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with a regional chemical transport model 1. Model
description and evaluation
Jianzhong Ma, Xiuji Zhou, D. Hauglustaine
To cite this version:
Jianzhong Ma, Xiuji Zhou, D. Hauglustaine. Summertime tropospheric ozone over China simulated with a regional chemical transport model 1. Model description and evaluation. Journal of Geophysical Research, American Geophysical Union, 2002, 107 (D22), �10.1029/2001JD001354�. �hal-03127181�
Summertime tropospheric ozone over China simulated
with a regional chemical transport model
1. Model description and evaluation
Jianzhong Ma and Hongli Liu
Chinese Academy of Meteorological Sciences, Beijing, People’s Republic of China
Didier Hauglustaine1
Service d’Ae´ronomie du Centre National de la Recherche Scientifique, Paris, France
Received 4 October 2001; revised 26 January 2002; accepted 5 March 2002; published 30 November 2002.
[1] A three-dimensional regional chemical transport model, extended from the Regional
Acid Deposition Model (RADM) and aimed at studying the distribution and budget of tropospheric ozone and its precursors over China, is presented. The model domain covers the China region with a horizontal resolution of 100 km. In the vertical, the model extends up to the pressure level of 10 mbar for meteorological simulation, and to the local thermal tropopause for chemical integration. The meteorological fields for the model run are provided with the Fifth-Generation National Center for Atmospheric Research (NCAR)/ Penn State Mesoscale Model (MM5). In addition to updated surface emissions, aircraft emissions and lightning NOxsources are taken into account. The initial fields and lateral
boundary conditions for most chemical tracers are provided with a global chemical transport model for ozone and related chemical tracers (MOZART). The model simulation is performed for the period July 1 –15, 1995, which appears to be representative of meteorological conditions in summertime over China. The model results are compared with surface measurements of ozone and its precursors in China, ozone soundings in Japan, and MOZART results for the China region. The daily variation as well as geographical and vertical distribution of O3concentration is generally well simulated by
the model. It is indicated that surface ozone is controlled by photochemistry in eastern China and by transport processes in western China. Large-scale transport of O3 and its
precursors from the highest-source-emission regions to remote areas and the free troposphere is simulated. INDEXTERMS: 0365 Atmospheric Composition and Structure: Troposphere—composition and chemistry; 0368 Atmospheric Composition and Structure: Troposphere— constituent transport and chemistry; 0322 Atmospheric Composition and Structure: Constituent sources and sinks; 0345 Atmospheric Composition and Structure: Pollution—urban and regional (0305); KEYWORDS: tropospheric ozone, 3-D model, NOxsources, regional pollution, transport, China
Citation: Ma, J., H. Liu, and D. Hauglustaine, Summertime tropospheric ozone over China simulated with a regional chemical transport model, 1, Model description and evaluation, J. Geophys. Res., 107(D22), 4660, doi:10.1029/2001JD001354, 2002.
1. Introduction
[2] Tropospheric ozone (O3) is an important trace gas.
Radiatively, ozone acts as a greenhouse gas, particularly in the upper troposphere [Lacis et al., 1990]. Chemically, ozone is the major precursor of hydroxyl radical (OH), which is responsible for the oxidation of a large number of atmos-pheric trace gases. As such, ozone plays a critical role in controlling the oxidizing capacity of the atmosphere [Thomp-son, 1992]. Near the surface, high concentrations of ozone are detrimental to public health and vegetation [National
Research Council, 1991]. Tropospheric ozone is produced from the oxidation of hydrocarbons and carbon monoxide (CO) in the presence of nitrogen oxides (NOx) and sunlight.
There is some evidence to suggest that ozone concentrations in the troposphere of the Northern Hemisphere have increased by a factor of two or more over the past 100 years [World Meteorological Organization (WMO), 1995]. This is mainly due to the increased anthropogenic emissions of ozone precursors, i.e., NOx, CO and hydrocarbons, in
indus-trial areas, such as North America, Europe and East Asia. [3] China, the most populous nation in the world, has
experienced a period of rapid economic expansion and industrial development. Correspondingly, anthropogenic emissions associated with the burning of fossil fuel from China have grown significantly in recent years, and are expected to become one of the major contributors of the 1
Also at Laboratoire des Sciences du Climate et de l’Environnement, Gif-sur-Yvette, France.
Copyright 2002 by the American Geophysical Union. 0148-0227/02/2001JD001354
global pollution sources [Benkovitz et al., 1996; Galloway et al., 1996; Bai, 1996; Ma and Zhou, 2000]. In order to assess the ozone level and its controlling factors in China, atmos-pheric chemical data were collected at four nonurban sites in China over a 12-month period beginning in mid-August 1994 in the Chinese Ozone Research Program (CORP) [Zhou, 1996]. Using 1-hour averages of the measurements of surface O3and sulfur dioxide (SO2), as well as limited measurements
of nitric oxide (NO), NOxand volatile organic compounds
(VOC), Luo et al. [2000] showed that nonurban ozone pollution episodes do occur in China. They also found, through modeling, that rural areas in southern China tend to be NOx-limited, and rural areas in northern China tend to
be VOC-limited. Using the same database and modeling approach, Yang et al. [1999] argued that surface O3 is
primarily determined by photochemistry in the eastern part of China, but is primarily determined by physical factors in the western part of China.
[4] While ozone pollution episodes are usually most
intense within high population/industrialized areas, enhanced ozone as well as its precursors can be transported thousands of kilometers away, even to the remote areas, thus making a large contribution to global tropospheric ozone [Liu et al., 1987; Jacob et al., 1993, 1996; Mauzerall et al., 1996]. Most emissions in China are dispersed along the eastern coast. Therefore, in addition to its impact on air quality in the continental boundary layer (CBL) near pollution sources, the effect of emissions from China on chemical characteristics of the whole troposphere, particularly over the western North Pacific, is subjected to be considerable [Berntsen et al., 1996; Elliott et al., 1997; Horowitz and Jacob, 1999; Jacob et al., 1999; Mauzerall et al., 2000]. Moreover, with comparison to the United States and Europe, China is closer to the tropics, where the effect on O3and OH concentrations would most
significantly perturb the oxidizing capacity of the atmosphere [Thompson, 1992].
[5] The purpose of our work presented here is to
investigate more explicitly the factors that control tropo-spheric ozone over China, especially the perturbation of ozone from fossil fuel burning, using an extended version of the Regional Acid Deposition Model (RADM) [Chang et al., 1987]. The focus of our study is on the transport and transformation of chemical trace gases on a regional scale instead of in situ air pollution in the urban areas. In this paper, we give a description of the updated model, and evaluate the model by comparison with available observations and other model results. In a companion paper [Ma et al., 2002b], we do a budget analysis for tropospheric ozone over China, and quantify the contribu-tions of various sources to NOx and ozone distributions
over the region.
2. Model Description
[6] The framework of the chemical transport model used
in this study is based on the Regional Acid Deposition Model (RADM) [Chang et al., 1987]. The domain covers the China region. The required meteorological information is provided off-line from the Fifth-Generation NCAR/Penn State Mesoscale Model (MM5) [Grell et al., 1994]. The same 3-D grids are used for RADM and MM5. In the horizontal, the model includes 53 48 grid cells in
the east-west and south-north directions having dimensions of 100 km on a side placed on a Lambert conformal map. In the vertical, the model is divided unequally into 30 layers, from the Earth’s surface to the pressure level of 10 mbar, using a s-coordinate system. The lowest seven layers are typically in the planetary boundary layer during the day. There are about 8 – 13 layers for the free tropo-sphere in the model. The vertical resolution in the vicinity of the tropopause is enhanced, typically in the range 0.8 – 1.0 km, in order to resolve the vertical distribution of aircraft emissions and the resulting chemical perturbations. The photochemical tendency of species is calculated merely for the model levels below the tropopause, where the upper boundary conditions of species are prescribed as discussed below. In our simulations, a time step (tad =
360 s) is used for both horizontal and vertical transport, followed by diffusion and dry deposition calculations with the same time step. Gas phase chemistry and emissions are solved together with a variable chemical step tc, which
is frequently smaller than tad with a minimum value of
120 s. In this study, we performed 15-day simulations for the year of 1995, over a period July 1 – 15.
[7] The RADM has been adapted and updated for the
studies of surface ozone and ozone pollution episodes in the China region [Yang et al., 1999; Luo et al., 2000]. In the present study, we attempt to evaluate the various factors that control tropospheric ozone over China, not only confined to surface ozone, but also with a focus on ozone in the free troposphere. As such, RADM was further extended and im-proved by updating surface emissions of NOx, CO and
nonmethane hydrocarbons, incorporating aircraft emissions and lightning NOxsources, adding the oxidation mechanism
of isoprene and acetone, and being constrained by a global chem ical transport model. The stratosphere-troposphere exchange of O3, NOxand HNO3at the thermal tropopause
was treated in the model for this study. Below we describe the model in detail, focusing on the adapted and extended parts of it, and present the preliminary results for the simulation period.
2.1. Meteorology
[8] Meteorological initial and boundary conditions for the
MM5 runs are provided by the National Center for Environ-mental Prediction (NCEP) global reanalyses, upgraded by incorporating at 24-hour intervals rawinsonde observational data from more than 200 meteorological stations in China. Figure 1 shows the averaged wind speed and humidity at 500 mbar and cloud cover over the model domain for the period July 1 – 15, 1995. As shown in the figure, strong west northwesterly inflow prevails north of 40N, and west and southwesterlies prevail in the southwestern part of the domain. Onshore winds dominate in southeastern China, due to anticyclone associated with the high pressure in the western Pacific. The winds converge in the eastern and northeastern China and Korea, where strong continental outflow occurs above 30N. Warm and wet air masses are brought to south China from the oceans. As a result, rainy and cloudy days frequently occur along the Yangtze River valley of southern China during the period. Correspond-ingly, photodissociation of chemical species is slow in the boundary layer of this region. The meteorological condition presented above for the simulation period is typical
charac-teristic of Asian Monsoon, which prevails over China in summertime [Ding, 1991].
2.2. Emissions
[9] In the papers by Yang et al. [1999] and Luo et al.
[2000], surface anthropogenic emissions in the model are
based on the annually averaged grid inventories of Bai [1996] for China and Kato and Akimoto [1992] for the rest of the model domain. The emission inventories given by Bai [1996] are based on statistic data of the year 1992. In this study, updated NOxand CO emissions for the year 1995 are
adopted (N. B. Bai, unpublished report, 2001). The source includes industrial sources as well as biomass burning due to agricultural activities. No diurnal variation of emissions is provided. The geographic distribution of surface NOx
emission used in the model is shown in Figure 2a. Anthro-pogenic emissions of nonmethane hydrocarbons (NMHC) were obtained by assuming the mass NMHC/NOx ratio as
unit where NOxwas expressed as equivalent NO2as given
by Berntsen et al. [1996]. The total anthropogenic NMHC sources were divided into the individual NMHC species using splitting factors given by Brost et al. [1988]. The surface emission rates of NOx and NMHC are given in
Table 1.
[10] In the present study, the biogenic emissions of
iso-prene and other natural NMHCs are included in the model based on Guenther et al. [1995]. The isoprene emission rates were assumed to change as a function of temperature and light according to Guenther et al. [1995]. The total natural NMHC source, other than isoprene, were divided into the individual NMHC species with the weighting factors deduced from the biogenic emission inventory given by Brasseur et al. [1998]. The daily-cycled aircraft emissions of CO, NMHCs, NOx, and SO2were incorporated into the
model according to the 3-D inventories developed by Ma and Zhou [2000]. The averaged aircraft NOx emissions
during the simulation period is shown in Figure 2b. [11] The lightning source of NOxin the model is varied in
space and time. This emission is parameterized using the scheme of Pickering et al. [1998] and predicted in the one-dimensional diagnostic cloud model of RADM. Based on the studies of Price and Rind [1992, 1994], frequencies of lightning flashes are computed in our model using the convective cloud-top height:
Fc¼ 3:44 105H4:90 ð1Þ
Fm¼ 6:40 104 H1:73 ð2Þ
where Fc and Fm are the lightning frequencies for
continental and marine thunderstorms (flashes per minute), respectively, and H the cloud-top height above ground (km). The intra-cloud (IC) to cloud-ground (CG) lightning flash ratio for a given grid square is computed according to Rutledge et al. [1992]:
IC=CG¼ 2:7 F0:5 ð3Þ
[12] Price et al. [1997] combined their estimates of
energy per flash with a selected best estimate of NO production per unit energy from available literature to obtain the NO production per flash for CG and IC lightning. Using the Price et al. [1997] values of 6.7 1026molecules
NO per CG flash and 6.7 1025molecules NO per IC flash, the NO yield, SNO, in a convective cloud for a given grid
square is computed as the following expression:
SNO¼ 6:7 1026F PICþ 6:7 1025F PCG ð4Þ
Figure 1. Average wind speed (a) and humidity (b) at 500 mbar and cloud cover (c) for July 1 – 15, 1995.
where PICand PCGare the weighting factors for IC and CG
clouds, respectively, and can be obtained with equation (3), and SNOis in unit of molecules NO per minute. Pickering et
al. [1998] derived the average profiles of lightning NOx(N)
mass in percent for the midlatitude continental, tropical marine and tropical continental conditions, respectively.
[13] The vertical distribution of NO production in the
model is scaled on the Pickering et al. [1998] profiles using the convective cloud-top height for a specific condition. Therefore, the lightning NOx source in the present
simu-lations is temporal and spatial variant and thus local dependent. Figure 2c shows the geographic distribution of the lightning NOxsource for the simulation period July 1 –
15, 1995.
[14] Stratosphere-troposphere exchange (STE) is also an
important source of NOxin the upper troposphere. Figure
2d shows the distribution of stratospheric NOx source for
the simulation period. In our model, the upper boundary condition for NOx is given by fixing its concentration,
instead of flux, at the tropopause. The negative value indicates the transport from the troposphere to the strato-sphere over the region. Table 1 summarizes the NOx
source strength over the model domain for the simulation period.
2.3. Chemistry
[15] The chemical gas-phase mechanism was initially
developed by Stockwell [1986] and modified by Ma et al. [2000]. For this study, acetone is also included as an addi-tional tracer because its import role as an HOx(= OH + HO2)
reservoir in the upper troposphere [Singh et al., 1995; Jaegle´ et al., 1997]. Following the approach of Zimmermann and Poppe [1996], a condensed chemical mechanism for
iso-Table 1. NOxSources and Anthropogenic NMHC Emissions in
the Model Domain
Sources Rate Surface NMHC (GgC/day) 39.63 Surface NOx(GgN/day) 17.48 Aviation NOx(GgN/day) 0.05 Lightning NOx(GgN/day) 2.62 Stratosphere NOx(GgN/day) 0.32 Total NOx(GgN/day)a 20.47 a
Horizontal transport from outside is not accounted for.
Figure 2. Source rates of NOxfrom surface emission (a), aviation (b), lightning (c), and stratosphere (d)
prene oxidation has been incorporated in the model with rate constants based on Madronich and Calvert [1990]. Hetero-geneous reactions of N2O5 and NO3, as well as those of
methylacrolein and methylvinylketone, on sulfate aerosols are parameterized using the empirical first-order loss rate used in IMAGES [Mu¨ller and Brasseur, 1995]. Aqueous phase HOxchemistry in clouds is not included in the present
model since its effects on O3and NOxappear to be
insig-nificant [Liang and Jacob, 1997]. Photolysis rates above and below clouds are corrected on-line in the model with the parameterization described by Chang et al. [1987]. Photol-ysis rates in clouds are obtained by interpolating the cloud top and bottom heights linearly with altitudes according to the studies of Madronich [1987] and Van Weele and Duyn-kerke [1993].
[16] The initial fields and lateral boundary conditions for
chemical tracers SO2, H2SO4 and aromatics in the model
are the same as the ones used by Luo et al. [2000]. However, in this study, the initial fields and lateral boun-dary conditions for O3 and other chemical tracers are
provided by the simulated results of a global chemical transport model for ozone and related chemical tracers (MOZART) [Brasseur et al., 1998; Hauglustaine et al., 1998]. The meteorological information for MOZART is supplied from the NCAR Community Climate Model (CCM) [Brasseur et al., 1998]. The simulated distributions of O3and its precursors by MOZART were evaluated by
comparisons with observational data including those over the western Pacific near the eastern coast of China [Hau-glustaine et al., 1998; Mauzerall et al., 2000]. MOZART can generally reproduce the distributions of many trace species, except for PAN in the upper troposphere over the biomass burning regions and O3at high latitudes above 300
mbar. Because of a different grid resolution between the RADM and MOZART (2.8latitude by 2.8longitude, 25 levels from the surface to the level of 3 mbar), the MOZART output data were interpolated into the grid cells of RADM.
[17] Ozone and nitrogen compounds (mainly NOx and
HNO3), formed in the stratosphere through a series of
chemical and photochemical reactions, may be transported to the troposphere across the extratropical tropopause [Hol-ton et al., 1995]. These stratospheric intrusions into the troposphere provide an import source of O3and NOx for
the troposphere. Stratospheric chemistry is not included in the present model, and discrepancies or shortcomings still exist in the MOZART results in this region. Therefore, observed concentrations of O3, NOx, and HNO3 in the
tropopause region derived from other parts of the world are used as the upper boundary conditions in RADM. Table 2 summarizes these O3, NOx, and HNO3 concentrations
used in RADM based on Logan [1999], Emmons et al. [1997] and Schneider et al. [1998], respectively. In the present work, we use the thermal definition of the tropo-pause, which is based on the vertical lapse rate and is given as the lowest point above 500 mbar where the lapse rate is less than 2C per kilometer for 2 km [Holton et al., 1995]. The tropopause altitude is computed on-line in the model. While the tropopause altitude varies temporally and spa-tially in the simulations, the concentrations of O3, NOx, and
HNO3at the levels of the tropopause and above are always
fixed at constant values.
[18] The stratosphere-troposphere exchange (STE) of a
species, e.g., O3, at the tropopause is calculated as the
following
MO3¼ Mð d MTuÞ
O½ 3 T Mð u MTdÞ O½ 3 BþMK O½ 3 T O½ 3 B
ð5Þ
where MO3is the net mass flux of O3transported across the
tropopause; Mdand Mu, MTuand MTd, and MKaccount for
the mass flux acting function for O3due to downward and
upward advection, tropopause up and down, and eddy diffusion, respectively; and [O3]Tand [O3]Brefer to the O3
mixing ratio at the tropopause and at the highest grid level below the tropopause, respectively. When wind comes down (Md> 0, Mu= 0), O3(at a value of [O3]T) is transported from
the stratosphere to the troposphere. When wind goes up (Mu
> 0, Md= 0), O3(at a value of [O3]B) is transported from the
troposphere to the stratosphere. When the tropopause shifts upwards (MTu > 0, MTd = 0), O3(at a value of [O3]T) is
transported from the stratosphere to the troposphere. When the tropopause shifts downwards (MTd> 0, MTu= 0), O3(at
a value of [O3]B) is transported from the troposphere to the
stratosphere. Therefore, the terms (Md MTu) and (Mu
MTd) can be considered as downward and upward advection
relative to the tropopause, respectively. In the present simulation there is a frequency of 11% by which the tropopause shifts hourly from one altitude to another for all model cells. Diffusion MK transports O3 from the
strato-sphere to the tropostrato-sphere since [O3]Tis typically larger than
[O3]B. It should be noted that a fixed O3concentration was
used for the tropopause in the present study. The STE treatment in our model would be improved in the future when we have observational data for the China region or stratosphere-troposphere coupled model results.
[19] Figure 3a shows the tropopause height over the
model domain averaged over the simulation period July 1 – 15, 1995. The dynamic definition of the tropopause is based on a value of potential vorticity (PV) [Holton, 1992]. There is no single accepted value for the tropopause, and accepted values for the PV tropopause range from 1.6 to 3.5 pvu (1 pvu = PV Units = 106 K m2 kg1 s1) [WMO, 1986]. The tropopause PV over the model domain is typically in a range of 1.5 – 4.5 pvu for the simulation period as indicated in Figure 3b. Higher PV values at 30 – 45N latitude band are due to the different latitudinal gradients between the thermal and dynamical tropopauses around subtropical summertime break in the summer hemi-sphere. Roelofs and Lelieveld [2000] showed that there is no apparent linear dependence between ozone and PV below 3 pvu and, for PV larger than 4 pvu, ozone appears to increase linearly with PV with a substantial scatter. It is indicated that the correlation between ozone and PV is not so significant in the upper troposphere as in the lower
Table 2. Tropopause O3, NOx, and HNO3 Concentrations
Imposed in the Model
Species Concentration
O3 120 ppbv
NOx 300 pptv
stratosphere. Figure 4 shows the scatterplot of model-simulated ozone versus PV at 300 mbar for July 1 – 15, 1995. It can be seen that stratospheric air masses have less influence in the upper troposphere below 30N. The corre-lation between ozone and PV at different latitudes, although weak, implies that the dynamic characteristic of strato-spheric ozone intrusion is reproduced in our model.
3. Model Evaluation
[20] In the studies of Yang et al. [1999] and Luo et al.
[2000], the earlier version of the model was evaluated using surface observational data at the three World Meteorological Organization (WMO) Global Watch Atmosphere (GAW) stations in China. The diurnal variation and pollution epi-sodes of ozone were reproduced, although the difference in magnitude between simulated and observed ozone exited. For this study, as described in the above section, we have modified the model to extend its ability of simulating photo-chemical processes in the free troposphere. In addition, the simulated period in the present study is different from those of Yang et al. [1999] and Luo et al. [2000]. Therefore, in this section we evaluate the present model by comparing its
results with surface measurements in China, ozone soundings near China, and MOZART results for China.
3.1. Surface Measurements
[21] Table 3 describes briefly the characteristics of the
three WMO/GAW stations in China. Measurements of sur-face ozone concentration were made at the three WMO/ GAW sites in the Chinese Ozone Research Program (CORP) [Zhou, 1996]. These observational data were used to study the nonurban ozone episode over eastern China during the fall and early winter [Luo et al., 2000]. On the other hand, measurements of ozone precursors CO, NMHCs and NOxare quite sparse. For NO, only data at Lin’an are
available for the simulation period. Figure 5 shows the observed and calculated diurnal cycles of surface NO concentration at the Lin’an station for July 1 – 15, 1995. Because of its short lifetime, the NO concentration is rather sensitive to the NO local emissions and in situ chemistry. The peak of NO in the afternoon is apparently due to
Figure 3. Average tropopause height (a) and PV values (b) for July 1 – 15, 1995.
Figure 4. Scatterplot of simulated O3 versus PV at 300
increased photolysis of NO2in the sunlight that favors the
NO2-to-NO conversion. At night, when the photolysis of
NO2is neglected, almost all NOxexist in the form of NO2
due to the reaction of NO with O3. The daily variation
pattern of NO is generally well simulated, but the observed NO concentration tends to be underestimated at night. The overestimation cannot be explained by emissions since the nighttime emissions are included in the model. Possibly some heterogeneous processes converting NO back from HNO3may have occurred but not included in the model.
[22] Figure 6 shows the diurnal cycle of simulated and
observed O3 concentrations during the period July 1 – 15,
1995. Because of the coarse grid resolution of the model, it is not suitable to choose the O3concentration simulated in
the lowest model layer to compare with the observation at the Waliguan station, which is located in an isolated mountain. Instead, we use simulated O3 concentrations of
the fifth model layer to match the height of the Waliguan site. The average daily variation of O3at Waliguan is very
small, implying that photochemistry is not a determining factor of O3 variation there. At the Lin’an and Longfeng
stations, the O3concentration is low at night, increases from
early morning, and reaches its maximum in the late after-noon, indicating that the ozone variation is photochemically controlled. The diurnal variation of NO at Lin’an (Figure 5) also shows that there are high chemical activities initiated by sunlight in the afternoon, which favor the production of ozone. The diurnal variation of O3 at the three stations is
generally well reproduced by the model. However, the model underestimates the observed O3 concentration at
Waliguan. Yang et al. [1999] simulated the same diurnal variation for O3at Waliguan for a 3-day period of August
14 – 16, 1994. Waliguan is just located at the northeastern corner of the Tibetan plateau, where the process of cloud convection may not be well simulated in the model. More-over, the western boundary condition of ozone can also influence the simulated results [Ma et al., 2002b]. The model underestimates the nighttime O3 concentration at
Longfeng, possibly due to an underestimate of O3
trans-ported to the region. The source attribution of diurnal O3
variation is presented in a companion paper [see Ma et al., 2002b, Figure 2].
3.2. Ozone Soundings
[23] Few aircraft measurements or soundings were made
over China, making it difficult to test our model results. In this study, we use ozone soundings at the two Japanese sites, Naha (26.2N, 127.7E) and Kagoshima (31.6N, 130.6E), to compare with the model (by ftp://www.msc-smc.ec.gc.ca/woudc). These two sounding sites are very close to, although not within, the eastern boundary of the model domain. Therefore, these soundings may provide important information for ozone, in particular, in the free troposphere. Figure 7 presents simulated O3distributions at
a longitudinal slice of 124E for the time of 06:00 UTC on
July 5, 7 and 12, respectively. The soundings data obtained at the same time over Naha and Kagoshima are also shown at the corresponding latitudes. The sounding data are available on July 5 at Naha, on July 7 at Kagoshima, and on July 12 at both Naha and Kagoshima. For July 5 and 7, low O3in the upper troposphere (less than 70 ppbv) over the
two sounding sites is well simulated by the model. The model also reproduces the peak O3at 300 mbar on July 7
over Kagoshima. Although the peak value is underesti-mated, it is illustrated that this peak is caused by STE at lower latitudes. For July 12, high O3 in the upper
tropo-sphere over the two sounding sites is poor simulated by the model. Even though the model reproduces the maximum O3
near the tropopause over Kagoshima, the observed peak O3
of 240 ppbv over Naha at 200 mbar is not simulated. The observed O3 peak may be caused by tropopause folding
occurring out of the model domain. It cannot be simulated in the present study as O3concentration in the stratosphere
is fixed at 120 ppbv. For all days the model overestimates surface O3at the same latitudes where Naha and Kagoshima
are located. This seems reasonable considering that the two sounding sites are further away from the continental coast. It is noted that higher levels of O3 are simulated at higher
latitudes north of 35 – 40N through the free troposphere. Such ozone distribution is associated with the Asian Mon-soon circulation in summertime (Figure 1). While STE is a dominant contributor to enhanced O3 levels at higher
latitudes, anthropogenic emissions followed by photochem-istry also makes a contribution [Ma et al., 2002b].
3.3. MOZART Results
[24] As described in section 2.3, MOZART provides the
initial and lateral boundary conditions of chemical species
Figure 5. Observed (circles) and calculated (solid line) mean diurnal cycles of surface NO concentration at the Lin’an station for July 1 – 15, 1995. Bars and dashed lines give the standard deviations for the observed and calculated values, respectively.
Table 3. The Three WMO GAW Stations in China
Waliguan Lin’an Longfeng
Characteristic Global baseline station Regional background station Regional background station Location 36180N, 100540E 30250N, 119440E 44440N, 127360E
Altitude (m a.s.l.) 3816 132 331
for our model runs. A comparison with MOZART is a useful method for the evaluation of the model (denoted as RADM) used in the present study. It should be noted that the period-mean values are compared considering that MOZART, which is driven by CCM, provides climatic information. Figure 8 presents the simulated averaged con-centration of NOxat 250 mbar, 500 mbar and at the surface
for July 1 – 15, 1995 by RADM and MOZART, respectively. Because the lifetime of NOxis of the order of a few days or
less, maximum NOxconcentrations are found in the
boun-dary layer of eastern China where most surface NOxsources
are located. The NOxconcentration decreases towards
west-ern China, reaching its minimum values on the Tibetan plateau. RADM and MOZART are generally in good agree-ment in simulating the distribution of NOxon the ground,
except that MOZART tends to overestimates NOxin remote
areas. In the free troposphere, the distribution of NOx is
quite uniform according to MOZART. However, RADM tends to underestimate NOx, possibly due to higher
humid-ity. An effective sink of NOx is the reaction of NO2with
OH, and OH is produced through the photolysis of ozone in the presence of water vapor. An interesting feature is that for
Figure 6. Observed (circles) and calculated (solid line) mean diurnal cycles of O3 at the WMO/GAW stations of
Waliguan (a), Lin’an (b) and Longfeng (c) for July 1 – 15, 1995. Bars and dashed lines give the standard deviations for the observed and calculated values, respectively.
Figure 7. Simulated O3distributions at a longitudinal slice
of 124E for the time of 06:00 UTC on July 5 (a), July 7 (b) and July 12 (c). Denoted numbers are O3soundings at the
two Japanese sites, Naha (26.2N, 127.7E) and Kagoshima (31.6N, 130.6E). Bright solid lines represent the thermal tropopause, and bright dotted lines the contour of 1.5 pvu.
Figure 8. Simulated averaged concentrations of NOxat 250 mbar (panels a1 and b1), 500 mbar (panels a2
and b2) and at the surface (panels a3 and b3) for July 1 – 15, 1995 by RADM (panels a1 – a3) and MOZART (panels b1 – b3). The three WMO/GAW stations are denoted by A, Waliguan; B, Lin’an; and C, Longfeng.
RADM maximum NOxconcentrations over eastern China
are also found at 500 mbar. This indicates that, with comparison to MOZART, RADM is more efficient in simulating the large-scale vertical transport of pollutants, at least for the simulation period.
[25] Figure 9 shows the net ozone production calculated
by RADM for July 1 – 15, 1995 in the boundary layer and free troposphere, respectively. The largest net ozone produc-tion is calculated in the polluted regions of eastern China, with maximum values of about 10 – 15 ppbv day1. Over the Tibetan plateau, net ozone destruction reaching 5 ppbv day1 is calculated, in agreement with the analysis of observational data measured at the Waliguan station [Ma et al., 2002a]. The net production of ozone in the boundary layer calculated by MOZART, as shown by Mauzerall et al. [2000], is more homogeneously distributed. They seemed to overestimate net production of ozone in rural and remote
areas. For example, a net ozone production of 5 ppbv day1 was calculated by Mauzerall et al. [2000] for the western and middle parts of China. In the free troposphere, net ozone production is also calculated in RADM over extremely polluted regions of eastern China, as a result of ozone precursors transported from the boundary layer. Deep con-vection plays an important role in the vertical redistribution of ozone and its precursors such as NOxin the troposphere
[Lelieveld and Crutzen, 1994]. Figure 10 presents the vertical redistribution of ozone caused by cloud convection in RADM for July 1 – 15, 1995 in the boundary layer and free troposphere, respectively. Over the Tibetan plateau, cloud convection is very efficient in transporting O3 from
the upper troposphere or lower stratosphere down to the boundary layer near the surface, enhancing surface ozone concentrations there. In this region the convection of ozone is typically 10 – 20 ppbv day1, with maximum value reach-Figure 9. Averaged daily net production of O3in the free troposphere (a) and in the boundary layer (b)
for July 1 – 15, 1995 calculated by RADM.
Figure 10. Averaged daily convection of O3in the free troposphere (a) and in the boundary layer (b) for
Figure 11. Simulated averaged concentrations of O3at 250 mbar (panels a1 and b1), 500 mbar (panels
a2 and b2) and at the surface (panels a3 and b3) for July 1 – 15, 1995 by RADM (panels a1 – a3) and MOZART (panels b1 – b3). The three WMO/GAW stations are denoted by A, Waliguan; B, Lin’an; and C, Longfeng.
ing approximately 40 – 50 ppbv day1. Therefore, it is indicated that surface ozone is controlled dominantly by photochemistry in eastern China and is determined primarily by cloud convection and synoptic transport in western China.
[26] Figure 11 presents the simulated averaged
concen-tration of O3at 250 mbar, 500 mbar and at the surface for
July 1 – 15, 1995 by RADM and MOZART, respectively. Both RADM and MOZART simulate high surface O3in the
middle part of eastern China, where pollutant emission rates are high, and on the Tibetan plateau, where altitudes fall in the middle troposphere. Highest concentrations of 40 – 50 ppbv are simulated with RADM in the downwind regions of plumes over the eastern China Sea, while maximum values are found close to the emission source in the continent according to MOZART. The surface O3 concentration in
southeastern China is generally below 20 ppbv as simulated by RADM, but is generally above 30 ppbv by MOZART. Asian Monsoon brings low ozone and wet air masses from the oceans to southern China. Moreover, associated rainy and cloudy weather conditions are not in favor of photo-chemical ozone production there. These are the cause of lower ozone in southeastern China simulated by RADM. The distribution of ozone at 500 mbar and 250 mbar simulated by RADM is also consistent with the Asian Monsoon circulation shown in Figure 1.
4. Conclusions
[27] We have presented a regional chemical transport
model, which provides the three-dimensional distribution and budget of ozone and its precursors in the troposphere over China. The model is extended from RADM [Chang et al., 1987] by updating surface emissions of NOx, CO and
nonmethane hydrocarbons, incorporating aircraft emissions and lightning NOxsources, adding the oxidation mechanism
of isoprene and acetone, treating STE of O3, NOx and
HNO3at the thermal tropopause. The model has been run
for a summertime period July 1 – 15, 1995, with chemical boundary conditions constrained by MOZART [Brasseur et al., 1998; Hauglustaine et al., 1998].
[28] The simulation period, prevailing with Asian
Mon-soon, is representative of summertime meteorological con-ditions over China. The model is evaluated by comparing surface measurements of ozone and its precursors in China, ozone soundings in Japan, and MOZART results for the China region. The results show that the model is able to simulate the distributions of ozone and other important species in the troposphere over China with fair agreement. Surface ozone is controlled by photochemistry in eastern China, and is determined primarily by cloud convection and synoptic transport in western China. Because of the Asian Monsoon Circulation, the surface O3concentration is
gen-erally below 20 ppbv in or near the tropics of southern China, and 20 – 30 ppbv at midlatitudes of eastern China. Highest concentrations of 40 – 50 ppbv are simulated in the downwind regions of plumes to the eastern China Sea.
[29] Most sources of ozone precursors NMHCs, CO and
NOxin our model are generally emitted near the surface in
the boundary layer of eastern China. We find that the concentration of NOx at 500 mbar over eastern China is
still higher in relative to those at the same altitudes over
other regions, implying that surface-emitted NOxis diluted
from the boundary layer to the free troposphere. The role of the vertical redistribution played by convective and synoptic transport on ozone precursors is also visible in the upper troposphere. The signature of O3 transported from the
polluted boundary layer to the free troposphere is smoothed out, since O3has a longer lifetime and is also transported
from the stratosphere. The contributions of various sources to the NOx and ozone concentrations in the troposphere
over China will be quantified in a companion paper [Ma et al., 2002b].
[30] Acknowledgments. This work was supported by the Natural Science Foundation of China (NSFC) under project 49805008. J. Ma was also supported by the Research Starting Foundation of the State Personnel Ministry and the State Education Ministry. We would like to thank the two anonymous reviewers for their helpful comments on the manuscript.
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