Characteristic And Influence Factors Of Carbonaceous Aerosols Using Online Analyzers In Typical Areas Of The Pearl River Delta Region

Carbonaceous aerosols（CAs）,mainly composed of elemental carbon（EC）and organic carbon（OC）,is a substantial fraction of atmospheric fine particle(PM_2.5)and can exert negative impacts on air quality,climate system and human health.The atmospheric loading of CAs is strongly associated with the human activities and atmospheric oxidation reactions.In recent years,its emission sources,chemical compositions,and the environmental impacts have been widely and thoroughly explored around the world.However,knowledges regarding the CAs in the downwind megacities remain poorly understood,especially the impacts of anthropogenic emissions reduction on the atmospheric concentration and chemical compositions of CAs.Hence,this thesis focuses on the typical areas of the Pearl River Delta（PRD）region,China using high-time resolution online OC/EC analyzers from 2019 to 2020.We clarified the annual,seasonal,and daily characteristics,the spatial distribution of EC,OC and secondary organic carbon（SOC）and the key factors influencing SOC formation in the downwind region.In addition,this thesis quantified the responses of CAs concentration decline due to the regional and local reduction during the periods of COVID-19 outburst and“100-day battle against air pollution”in the PRD region.The main findings are as follows:（1）The annual mean EC and OC concentrations at the regional HS site in the PRD region（1.45±0.82μg/m³ and 4.94±3.34μg/m³）are lower than those in the typical urban areas（1.59±0.90μg/m³ and 6.16±3.39μg/m³ at DG site,1.60±1.00μg/m³ and 5.89±3.32μg/m³ at ZQ site,respectively）.Overall,the CAs concentrations in the typical urban areas are 16%higher than those at the regional HS site,indicating that the strong human activities can lead to high levels of both OC and EC concentration in the typical urban areas of PRD.（2）The seasonal trend of OC and EC concentrations between Autumn and Winter in the downwind area of the PRD region（Autumn>Winter）is different from in the upwind urban areas（Winter>Autumn）.Particularly,the OC concentration in the downwind area is higher than or close to that in the upwind urban areas in autumn.Moreover,a strong enhancement phenomenon of OC concentration in the downwind area is observed in the autumn afternoon.All these findings are much different from the upwind area.Also,the minimum R squared（MRS）optimized method analysis shows that the local and regional combustion-related SOC(SOC_com)is the key factor that regulates the enhancement of OC in the autumn afternoon in the downwind region.（3）The Grey Relation Analysis（GRA）and the online observational data show that the relevance degree of SOC_com between atmospheric oxidation（r=0.888）and temperature（r=0.848）are significantly higher than those with PM_2.5/PM₁₀ ratio（r=0.836）and relative humidity（0.784）,reflecting that the SOC_com formation is possibly controlled by atmospheric oxidation reactions.Furthermore,we proved that the Zhaoqing-Foshan-Jiangmen belt is a hotspot of atmospheric photochemistry with the high pollution level of SOC_com in autumn in the downwind PRD region using the analysis of K-mean cluster method based on metrological observation and SOC_com concentration.In addition,the SOC_com fraction that transported from the north-east and the east of the PRD region also can not be ignored.（4）The radar chart is employed as the diagnostic tool to identify emission sources in this thesis.We found that the SOC_com enhancement in the autumn afternoon in the downwind area of the PRD region are mainly associated with fossil combustion sources emission from the upwind cities,including vehicle exhaust,coal combustion and industrial combustion.Moreover,the ternary plot is used as the diagnostic tool for identifying the chemical components proportions.We identified enhanced proportions of sulfate and nitrate with the SOC_com concentration decline.Besides,the multiple regression model is taken as the diagnostic tool to evaluate the potential capacity for co-control pollution.We found that the response model between SOC_com and combustion emission reduction(characterized by EC_2020/2019)displays a strong linear relationship,suggesting that it is important for co-control of fossil combustion sources emission in the upwind areas of the PRD region to reduce the impact on SOC_com in the downwind area of PRD region in autumn.（5）We found that SOC and atmospheric oxidation show a common decrease trend during the periods of the COVID-19 epidemic outburst based on the online observations.Combined with the simulation evaluation of the“100-day battle against air pollution”,we inferred that super-regional and local air pollution reduction contributes for the CAs decline in the downwind area of PRD region.Furthermore,the sensitivity of OC decline to variable anthropogenic emissions is lower than EC.This is probably owing to that the local formed and transported SOC can offset the primary emissions reduction for the sources in the PRD region.This thesis provides solid and reliable database and methods for studying the emission soruces,chemical composition,and formation mechanism of atmoshperic CAs in the megacities.Moreover,it offers a useful reference for future researchs regarding the impacts and control strategies of atmospheric CAs.