Current Situation And Causes Of Ozone Pollution And Its Responses To Reductions Of Precursors Emissions In Nanjing

With the development of economy,the ozone pollution in Nanjing is serious.The annual averages of observed maximum daily 8?hour average ozone（MDA8 O₃）in Nanjing from 2013to 2019 increase by about 4μg m^?3 yr^?1.In order to explore the causes of ozone pollution in Nanjing,the sensitivity of ozone to its precursors,pollution sources and ozone budget in Nanjing under typical circulation patterns are studied by using the hourly observations of the China National Environmental Monitoring Network in Nanjing,GEOS?Chem and its adjoint model.And combined with the sensitivity simulations of emission perturbation in GEOS?Chem,the random forest algorithm is used to efficiently predicted under different reduction scenarios of anthropogenic emissions in Nanjing.The main conclusions are as follows:（1）The Multi-resolution Emission Inventory for China（MEIC）of Tsinghua University and 0.25°×0.3125°nested grids are adopted in GEOS?Chem.For the GEOS?Chem adjoint model,the correlation coefficient（r）,normalized mean bias（NMB）and normalized mean error（NME）of the observed and simulated surface MDA8 O₃ from March to July 2017 in Nanjing are 0.65（95%confidence level）,5.31%and 21.01%,respectively.（2）The T?model principal component analysis（T?PCA）is used to classify the typical circulation patterns during heavy ozone pollution days(defined as the observed MDA8 O₃≥160μg m^?3),light ozone pollution days(100μg m^?3≤MDA8 O₃<?160μg m^?3)and ozone clean days(MDA8 O₃<100μg m^?3)in Nanjing from 2013 to 2019,which are divided into four types,four types and five types,respectively.The main circulation patterns of pollution days are Siberian Low,Lake Balkhash High,Northeast China Low,and Yellow Sea High and Subtropical High.In ozone pollution days,the weak high or ridge of high pressure is always located over the sea near Nanjing.And Nanjing is frequently affected by the southeast wind at the surface.In ozone clean days,Nanjing is mostly controlled by the terrestrial circulation patterns and affected by the northeast wind at the surface.（3）The ozone pollution events under typical circulation patterns in Nanjing are selected to analyze the sensitivity of ozone to the emission of its precursors（nitrogen oxides（NO_x）,carbon monoxide（CO）and non-methane volatile organic compounds（NMVOCs））in China by using the GEOS?Chem adjoint model.In the several pollution events,the contributions of ozone precursors emitted from Jiangsu and Zhejiang areas to ozone pollution in Nanjing are high.The 10%reduction of anthropogenic NO_x emission in Jiangsu and Zhejiang areas could reduce the ozone concentrations in Nanjing by up to 3.40μg m^?3.And the 10%reduction of anthropogenic NMVOCs emission in Jiangsu and Zhejiang areas could reduce the ozone concentrations in Nanjing by 0.77?0.96μg m^?3.However,the ozone pollution in Nanjing is mostly affected by the emissions from southern Anhui,Jiangxi and southeastern coastal areas of China because of Subtropical High.In the pollution events,the emissions of local NMVOCs in Nanjing are reduced by 10%,and the ozone concentrations would be reduced by 0.01?0.07μg m^?3.The reduction of local emissions of NO_x would increase the ozone concentrations in Nanjing.For sector emissions,the contribution of industry emissions is more than 40%,followed by the contribution of transportation emissions is about 30%.Meanwhile,the results of ozone budget analysis show that chemical productions are the main sources of ozone in Nanjing,and the highest contribution to surface ozone formation is 0.42 Gg day^?1.Exploring the sources and causes of ozone pollution events in Nanjing under the typical circulation patterns could provide the scientific basis for forecasting ozone pollution events and accurately formulating strategies of emission reduction.（4）In addition,for different emission control strategies,this study applies a method of the random forest model to learn the simulations of GEOS?Chem,which would efficiently obtain the response of ozone pollution in Nanjing to different strategies of emission reduction.The results show that in the ozone pollution in the summer of 2019,Nanjing is in the transition regime.In the other seasons,Nanjing is in the NMVOCs?limited regime.The simulations could provide the scientific support for timely formulation and adjustment of emissions control strategies.