Anthropogenic Sulfate Aerosols in China: Distribution and Implied Climate Impacts. Guest contribution. Y. Zhang''*, Q.X. Gao\ Z.H. Ren^ and Y.H.

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1 Anthropogenic Sulfate Aerosols in China: Distribution and Implied Climate Impacts Guest contribution Y. Zhang''*, Q.X. Gao\ Z.H. Ren^ and Y.H. Ding' ^Institute For Atmospheric Physics, GKSS, Germany National Climate Center, Beijing, China Climate Change Research Center, Beijing, China Introduction Combustion of coal, which is the major energy source in China, is one of the most important anthropogenic processes to produce SC^. Chen et al (1995) estimated that production of SO: that is the precursor of sulfate aerosols is twice as high as that of NOx by the coal combustion in China. It was indicated also in their work, that the percentage of 864 is much higher than that of NOx in the rainfall in China. Therefore, the dominant chemical component of anthropogenic aerosols from coal combustion in China is sulfate. Previous researche revealed that sulfate aerosols have an important radiative impact on the climate, due to their strong scattering ability (e.g. Hegerl et al., 1996). In this work, preliminary results about seasonal distributions of anthropogenic sulfate aerosols in China for the year 2020 under 'Business As Usual (BAU)' scenario have been obtained using a tropospheric chemical transportation model. Further, sulfate aerosol radiative forcing, a perturbation of radiative balance at the top of the atmosphere caused by a change of the aerosols, has been calculated using a two-steam radiative transfer model, based on distributions of the aerosol concentration, surface albedo, solar zenith angle and atmospheric parameter profiles. Models Tropospheric chemical transportation model The chemistry transport model used for the estimation was developed and described by Jiang et al. (1997), in which physical and chemical processes such as dry and wet depositions, gas-to-particle transition, dynamic transportation are involved. For the calculation of aerosol horizontal transportation, the typical patterns in lower tropospheric wind field in China were analysed and classified. Proceedings ofeurotrac Symposium '98 Editors: P.M. Borrell and P. Borrell 1999: WITPRESS, Southampton

2 556 Y. Zhang et al The criterion of the pattern classification is the similarity of the main streams in 2/3 of the area of a concerned region. Using these wind field patterns and their frequencies, distribution of the aerosol concentration by the horizontal wind transportation was estimated. The grids for the modelling are in 0.5 x 0.5. Aerosol-radiation model The aerosol-radiation model used here is based on the work of Kerschgens et al (1978), with a two-stream transfer scheme. In the solar wavelength range between 0.2 and jam, radiation transfer is calculated in 50 intervals. The radiation fluxes were calculated for the atmosphere up to 50 km. Scattering of the aerosols was taken into account. The optical parameters, extinction parameter, single scattering albedo and asymmetry factor of sulfate particles were computed using the Mie calculation method. Spatial distribution of seasonal sulfate aerosol concentration anomalies in China Anthropogenic sulfate aerosol concentration anomalies (the seasonal mean minus the annual mean) in the China region (18N-45N, 75E-123E) were calculated for the year of 2020 given by the chemical transportation modelling (Jiang etal, 1997). Fig. 1 shows the seasonal distributions of near-ground-surface sulfate concentration anomalies estimated for the year of 2020 under a 'Business As Usual' scenario. The concentration of aerosols would be much higher by using a BAU scenario than that by using any restrained emission scenario. The sulfate aerosols near the ground are concentrated in the eastern region of China in all seasons since the majority of the emission sources are located in this region. Owing to the monsoon climate controlling the eastern China region, there are distinct seasonal differences in climatological patterns of the wind fields in this region, i.e. northerly winds in winter and southerly winds in summer. The sulfate concentration has also very distinct seasonal changes due to the monsoon climate of the region. It can be seen in Fig. 1 that the concentration of the aerosols is higher in summer than in winter above the northeastern part of the continent, and is lower above the coastal region in southeastern China, the East China Sea and the South China Sea. Sulfate aerosol radiative forcing in China Radiative forcing of anthropogenic sulfate aerosols, which expresses potential climate impacts of the aerosols, was calculated based on the distribution of the aerosol concentration, ground surface albedo, solar zenith angle and atmospheric parameter profiles (shown in Fig. 2). The data of surface albedo

3 Sulfate Aerosols in China 557 were taken from Global Data Sets for Land-Atmosphere Models, ISLSCP Initiative I (Meeson et 0/., 1995; Sellers et a/., 1995). In general, radiative forcing by the sulfate aerosols is negative that implies a 'cooling effect' of the aerosols in most areas of China, except in snow covered northern areas during winter which exhibit higher surface albedo. Sulfate Mass Concentration Anomaly (Summer) (microgr m-3) 20N Sulfate Mass Concentration Anomaly (Winter) (microgr m-3) Fig. 1: Winter (lower) and summer (upper) sulfate aerosol concentration anomalies (the seasonal mean minus the annual mean) near the ground surface (in p,gm^) for the year 2020 under a 'Business As Usual' scenario.

4 558 Y. Zhang et al. 45N January Mean Sulfate Radiative Forcing (Wm-2) 40N 35N 30N 25N 20N 45N July Mean Sulfate Radiative Forcing (Wm-2) 40 N 35N- 30 N 25N 20N Fig. 2: January (upper) and July (lower) average distribution of sulfate aerosol radiative forcing for the year 2020 under a 'Business As Usual' scenario. Conclusions 1. Sulfate aerosols near the ground surface are concentrated in the eastern region of China 2. Owing to the Monsoon climate, the concentration of sulfate aerosols above the northeastern part of the continent is higher in summer than in winter, but is lower above the southeastern coastal region, the East China Sea and the South China Sea regions.

5 Sulfate Aerosols in China The sulfate aerosol radiative forcing depends on the ground albedo, solar zenith angle as well as on the aerosol concentration. The sulfate aerosol radiative forcing is negative in most areas of China, except for a small region in the north of China which is covered with ice and snow and has a high surface albedo in winter. References Chen, F. and F.H. Chai; (in Chinese). Research ofenvironmental Science. 10 (1997) Hegerl, G.C., K. Hasselmann, U. Cubasch, J.F.B. Mitchell, E. Roceckner, R. Voss and J. Waszkewitz; MPIfuerMeteorologie, Report 27 (1996) 70pp. Jiang, Z.Y., Q.X. Gao, S.S. Liu and Z.H. Ren; (in Chinese). Research of Environmental Science. 10(1997) Kerschgens, M, U. Pilz and E. Raschke; Tellus, 30 (1978) Meeson, B.W., F.E. Corprew, J.M.P. McManus, D.M. Myers, J.W. Closs, K.-J. Sun, D.J. Sunday, P.J. Sellers; Published on CD by NASA, 1-5 (1995). Sellers, P.J., B.W. Meeson, J. Closs, J. Collate, F. Corprew, D. Dazlich, F. G. Hall, Y. Kerr, R. Koster, S. Los, K. Mitchell, J. McManus, D. Myers, K. -J. Sun, P. Try;. Published on CD by NASA, 1 (1995).