| The Chinese government introduced in 2013 the Clean Air Action to aggressively control anthropogenic emissions,and emission reduction rates markedly accelerated since then.During the same time,annual mean PM2.5 across China show a general 30-50%decrease over the 2013-2018 period.Concurrent 2013-2018 observations of sulfur dioxide(SO2)and CO show that the declines in PM2.5 are qualitatively consistent with drastic controls of emissions from coal combustion.However,there is also a large meteorologically driven interannual variability of PM2.5 that complicates trend attribution.Secondary inorganic aerosols(sulfate,nitrate and ammonium)constitute more than half of China PM2.5 mass concentrations,and they together show the steepest decline among PM2.5 compositions.The formation of sulfate-nitrate-ammonium from their gas precursors are governed by strong chemical feedbacks.Understanding the relationship between secondary inorganic aerosols and their gas precursors is of critical importance.In the overall improving air quality in China,severe PM2.5pollution episodes still happen frequently.It is a major issue to determine critical emission source regions and time periods.Based on the above background,by combining multi-source data statistical analysis and numerical models,this study quantitatively evaluated the impacts of meteorology variations and emission controls on multi-timescale(interannual,seasonal and pollution episodic)changes of China PM2.5 and its compositions.Three main parts are:In the first part of this research,we used a stepwise multiple linear regression(MLR)model to quantify meteorology contribution to the PM2.5 trends across China.The MLR model correlates the 10-day PM2.5 anomalies to near surface wind speed,precipitation,relative humidity,temperature,and 850 h Pa meridional wind velocity(V850).We find that meteorology made a minor but significant contribution to the observed 2013-2018 PM2.5trends across China and that removing this influence reduces the uncertainty on the residual anthropogenic trends.The mean PM2.5 decrease across China is 4.6?g m-3 a-1 in the meteorology-corrected data,12%weaker than in the original data.The residual trends in the five megacity clusters attributable to changes in anthropogenic emissions are 8.0±1.1?g m-3a-1 for Beijing-Tianjin-Hebei(14%weaker than the observed trend),6.3±0.9?g m-3 a-1 for Yangtze River Delta(3%stronger),2.2±0.5?g m-3 a-1 for Pearl River Delta(19%weaker),4.9±0.9?g m-3 a-1 for Sichuan Basin(27%weaker),and 4.9±1.9?g m-3 a-1 for Fenwei Plain(Xi'an;25%weaker).This indicates that emission control plays a dominant role(88%)in the linear PM2.5decreasing trend,while meteorological variations to a certain extent(12%)accelerated this PM2.5decreasing trend.2015-2017 observations of flattening PM2.5 in the Pearl River Delta,and increase in the Fenwei Plain,can be attributed to meteorology rather than to relaxation of emission controls.Under the conclusion that“emission reduction plays a dominant role in China PM2.5decline”,the second part of this research interpreted 2013-2017 changes in China PM2.5and its compositions as a result of emission control using an updated version of GEOS-Chem.This part first refers to the up-to-date constraints on GEOS-Chem,and updated?(N2O5),?(NO3),and dry deposition velocity of HNO3 and p NO3-in GEOS-Chem v12.0.0.The updated GEOS-Chem model shows a much better overall performance in reproducing China sulfate,nitrate and ammonium PM2.5.GEOS-Chem simulation results show that sulfate,nitrate,ammonium,BC and OA significantly decreased across China from2013-2017 as a result of the Clean Air Action.Of all the PM2.5 components,sulfate has the largest decreasing ratio,which accounts 32-43%of total PM2.5 decrease.Sulfate,nitrate and ammonium together account 58-71%of total PM2.5decrease.With the decrease of SO2emissions,aqueous-phase SO2 oxidation efficiency increase,thus leading to weaker sulfate reductions than SO2emission reductions,especially in winter.In the overall decreasing particulate nitrate,North Shanxi,some spots in north Beijing and Pearl River Delta witnessed a stagnant or increasing(for some model grids)particulate nitrate change,despite of the decreasing NOx emissions.This is because over these regions,particulate nitrate is very sensitive to free NH3,and the decrease of sulfate releases free NH3,thus facilitating nitrate partitioning to its aerosol phase.The above results indicate that in order to achieve equivalent sulfate decrease in winter and summer,winter should have larger SO2 emissions reduction extent than summer.It also indicates that,as sulfate continues to decrease,China PM2.5 will become increasingly organic,which indicates the joint potential in reducing O3 and PM2.5 by cutting NMVOCs emissions,which also lead new challenges in achieving continuing PM2.5reductions.As the first two parts pointed out the importance of emission control and the driving role of meteorology in the formation of severe pollution episodes,the last part of this research will focus on the determination of the critical emission source regions and time periods during a typical PM2.5 pollution episode in Beijing.This part first developed an adjoint model of the GRAPES-CUACE aerosol module.Then,a typical PM2.5pollution episode in Beijing was selected,and the changes in meteorological conditions and pollutant concentration during the pollution process were analyzed in detail.In combination with the GRAPES-CUACE aerosol adjoint model,the peak PM2.5 concentration is set as the objective function to determine the key emission source area and emission period of the pollution episode.This part points out that narrowing the emission sources reduction scope to target critical source zones(zones detected by an adjoint model),rather than emission-intensive regions,2 to 3 days prior to unfavorable meteorological conditions can effectively decrease PM2.5concentrations and improve the efficiency of PM2.5reduction measures. |
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