Study On The Formation Of Secondary Inorganic Aerosols Under Different Pollution Types In Nanjing In Winte

The Yangtze River Delta is the core region of China economic belt,with high urban den-sity and developed industry,but with serious man-made pollution and frequent excessive PM_2.5.Nanjing,as the central city of the Yangtze River Delta,has its unique geographical environment and humid and warm meteorological conditions,which makes the air pollution problem of Nan-jing cannot be ignored.In order to analyze the pollution characteristics,sources and potential contribution of water-soluble inorganic ions in fine particulate matter PM_2.5 in Nanjing,PM_2.5 samples were collected from December 2015 to April 2016,and the concentrations of eight water-soluble inorganic ions(NO₃^-、SO₄^2-、NH₄⁺、Cl^-、K⁺、Mg²⁺、Ca²⁺、Na⁺)were measured.Positive matrix fac-torization（PMF）method was used to explore the source of PM_2.5,combined with Backward trajectory model（HYSPLIT）,the potential source contribution function（PSCF）analysis and the concentration weighted trajectory（CWT）analysis,the potential source area of each com-ponent was determined,and the pollution contribution was calculated quantitatively.The results showed that under different pollution levels,SO₄^2-,NO₃^-and NH₄⁺（SNA）were the main components of PM_2.5,and high relative humidity（RH>50%）and low temperature（T<8℃）were conducive to improving the generation rate of secondary inorganic aerosols.PMF source analysis found that secondary conversion sources contributed the most significantly,followed by dust sources,biomass burning sources and Marine sources.The distribution of potential source regions of SNA is similar,but the peak values of NO₃^-CWT was over 24μg·m^-3,and the range of high value of CWT was large,which further indicated that vehicle exhaust emis-sions and fuel combustion were the main potential sources during the observation period.At the same time,WRF-Chem model was used to study the change characteristics of sec-ondary inorganic aerosol and its precursors,and it was found that when statically stable pollu-tion occurred,stable weather situation was not conducive to the diffusion of pollutants,which led to the occurrence of pollution.At this time,the high value center of high concentration of NO₃^-and SO₄^2-mainly appeared near the ground.The contribution of chemical generation process to NO₃^-is 53.53%,but for SO₄^2-,the vertical mixing process,as the most important contribution process,is 59.35%.When cold front type pollution occurs,the PM_2.5 concentration in Nanjing will show a trend of rapid rise and then rapid decline.With the transit of cold front,The high value area of pollutant concentration moves southbound gradually,which indicates that the high concentration of NO₃^-and SO₄^2-are distributed along the moving path of the cold front.Due to the continuous movement of the cold front,the warm and wet polluted air mass in the front of the front is forced to rise to the free troposphere along the front of the front,so that the secondary inorganic aerosol is lifted to the upper atmosphere.In addition,the con-tribution of the chemical formation process to the NO₃^-and SO₄^2-in the high air accounted for 27.91%and 40.33%.