Study On Molecular Composition Of Secondary Organic Aerosol

Secondary organic aerosol（SOA）affects regional air quality,atmospheric visibility and human health,thus has attracted increasing interest.The Pearl River Delta（PRD）region is characterized by a subtropical monsoonal climate,with rich botanical resources as well as rapid growth of population and economy.The conditions facilitate the emissions of biogenic and anthropogenic volatile organic compounds（VOCs）and gaseous pollutants,which could influence the composition and concentration of SOA in the PRD region and over the South China Sea（SCS）.Until now,the seasonal cycles and influencing factors of SOA in the PRD region,as well as its sources and distributions over the SCS have not been fully studied.In this study,SOA tracers in fine particulate matter(PM_2.5)were observed at a regional site,Tianhu,Guangzhou,from March 2012 to February 2013.In addition,dicarboxylic acids（DCAs）and related compounds in total suspended particle（TSP）samples were analyzed during two cruises over the northern and western SCS（NSCS and WSCS）,respectively.The main conclusions are drawn as follows,The characteristics including seasonal cycles and the factors influencing SOA formation in this region were studied.The seasonal patterns of isoprene（SOA_I）,monoterpene（SOA_M）and sesquiterpene（SOA_S）SOA tracers were characterized over three enhancement periods in summer,autumn and winter,while the elevations of aromatic SOA（SOA_A）tracer（i.e.,2,3-dihydroxy-4-oxopentanoic acid,DHOPA）were observed in autumn and winter.At Tianhu,Guangzhou,biogenic SOA formation could be influenced by several factors in a seasonal cycle.Precursor emissions controlled SOA_I formation throughout the year,but just facilitated SOA_M and SOA_S formation in summer rather than in other seasons.Atmospheric oxidants（O_x）were found to enhance SOA_M formation in autumn and winter.The formation of SOA_S was influenced by the precursor emissions in summer,atmospheric oxidation in autumn and probably also by biomass burning in both summer and winter.Good correlations between observed anthropogenic SOA tracer,DHOPA and O_x as well as[NO₂][O₃]suggest the involvement of both ozone（O₃）and nitrogen dioxide（NO₂）in the formation of DHOPA.The results showed that regional anthropogenic pollution may not only increase the emissions of aromatic precursors but also can greatly promote the formation processes.Nine straight-chain（C₂-C₉）and 5 unsaturated DCAs were detected in marine aerosol over the SCS.The concentrations of total DCAs ranged from 373 to 882 ng m^-3(ave.528 ng m^-3)and 286 to 984 ng m^-3(524 ng m^-3)in marine aerosol over the NSCS and WSCS.The molecular distribution of DCAs was characterized as C₂>C₄>C₃.C₉ is the predominant DCA in C₇-C₁₀.Meanwhile,phthalic acid（Ph）was found the predominant aromatic acids.These results suggested that biogenic and anthropogenic emissions are important sources of DCAs in marine aerosol over the SCS.The concentrations of total carbon（TC）and DCAs,air mass back trajectory and the contribution of DCAs to TC were analyzed to find the potential sources of DCAs in marine aerosol over the SCS.We found that continental-outflow and marine emission were important sources of DCAs over the SCS,and photochemical oxidation also contributed to DCAs both over the NSCS and WSCS.The concentration of total DCAs showed good correlations with levoglucosan in marine aerosol over both NSCS and WSCS,suggesting that biomass burning is an important source of DCAs.C₃-C₆ and C_18:1 showed significant positive correlations with C₉,indicating the photochemical degradation processes of DCAs.The results of selected diagnose ratios indicated that DCAs in marine aerosol over the SCS could not only be affected by both continental and marine emissions,but also by secondary photooxidation.