Assessment Of The Impacts Of Cloud Chemistry On Sulfate And Its Precursor Gases In Typical Polluted Regions In China

Cloud chemistry processes are an important pathway for secondary generation of atmospheric aerosols and affect the spatial and temporal distribution of gases and aerosols in the atmosphere.However,the influence of cloud chemistry in China’s heavy pollution weather and different seasons is not well studied,and there are a few models assessing the contribution of cloud chemistry in China,especially the quantitative assessment of cloud chemistry on sulfate,SO₂and its key oxidants in China’s regions and different seasons.Therefore,a regional online chemical weather model,Weather Research and Forecasting（WRF）/China Meteorological Administration Unified Atmospheric Chemistry Environment（CUACE）,is used to assess the contributions of cloud chemistry to the SO2 and sulfate levels in typical regions of China.The cloud chemistry of CUACE includes two oxidation mechanisms,O₃and H₂O₂,for the conversion of SO₂to sulfate.In this study,multiple consecutive time series containing complete data on SO₂,H₂O₂,O₃,and sulfate in cloud chemistry observations with clouds from June 24,2015 to August 7,2015 and June 25,2018 to July 28,2018 at Mount Tai,Shandong,China,are first selected for comparison with the corresponding these four elements of cloud chemistry in WRF/CUACE as a way to verify and evaluate the CUACE cloud chemistry mechanism.It is found that the CUACE cloud chemistry mechanism can reproduce the cloud consumption of H₂O₂,O₃,and SO₂as well as sulfate production,and thus can be well used to assess the contribution of cloud chemistry to SO₂and sulfate in typical regions of China.In this study,a typical heavy pollution episode from December 16 to 21,2016 is selected to analyze the characteristics of the regional contribution of cloud chemistry in typical regions of China.It is found that cloud chemistry contributes significantly to SO₂consumption and sulfate production in east-central China relative to the absence of cloud chemistry effects.Sulfate production increases by 60-95%and SO₂decreases by more than 80%during cloudy in heavy pollution events.Among the four typical polluted regions of the North China Plain,Yangtze River Delta,Pearl River Delta,and Sichuan Basin,the impact of cloud chemistry is the largest in the Sichuan Basin,with an average reduction of SO₂of about 1.0-15.0 ppb and an increase of sulfate of about 10.0-70.0μg/m³,followed by the Yangtze River Delta with a reduction of SO₂of about 1.0-3.0 ppb and an increase of sulfate of about 10.0-30.0μg/m³.However,the cloud chemistry contribution is not significant in the PRD due to lighter pollution and in the North China Plain due to less water vapor.It is also found that cloud chemistry mainly occurs in the lower and middle troposphere below 5 km and in the boundary layer.In addition,the consumption of SO₂and the increase of sulfate during the pollution episode are both higher than the monthly average in cloud chemistry.In this study,January,April,July,and October 2017 are selected to represent winter,spring,summer,and autumn to investigate the seasonal regional effects of cloud chemistry on the concentrations of two oxidants,H₂O₂and O₃,SO₂,and sulfate aerosol.It is found that H₂O₂,the main oxidant in cloud chemical processes,consumes a maximum of 1.0-2.5 ppb in the Sichuan Basin and southeastern North China Plain in summer.Winter,spring and autumn in the Sichuan Basin,summer in the Yangtze River Delta,and spring and autumn in the Pearl River Delta are also regions with high values of H₂O₂consumption at 1.0-1.5 ppb.Cloud chemical processes consume 0.0-1.0 ppb of O₃in most of the Middle East of China,with relatively high consumption in autumn and winter.Sulfate increase and SO₂consumption do not vary much in all seasons,with SO₂consumption ranging from 0.5to 3.0 ppb and sulfate increase 3.0-30.0μg/m³in the four major pollution regions.The distribution of sulfate increase region,SO₂consumption region and H₂O₂consumption region basically coincide.The Sichuan basin is a region of high value of increase or consumption in all seasons and more in summer.In addition,due to the high SO₂concentration in winter,although H₂O₂consumption is less,SO₂consumption is still relatively high,which makes the increase of sulfate also more.The influence of cloud chemistry is small in winter,spring and autumn in Beijing and its northwestern region in the North China Plain,which is the same conclusion as in typical individual cases of heavy pollution.Cloud chemistry has a significant effect in summer in the whole North China Plain region,which also suggests that the low effect in winter,spring and autumn in the northwestern part of the North China Plain is due to the low ambient liquid water content,which makes the cloud-water environment unsupportive for cloud chemistry processes to occur.