Study On Atmospheric Pollution Process And Carbonaceous Aerosol Distribution Characteristics In Nanjin

The current air pollution problem in China is characterized by regionalization,which poses a great challenge to effectively control regional pollution.In order to better understand the current atmospheric pollution process and the distribution characteristics of carbon aerosols,and formulate effective treatment plans,this article discusses in detail the black carbon（BC）pollution characteristics and influencing factors under different primary pollutants through long-term observation,and further discusses the carbon aerosol pollution characteristics and influencing factors under different air quality levels and different O₃pollution processes in Nanjing in spring and summer,Finally,combining observation with the WRF-Chem model,we selected two typical O₃pollution processes（the correlation coefficients between O₃and secondary organic carbon aerosols（SOC）were-0.64 and 0.63,respectively）to explore the differences in formation mechanisms.The results show that the air quality in Nanjing showed a significant improvement trend from 2015 to 2020,with the number of days with excellent air quality increasing at a rate of38.2%from 2015 to 2020.The number of days in which the primary pollutant is PM_2.5has decreased year by year,from 89 days in 2015 to only 18 days in 2020,with an annual decrease rate of 16.0%;The number of days when the primary pollutant is O₃shows an upward trend,with a middle-aged increase rate of 11.1%from 2015 to 2019,indicating that the primary pollutant of atmospheric pollution in Nanjing gradually changes from PM_2.5pollution to O₃pollution.When the primary pollutant is O₃,the annual change in BC mass concentration significantly decreased during 2015-2016 for both mild and moderate pollution,followed by an annual upward trend in 2016-2019,with annual growth rates of 112.2%and138.6%,respectively.After reaching the maximum value in 2019,BC mass concentration began to decrease,reaching 3.0μg·m^-3in 2020 and 3.2μg·m^-3The mass concentrations of BC have a significant negative correlation with wind speed,with correlation coefficients of-0.79and-0.68,respectively,and are less affected by the inversion layer.At the same time,the seasonal variation of BC mass concentration is mainly influenced by wind speed in autumn,summer,and spring with light pollution,and in spring,summer,and autumn with moderate pollution.In 2019,where O₃pollution occurred the most frequently in 6 years（with 78 days of O₃pollution）,further research found that the spring and summer in Nanjing were the most prone to O₃pollution.The average mass concentrations of BC and primary organic carbon（POC）in summer are 3.1μg·m^-3and 2.1μg·m^-3was 38.0%and 30.0%higher than that in spring,respectively;The average mass concentrations of organic carbon（OC）and SOC in spring are6.2μg·m^-3and 4.6μg·m^-3is 26.5%and 56.5%higher than that in summer,respectively.The diurnal variation characteristics of different carbon aerosols under different air quality levels are different.The daily variation of OC and SOC is gentle when the air quality is optimal,with a single peak distribution in good times,and a three peak distribution in mild pollution,with peaks appearing at 1:00-3:00,6:00-8:00,and 22:00-24:00,respectively.Lower temperatures and wind speeds are not conducive to OC and SOC diffusion,while higher relative humidity has a significant impact on the increase in the mass concentration of OC and SOC.Both BC and POC have a bimodal distribution in optimal and moderate daily variation,while under both good and mild pollution,their daily variation has a single peak distribution,with peaks occurring from 4:00 to 7:00,mainly affected by vehicle exhaust.Lower wind speeds inhibit the diffusion of pollutants,which is the main reason for BC to peak from 5:00to 7:00 in the morning in light pollution.The daily variation characteristics of carbon aerosols during three times of O₃pollution are different.The diurnal variation of OC and SOC in Process 1 and Process 2 showed a characteristic of high at night and low during the day;However,there was no significant difference in diurnal concentration in process three.In Process 1 and Process 2,BC and POC both exhibit a single peak distribution,with peaks occurring between 4:00 and 6:00,but in Process 3,they exhibit a significant dual peak distribution,with peaks occurring between 5:00 and 7:00 and 21:00-24:00,respectively.The occurrence of 21:00-24:00 peaks is mainly related to the decrease in atmospheric boundary layer height and the addition of late peak motor vehicle emissions.In the first two processes,O₃and SOC were negatively correlated,with correlation coefficients of-0.64 and-0.66,respectively,while in the third process,O₃and SOC were positively correlated,with correlation coefficients of 0.63.At the same time,the correlation coefficients of OC and BC in the three pollution processes were 0.87,0.80 and 0.40,respectively.The good correlation between the first two processes indicates that the previous two pollution processes have similar sources and are more affected by local accumulation,while the third pollution process is also affected by external transportation,The last two processes were selected as typical O₃pollution processes for WRF-Chem model simulation.It was found that in terms of generation mechanism,the high concentration of near-surface O₃mainly depended on vertical mixing in O₃pollution based on local accumulation,with a total contribution of 451.7μg·m^-3within 96 hours accounts for 66.2%of the total process volume,while O₃pollution based on external transport mainly relies on horizontal advection and vertical mixing,with a total contribution of 1050.2μg·m^-3and 799.1μg·m^-3within 72 hours and 799.1μg·m^-3,accounting for 61.5%of the total process volume.Horizontal advection also allows a large number of VOCs from upstream regions to be transported to promote the formation of SOC photochemical reactions,resulting in simultaneous increases in the mass concentrations of SOC and O₃.