Precipitation Scavenging,Source Analysis And Contribution Of Carbon Sink Of Aerosol Carbon In Nanchang Area

From April 2018 to February 2019,samples were collected at the top of the environmental protection building of Qianhu Campus,Nanchang University,Honggutan New District,Nanchang City.The APS-3A Precipitation Automatic Sampler（Changsha Xianglan Scientific Instrument Co.,Ltd.）was used to collected the segmented precipitation.Aerosol particles(PM_2.5 and PM₁₀)were collected by TH-150F intelligent medium-flow atmospheric sampler（Wuhan Tianhong Instrument Co.,Ltd.）before precipitation,during precipitation and after precipitation,and collected dry and wet deposition（precipitation and dust reduction complex）monthly.Determined the organic carbon（OC）and elemental carbon（EC）in collected segmented precipitation samples,aerosol samples,and dry and wet deposition samples,discussing the distribution characteristics,analyzing the sources and estimate carbon sinks.The main conclusions were as follows:（1）The distribution of OC,EC in the segmented precipitation and wet precipitation scavenging of carbonaceous aerosol.The OC and EC concentrations in the segmented precipitation gradually decreased with the precipitation process.The concentration of OC in the segmented precipitation ranged from 0.0010 to 3.5548 mg/L（average 0.6291 mg/L）,and the EC concentration ranged from 0 to 2.3038 mg/L（average 0.3050 mg/L）.The concentration of OC、EC gradually decreased with the progress of precipitation,while a small part of the precipitation occurred rise in the middle or tail.The influencing factors were related to rainfall,meteorological factors,accumulation of dyes and cloud itself.The ratio of OC/EC in the front and middle of the segmented precipitation did not changed much,because the OC and EC in the segmented precipitation samples were mainly from wash-out process.While the ratio of OC/EC in the tail-part of the segmented precipitation occasionally rised sharply,because the concentration of matters in the final stage of segmented precipitation samples was mainly determined by the cloud water removal process.OC was water-soluble and it was one of the main cloud condensation nuclei,the EC concentration in the cloud water is lower than OC,and it led to the sharp rise of the ratio of OC/EC at the final stage of the segmented precipitation.The carbonaceous aerosol in precipitation is mainly derived from below-cloud scavenging.The contribution of below-cloud scavenging to precipitation OC ranged from 47.72 to 95.27%（average 78.77%）,and the contribution to EC ranged from56.38 to 100%（average 89.34%）,indicating that the precipitation of carbonaceous aerosol mainly comes from below-cloud scavenging.The contribution of cloud water to OC and EC in precipitation is 4.73⁵2.28%（average 21.23%）and 0⁴3.62%（average 10.66%）,which is related to the involvement of OC in the cloud condensation process and less involvement of EC.（2）The distribution of OC,EC in PM_2.5,PM₁₀ during whole precipitation process and precipitation scavengingPrecipitation plays an important role in atmospheric cleaning.The concentration of OC and EC in PM_2.5 is as follows:before precipitation>during precipitation and after precipitation,the concentration of OC and EC gradually decreased with the process of precipitation,and there had a slight rose in the final stage of precipitation occasionally.The variation of concentration of OC and EC in PM₁₀ was:the highest before precipitation,decrease during precipitation,and rise after precipitation.During the sampling period in Nanchang,the mean concentration of OC and EC in PM_2.5before precipitation was 16.37 g·m^-3 and 5.37 g·m^-3,respectively.The mean mass concentration of OC and EC in PM_2.5 after precipitation was 9.66 g·m^-3 and 3.07g·m^-3,respectively.Precipitation had cleaning effect on aerosol carbon（OC,EC）.Precipitation scavenging plays an important role in atmospheric cleanliness.The average concentrations of OC and EC in PM_2.5 before precipitation in Nanchang City were 16.37μg·m^-3 and 5.37μg·m^-3,respectively.The concentration of OC and EC in Nanchang aerosol was at a medium level.The average concentration of OC and EC in PM_2.5 after precipitation was 9.66μg·m^-3 and 3.07μg·m^-3 respectively.The concentration of OC and EC in aerosols in Nanchang was relatively clean.Based on the ratio of OC/EC and the distribution characteristics of carbon components of PM_2.5 and PM₁₀ in atmosphere,it is indicated that carbon aerosols of atmospheric PM_2.5 and PM₁₀ in nanchang region in winter mainly came from coal burning emissions,followed by gasoline vehicle exhaust emissions and biomass combustion.The correlation between OC and EC in segmented precipitation was higher than that between OC and EC in atmospheric PM_2.5 and PM₁₀ before,during and after precipitation.The correlation between OC and EC in segmented precipitation was significant,reflecting the homology of the particles OC and EC washed by the cloud into the precipitation samples.The correlation between OC and EC in atmospheric PM_2.5.5 and PM₁₀ was not obvious,indicating that the source of OC and EC in atmospheric PM_2.5 and PM₁₀ were complicated.Precipitation had different scavenging effects on 8 carbon components in PM_2.5and PM₁₀.Precipitation significantly reduced OC1,EC1 and EC2 in PM_2.5 and PM₁₀,but OC1 recovered quickly after rainfall;OC3 accounted for 47⁹0%of the total carbon in the segmented precipitation,and the removal effect of precipitation on OC3was not obvious.（3）The dry and wet deposition of OC and EC and the regional carbon sinkThe trend of dry and wet deposition of OC and EC is similar in different months of the year:they gradually decrease from September 2018 to January 2019,and rises in February 2019.During the sampling period,the average dry and wet deposition of OC and EC was 221.57mg/m² and 87.42mg/m²,respectively.The source analysis results of the dry and wet deposition samples in Nanchang area show that the dry and wet deposition of carbon mainly from the exhaust gas of automobile exhaust emissions change to coal-fired emissions.The dry and wet deposition flux of OC in nanchang area was 221.57kg/（km²·month）,the dry and wet deposition flux of EC was 87.42kg/（km²·month）,then the dry and wet deposition of OC contributes 19680.73t/a to the regional carbon sink,and the dry and wet deposition of EC contributes 7764.99t/a to the regional carbon sink.