution monitoring stations and 31.8% of the automobile exhaust monitoring stations in 1988. (Exceedence was especially frequent at the stations in Tokyo, Yokohama and Osaka). Figure l(c) shows the observed NO;1 concentration in relation to its EQS. Direct emissions of NO2 from all sources amounts to only 10% of total NO, emitted, however, the per-
*Author to whom correspondence should be addressed. 703
cent of NO2 in the atmosphere is much higher. The increase is due to oxidation of NO in the atmosphere. Thus, knowledge of atmospheric oxidation processes is important to understand the formation of NO2 better and to formulate control strategies better. In this paper, high wintertime concentrations of NOz are analyzed by using air quality monitoring station data in the TMA in conjunction with a photochemical box model (PBM) and a photochemical grid model. First, based on analysis of monitoring data, the typical variation with time of NO, NO2 and OJ observed on days with high NO2 concentrations will be presented. Then a PBM is applied to the Tokyo downtown area using typical emission patterns for NO, and non-methane hydrocarbons (NMHC) and characteristic meteorological conditions (of temperature, humidity and mixed layer height). Finally, a sensitivity analysis based on a photochemical grid model and tendency analysis of NO2 production based on the photochemical grid model, are described.
WINTERTIME LOCAL WINDS IN THE TMA
The complex topography of the Kanto Plain (which contains the TMA) is shown in Fig. 2. Because of the complex topography relatively complex local wind circulation patterns (such as sea/land breezes, mountain/valley winds, and combinations thereof) often develop even in winter. Fujibe (1985) and Ohara et al. (1989, 1990) showed that the sea/land breeze plays an
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704
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(General environment air monitoring swtions)
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monitoring stations (Readings at left)
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Yearly variation of average annual values of (a) NO, (b) NO,; and (c) the relation with the environmental quality standard for NO2 (Japan Environmental Agency, 1990).
Wintertime NO2 pollution in Tokyo
705
TMA. Another cause is the topographical blocking effect of the central mountainous area in Japan which occurs in early winter (Mizuno and Kondo, 1992).
ANALYSIS OF MONlTORING
50 km
I
Fig. 2. Topography of the Kanto District. Elevation contours are 200, 500, 1000,2000 m. important role in the formation of high air pollutant concentrations in the TMA. Figure 3 shows a wintertime wind pattern over the Kanto Plain observed on 27 January 1983. The variation with time of surface winds in the southern Kanto district (plotted at four hour intervals starting from 1700 JST on 27 January 1983) clearly shows the sea/land breeze change. At 1700 JST sea breeze penetration from Sagami Bay and Tokyo Bay prevailed; at 2100 JST the sea breeze weakened on the west coast of Tokyo Bay, while on the east coast SE winds developed (land breeze and/or down slope winds); and by 0100 JST on 28 January land breeze covered the entire Kanto district. Figure 4 shows air trajectories for the same day as plotted in Fig. 3, and Fig. 5 shows the variation with time of NO, NO, and NO2 along the trajectory paths. Both figures clearly reveal the influence of local wind circulation on the transport of pollutants. Closed wind circulation patterns such as those just described are not the only causes of high pollution levels in the
STATION DATA
Days with a high concentration of NO2 (October-March) were analyzed using observational data from the general air quality monitoring stations in Tokyo, Kawasaki and Yokohama. A day with “high concentration” is defined as a day when 80% or more of the monitoring stations record hourly NO2 concentrations of 60 ppb or more, and hourly wind speeds of 2 m s- 1 or less. A total of 72 d were selected within the time period 1983-1989 (70% of the selected days occurred in November and December). Based on the data recorded during these 72 d from 23 monitoring stations located in the TMA, the average variation with time of NO, NO2 and 0~ was calculated and was assumed to be representative of TMA concentration patterns (Uno and Wakamatsu, 1992). The set of 72 d was divided into two categories of days, fine and cloudy. Analysis of the monitoring data was conducted for the fine days only. Figure 6a shows the variation with time of NO, NO2 and 03 for the fine weather days (53 out of the 72 days). To indicate seasonal differences, Fig. 6b shows a typical summertime pattern for the TMA. In winter, NO exhibits peaks in the morning (110 ppb between 0700 and 0800 JST) and at night (175 ppb between 2000 and 2200 JST). The NO minimum in the afternoon (26 ppb) is reached at 1400 JST. NO2 concentration increases from 36 ppb to a high of 50 ppb at 0800 JST (a l-h delay compared to NO), and then remains more or less constant thereafter until 1500 JST, except for a small dip (120@-1400 JST). From 1600 to 1900 PST, the NOz concentration increases rapidly, reaching a maximum of 82 ppb at 1900 JST. The increase of NOz concentration in the afternoon starts earlier (by
1983 OlOOJST
: 28 Jan,
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Fig. 3. Surface wind vectors in the southern Kanto district at 4-h intervals starting from 1700 JST on 27 January 1983. Dashed lines are borders of prefectures (Ohara et al., 1989).
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APPLICATION
\
OF A PHOTOCHEMICAL
k.1 ‘i 1 ‘. ’ ‘1.
BOX MODEL
(PBM)
I ‘:
To investigate the basic behavior of pollutants in winter, the photochemical box model (PBM) developed by Schere and Demerjian (1984) was applied to the Tokyo area. The PBM is a single-box model in which the vertical depth is that of the mixed layer height. Figure 7 shows a schematic of the PBM. The basic equation is
ac. -_1= at
us+aziacj+Qi
ax
+
at aZ
zi
..jcn)
Rj(Cl,C2,.
where Cl is the jth species concentration, U is volume average wind speed, Zi is the mixed layer height, Qj is jth species emission intensity, and Rj is jth species production rate by chemical reaction. The carbon bond mechanism, version 4 (CBM-4) set of chemical reactions (Gery et al., 1989) was used in the model. Removal by dry deposition is not included in the PBM. Typical NO, and NMHC emission patterns observed in the central part of Tokyo area and characteristic meteorological conditions, were used in the model calculations. A modified version of the source fingerprints of NMHC developed by Wadden et al. (1984) was used to specify the CBM-4’s HC categories. Figure 8 shows the observed variation with time of the mixed layer height (squares) using data from Matsui (1990), and the resulting calculated variation (solid line) which was used in the PBM. (Mie scattering laser-radar was used to measure the time variation of Z,.) The days shown in the figure were chosen from those used in Fig. 6a with a wintertime high concentration of NO*. Initial concentrations of the pollutant species were taken from the average time variations shown in Fig. 6a. Based on aircraft measurements, the 03 concentration above the mixed layer height (i.e. boundary condition at the top of the box) was set to
Fig. 4. Trajectories passing through site A at 2200 JST. Numbers denote the time (JST). S1 and Sz are the starting points of wind trajectories (Ohara et al., 1990).
about 2 h) compared to NO. An 0~ maximum (20 ppb) is observed at 1400 JST. In summer, NO, NO* and 03 show time variations which are typical for photochemically dominant reactions (Fig. 6b). Because of the high emissions of NO, in Tokyo, 03 has a maximum of only 74 ppb. The highest concentrations of 03 are typically observed over the inland part of the Kanto plain (Wakamatsu et al., 1983; Uno et al., 1984). The wintertime variation shown in Fig. 6a is typical for the TMA. Its dynamics are basically controlled by (1) mixed layer height, (2) time variation of emissions, (3) chemical production/removal processes and (4) local wind systems (e.g. sea/land breeze).
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Fig. 5. Time variation of NO, NO* and NO concentrations on trajectory Sz to A in Fig. 4 (Ohara et al., 1990).
Wintertime NO2 pollution in Tokyo 2OOL ,
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Fig. 6. Average variation with time of NO, NO2 and O3 for (a) winter and (b) summer (calculated from monitoring station data, see text). SR and SS in figure indicate the range of sunrise and sunset for the selected days. 60 ppb. Furthermore, it was assumed that the volume average wind speed was zero. This is because only wintertime high concentration days with very light winds were selected for the model calculations. Figure 9 shows the results of the PBM calculations
for a typical winter day. The “obs” symbols in the figure represent observational data points (typical averaged time series of observed concentrations) taken from the graphs in Fig. 6a. Model calculations agree well with observations during the daytime, however, agreement is poor after 1700 JST, especially for NO. This may result from the fact that the PBM assumes a well-mixed box. Though this is a good approximation during the daytime, it is not a good approximation when the surface inversion layer becomes important during the night-time. Reproduction of NO2 concentrations, including the observed midafternoon concentration dip, is very good. It appears that the PBM is able to simulate the various processes which result in high NO2 concentrations. A sensitivity analysis consisting of a set of special case simulations was subsequently performed. The cases simulated include: (1) a case eliminating reaction
chemistry (no-reaction case), (2) a case without any photochemical reactions but including the dark reaction (dark-reaction-only case) and (3) a control-run case (control run). Figure 10 shows the results from the sensitivity analysis runs. The results of the control run and no-reaction cases clearly indicate that the photochemical reactions (oxidation of NO to NO4 and the transformation of NO, to PAN and HNOJ are important, even in winter (these graphs are not shown here). The reactions decrease the mid-afternoon concentration of NO2 (the dips shown in Figs 9 and 10). These dips are not reproduced in the case without photolysis reactions (dark reaction only). The detailed kinetics of NO2 production are discussed further in the following section.
APPLICATION
OF A PROGNOSTIC
AND PHOTOCHEMfCAL
METEOROLOGICAL
GRID MODEL
The PBM is useful in estimating the time variatilon of volume averaged concentrations. However, knowledge of the vertical concentration profiles is impor-
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Fig. 7. Schematic illustration of the photochemical box model (PBM) (Schere and Demerjian, 1984).
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Fig. 8. Variation with time of observed mixed layer height Zi. Squares indicate observational data (Matsui, 1990), and solid line indicates the average used in the PBM application.
tant to understand detailed transport mechanisms better. A prognostic
and dispersion meteorological
model coupled to a transport/reaction/removal model was applied with a 2D configuration to examine the importance of chemical reactions for wintertime NO2 production. As a prognostic mesoscale meteorological model, the Colorado State University Mesoscale Model
(CSUMM) was used to generate a flow field (Pielke, 1974; Ulrickson and Mass, 1990). The model is hydrostatic and consists of the equations of motion, moisture and continuity within a 3D, terrain-following coordinate system. It includes a thermodynamic equation, a diagnostic equation for pressure, and a surface heat budget. The output of CSUMM was used as input to the transport/reaction/removal
Wintertime NO2 pollution in Tokyo
709
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(PBM)
0
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12
14 Time (JST)
16
18
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Fig. 9. Average variation with time of NO, NO2 and O3 for winter days. Observational data and PBM calculations are plotted using symbols and lines, respectively.
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Fig. 10. Winter day sensitivity analysis for NO2 by PBM.
model (photochemical grid model). The basic model equation (with z* coordinate system) is as follows:
where S is the model top height, Kh is the eddy diffusivity for pollutants, and Ri and Qi are the same as in the basic PBM equation (1). The surface boundary condition is given by Xi = Qi(z = 0) - ud,/Ci Kh dz. SurfWe
(3)
where u&i is a dry deposition velocity. The CBM-4 chemical mechanism was used as the reaction module. The top boundary concentration was kept constant during the calculations.
The main purpose of this model application was to understand the wintertime NO2 production mechanisms quantitatively. Figure 11 shows a 2D calculation domain and the location of an urban area (the emission source area). A relatively small mountainous area was placed at the center of the calculation domain. The urban area was located on the east side of the mountain from x = 300 km to x = 330 km. The sea was placed on both sides of the region. The roughness length (~0) was set at 0.15 m over the land. A horizontal grid size of Ax = 3 km, with 23 vertical levels (with fine resolution near the surface, and coarser resolution near the top), was used. The same grid resolution was used in both the CSUMM and photochemical grid model. The sea surface temperature was kept constant (T.,. = 13°C) during the calculations. The initial condition for the wind profile was set to
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x (km) Fig. 11. (a) Topography of modeling region for use in prognostic meteorological model and photochemical grid model. Also shown is the location of an urban area (emission area), and (b) the detailed topography
near urban area.
calm, the gradient of potential temperature was 0.04 K/100 m, and the relative humidity was assumed to be 65%. Average diurnal emission intensities of NO, NO2 and hydrocarbons (NMHC) were taken from the TMA emission inventories, the same as used in the PBM application. Initial concentrations for all species except 0s were set to 0 ppb (and 03 to 35 ppb). Dry deposition velocities were taken from Chang et al. (1989). The numerical calculation was started on 1 December and run for 3 d. The third simulation day’s result were used for analysis. Figure 12 shows the time-height cross-section of NO, NO2 and 03 over the city (Point A in Fig. 11). The typical wind direction reversal of the sea/land breeze and the diurnal development of the stable and the mixed layer were well simulated (not shown). The NO2 isopleth of 30-40 ppb agrees well with the development of the mixed layer height from 1000 to 1600 JST. NO2 concentration displays an almost uniform concentration profile within the daytime mixed layer, which validates the PBM assumption. This uniform vertical distribution results from the nonlinear interaction among vertical diffusion, emission intensity, deposition, and complex chemical reactions. This high concentration region aloft is maintained until the next morning. Concentration levels near the surface decrease because deposition becomes important after 1900 JST. The behavior of NO is different from that of
NO*. O3 shows an inverse relation with NO. The surface level maximum concentration of 03 was 25 ppb at 1400 JST and almost zero at night. The simulated concentration levels are slightly different from the observed ones shown in Fig. 6a, however, the characteristic variation of the pollutants is qualitatively well simulated. The identification of the relative contribution of meteorological effects and chemical reactions is important to know for air pollution control strategy. The tendency equation of NOz change is
&
J3ADVZ
&
Overall
ADVX
-i-c
Y
DIFF( + DEPO)
+
QNO,
+
RNO,
*L1-’ SOURCE
(4)
REACT
where Overall, ADVX, ADVZ, DIFF, SOURCE and REACT indicate the overall time variation rate of N02, and the changes by horizontal advection, by vertical advection, by vertical diffusion (first vertical grid level includes dry deposition), by emissions, and
Wintertime NO, pollution in Tokyo
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Fig. 12. Time-height cross-section (over the city; x = 306 km) of (a) NO, (b) NOz and (c) 0s calculated by the 2D version of photochemical grid model. The data represent OGOO JST on 3rd simulation day (3 December). Arrows in figure are wind vectors (horizontal and vertical direction). (Note that the vertical wind scaling is multiplied by factor 30.)
I. UN0 et al.
712
radical reaction). The first, second and third terms of equation (5) are the major contributors to the changes in NO2 concentrations. Figure 13 shows the time variation of the terms in equations (4) and (5) at the surface (z = 10 m) for x = 306 km (Point A). Figure 13a shows the meteorological and source emission terms, and Met.-total in the figure indicates the sum of ADVX, ADVZ, DIFF and SOURCE. SOURCE shows an almost constant value from 900 to 1900 JST. ADVX and ADVZ show a good balance. The Overall value is almost zero from 0400 to 0800 JST. After 0900 JST, the DIFF term becomes dominant, and Met.-total becomes
by chemical reactions (ppbv min- I), respectively. The chemical reaction term RNO~can be expressed by, R NO2
=
-
kiCN021+ kdNWW
v
Net-Photo +
RIHO,]
-t
R[othcr radiacP,l
(5)
where the first term indicates the change of NO2 photolysis, the second term the change by NO + 03 reaction, the third term the change by NO + HO2 reaction, and the last term the sum of the other NO1 production/destruction reactions. The sum of the first and second terms (Net-Photo) is the net tendency of NO2 change by the NO-NO*-03 reaction (without
0
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Fig. 13. Time variation of the terms in the NO* budget equation at x = 306 km calculated at z = 10 m. (a) Meteorological terms for NO2 budget equation, (b) Overall, REACT, Met.-total and NO2 concentration, (c) major chemical reaction temw.
Wintertime
NO,
- 0.2 ppbvmin-’ from 1000 to 1800 JST. This indicates that meteorological processes tend to diminish the NO2 concentration level. Figure 13b shows the terms Overall, REACT, Met.-total and NO* concentration. Overall shows two peaks at 1000 and 1700 JST because REACT overcomes the Met.-total during the daytime. The time variation of Overall accounts for that of NOz, and indicates that REACT plays a significant role in the rapid increase of NO2 concentration in the morning (090&l 100 JST) and the evening (1600-1700 JST). Figure 13c shows the time variation of the terms in REACT ( = RNo2 in equation (5)). This figure indicates that the reaction of HO1 + NO plays a dominant role in the morning hours and the reaction of NO + O3 is important in the evening (because the Net-Photo becomes dominant in the REACT term). This detailed examination of NO2 time variation clearly indicates that the photochemical reactions plays an important role in NO2 production, and that the nonlinear interaction between meteorological processes (e.g. advection and diffusion) and chemical reactions should be carefully considered when planning controls strategies for NO1 in winter.
CONCLUDING
REMARKS
The high wintertime concentration of NO2 is an important environmental issue in Japan. In this paper the dynamics of the high NO2 concentrations were explored through (1) analysis of air quality monitoring station data, (2) a photochemical box model (PBM), and (3) a photochemical grid model. The typical variation with time of NO, NO2 and 03 for days with high concentrations (determined from the air quality monitoring data) was presented. The application of a PBM and 2D versions of a photochemical grid model successfully reproduced the pattern of observational data using typical emission intensities for NO, and NMHC from the Tokyo downtown area. It was found that photochemical reactions are important in determining NO2 concentrations. The high concentrations of NO2 in winter (exceeding the Japan environmental quality standard) mainly result from photochemical oxidation of NO to NO*.
Acknowledgements-The thanks to Mr Kenneth
authors want to express their L. Schere and Mr James M.
pollution
in Tokyo
713
Godowitch of U.S. EPA (Atmospheric Research and Exposure Assessment Laboratory) for their kind permission to use the photochemical box model (PBM) and the prognostic meteorological model (CSUMM).
REFERENCES
Chang Y. S., Carmichael G. R., Kurita H. and Ueda H. (1989) The transport and formation of photochemical oxidants in central Japan. Atmospheric Environment 23, 363-393. Fujibe F. (1985) Air pollution in the surface layer accompanying a local front at the onset of the land breeze. J. Met. Sot. Japan 53, 226-237. Grey M. W., Whitlen G. Z., Killus J. P. and Dodge M. C. (1989) A photochemical kinetics mechanism for urban and regional scale computer modeling. J. geophys. Rex 94, 12,925-12,956. Japan Environmental Agency (1990) Quality ofthe Enuirontnent in Japan 1990. Matsui I. (1990) Observation of the lower atmospheric structures by Mie scattering laser radar in urban area (in Japanese). Kogaku 19,438&446. Mizuno T. and Kondo H. (1992) Generation of a local front and high levels of air pollution on the Kanto Plain in early winter. Atmospheric Environment 26A, 137-143. Ohara T., Uno I. and Wakamatsu S. (1989) Observed structure of a land breeze head in the Tokyo metropolitan area. J. appl. Met. 28, 693-704. Ohara T., Uno I. and Wakamatsu S. (1990) Observational study of high concentration of NO, accompanied by the passage of land breeze front (in Japanese). J. Japan Air Pollut. Ass. 2S, 66-76. Pielke R. A. (1974) A three-dimensional numerical model of the sea breeze over south Florida. Mon. Weath. Rev. 102, 115-134. Pielke R. A. (1984) Mesoscale Meteorological Model. Academic Press, New York. Schere K. L. and Demerjian K. L. (1984) User’s Guide&r the Phorochemical Box Model (PBM). EPA-600/8-84-022a. Ulrickson B. L. and Mass C. F. (1990) Numerical investigation of mesoscale circulations over the Los Angles basin. Part I. A verification study. Mon. Weath. Reo. 118, 2138-2161. Uno I. and Wakamatsu S. (1992) Analysis of wintertime high concentration of NO* using a photochemical box model (in Japanese). J. Japan Air Pollut. Ass. 27, 179-195. Uno I., Wakamatsu S., Suzuki M. and Ogawa Y. (1984) Three-dimensional behavior of photochemical pollutant over the Tokyo metropolitan area. Atmospheric Environment 18, 751-761. Wadden R. A., Uno I. and Wakamatsu S. (1986) Source discrimination of short-term hydrocarbon samples measured aloft. Enoir. Sci. Technol. 20, 473-483 Wakamatsu S.; Ogawa Y., Murano K., Goi K. and Aburamoto Y. (1983) Aircraft survey of the secondary photochemical pollutants covering the Tokyo metropolitan area. Atmospheric Environment 17, 827-835.