Quantitative Studies By Observations And Simulations On The Impact Of Meteorological Conditions On The Changed Aerosol Pollution In Winter In China

Heavy aerosol pollution usually occurs in the autumn or winter in China,especially in winter,pollutant emissions can be regarded as unchanged,but why does aerosol pollution occur again and again and heavy pollution episodes lasting over three days occur frequently?The role of unfavorable meteorological conditions is worth studying.Since the Chinese government issued the"Atmospheric Pollution Prevention Action Plan"in 2013,also known as“Ten Statements of Atmosphere”,China's air quality has improved year by year through efforts of emission reductions during several years.In the winter of 2016 and 2017,PM_2.5 concentration reached the lowest value since 2013 in Central and Eastern China.However,under such large reductions in emission,did heavy pollution episodes continue to occur in Central and Eastern China in the winter of 2016 and 2017,and did similar phenomenon occur in Western China where pollutant emissions are significantly lower?What was the role of unfavorable meteorological conditions in heavy pollution episodes?What was the degree of interaction between PM_2.5concentration and unfavorable meteorological conditions?This is an important issue which the government and the people pay special attentions to.Four sensitivity experiments based on the Global-Regional Assimilation and Prediction System coupled with the Chinese Unified Atmospheric Chemistry Environment?GRAPES-CUACE?model,together with PM_2.5concentration,observed meteorological elements,ECMWF and NCEP reanalysis data,radiation data,emission inventories in 2016 and 2017,and PLAM are used to evaluate the contributions of meteorology to the substantial reductions of PM_2.5 concentration in Central and Eastern China with relatively higher pollutant emissions and the changes of PM_2.5 concentration in Northwest China with relatively lower pollutant emissions in winter in 2016.The results show that,in the entire Central and Eastern China,the meteorological conditions improved and pollutant emissions reduced in December 2017,compared with December 2016,which were both conducive to the reductions of PM_2.5 concentration.The more favorable meteorological conditions contributed 22%of the total decrease of PM_2.5 concentration,indicating that the emission reductions were still the dominant factors of the improvement of overall aerosol pollution.In different cities in Central and Eastern China,changes of meteorological conditions contributed 12-51%of the total decrease of PM_2.5 concentration,which was due to topography and geographical locations.The contributions of meteorological conditions in Beijing?51%?were greater than those in other cities,which directly led to the most substantial decrease in PM_2.5 concentration.In addition,during different pollution periods,PM_2.5 concentration also showed a downward trend,and changes of meteorological conditions contributed 98%,28%and19%of the total decrease during clean periods,light pollution periods and heavy pollution periods,respectively.In the Northwest region with relatively lower pollutant emissions in winter in 2016 when the pollutant emissions changed a little,the stable zonal westerly airflow or high-pressure ridge often appeared at 500 h Pa,accompanied with lower boundary layers,which caused accumulation of PM_2.5 concentration;when PM_2.5 accumulates to a certain extent(above75?g m^-3 in Lanzhou and 100?g m^-3 in Urumqi),it would create or strengthen inversion,lead to humidification near the surface and worsen other meteorological conditions;then the worsened meteorological conditions could lead to a further increase of PM_2.5 concentration or“explosive growth”;the worsened meteorological conditions explained 50-85%of the explosive growth of PM_2.5 by using PLAM.Although emission is still the main reason of PM_2.5concentration changes in China,PM_2.5 concentration changes are not decoupled from unfavorable meteorological conditions,which will still lead to accumulation of PM_2.5,in the Western China where the basic emissions are not high.When PM_2.5 accumulated to a certain extent,the sustainability of aerosol pollution will be strengthened.These results show that emission reductions are the necessary measures to reduce aerosol pollution.However,when the degree of emission reductions can't decouple PM_2.5 from unfavorable meteorological conditions,the unfavorable meteorological conditions are still necessary external conditions of the formation of aerosol pollution in winter;even under favorable meteorological conditions,it is not advisable to allow unrestricted discharge of pollution,because the increase of pollutant will significantly affect the meteorological conditions in the boundary layer and form a"vicious circle"of interaction between pollution and meteorological conditions.