Uncertainties Of The Correlation Between Aerosol Optical Thickness（AOD）and PM_2.5 In Nanjing And Beijing

With the acceleration of economic development and urbanization,the emission of various air pollutants is increasing,and the air quality problem is becoming more and more serious.In recent years,the frequent occurrence of haze weather has seriously affected people’s travel,life and health.The pollution problem of PM_2.5 has attracted extensive attention from the public and the government.The temporal analysis and monitoring of fine particles are very important for the prevention and treatment of pollution.Based on the observational data of PM_2.5 mass concentration,aerosol optical depth（AOD）data and meteorological elements from MICAPS data in Nanjing and Beijing,during May1,2014～October 31,2019,the variations of PM_2.5 mass concentration in two cities and its relationship with AOD,meteorological factors were analyzed and discussed.Based on observational studies of Nanjing area,after using data in December and January of 2013～2018,a total of 6 months of total cloud cover observational data,the CE-318 sun photometer and MODIS aerosol optical depth（AOD）inversion of Nanjing,combined with the concentration of PM_2.5 hourly data provided by environmental protection,to verify the analysis the influence of PM_2.5 concentrations for AOD.The results showed that:（1）The PM_2.5 mass concentration in Nanjing and Beijing was significantly higher in winter than in summer,and the AOD was smaller in winter than in summer.By contrast,the average monthly concentration of PM_2.5 in Beijing is higher than that in Nanjing.（2）The PM_2.5 concentration in Nanjing and Beijing is positively correlated with AOD,and there is a significant seasonal difference between PM_2.5concentration and AOD,which mainly shows that the correlation in summer is greater than that in winter.（3）At the same AOD level,the higher the relative humidity in the two places is,the higher the PM_2.5 concentration is;（4）The R² of PM_2.5concentration and visibility in Nanjing is0.32,while that in Beijing is 0.69.The correlation between PM_2.5 concentration and visibility in the two places is better in winter than in summer.In the case of high relative humidity and the same PM_2.5 concentration level,visibility in Nanjing is better than that in Beijing.（5）In the case of high pollution weather in Nanjing,the cloud cover is not particularly high,and the high concentration of PM_2.5 has a great impact on AOD,which easily leads to the loss of AOD inversion and data reduction.In the case of good weather in Nanjing,PM_2.5 concentration has a small impact on AOD inversion and the loss of AOD data is reduced.