Evaluation On The Contribution Of Volatile Organic Compounds To The Formation Of Secondary Organic Aerosols During PM_2.5 Pollution Episodes In Typical Urban Area Of Beijing

Fine particles(PM_2.5)will become part of the sediment after being transported and deposited in the air,and will become part of the sedimentary rock after diagenesis.Before the final sedimentation,PM_2.5 can float and transport continuously in the air,affecting the ambient air quality and global climate change,damaging human health.PM_2.5 is composed of sulfate,nitrate,ammonium,elements,organic matter and so on.Secondary organic aerosols(SOA)are important part of organics in PM_2.5.As important precursors of the SOA,anthropogenic volatile organic compounds(VOCs)mainly come from the emissions of organic mineral resources in industrial applications(fossil fuel combustion,industrial process,solvent use,et al),transportation,and civilian use(catering,cosmetics,decoration,et al).Studies on the characteristics of SOA and contributions of ambient VOCs to the formation of SOA in PM_2.5 pollution episodes are of great significance for clarifying the impact of VOCs emissions form organic mineral resources to the SOA formation in PM_2.5 pollution episodes and formulating effective policies for PM_2.5 pollution prevention.In order to study the characteristics of SOA and contributions of VOCs to the formation of SOA in the PM_2.5 pollution episodes in the typical urban area of Beijing in four seasons,one or two PM_2.5 pollution episodes were selected in each season from March 2018 to January 2019,and enhanced observations of PM_2.5 and VOCs were carried out at the Supersite for Urban Air Comprehensive Observation and Research of the Chinese Research Academy of Environmental Sciences,which located in Chaoyang District,Beijing.The characteristics of PM_2.5,deposition flux,SOA in PM_2.5,SOA from VOCs oxidation(SOAvoc),SOA from specific VOCs(SOAvoc')were analyzed,the differences of SOA,SOAvoc and SOAvoc'were compared.The contributions of VOCs to the formation of SOA and the contributions of SOA to the formation of PM_2.5 pollution were discussed.The key VOCs species that generate SOA and the main sources of key VOCs species were identified.The main conclusions are as follows:1.Although the concentration of SOA was significantly lower than that in 2013,it was still an important organic component of PM_2.5During the PM_2.5 pollution episodes in Beijing,the mass concentration of SOA decreased by about 50%compared with that in 2013,but it still accounted for 58%and 46%of OA and 13%and 11%of PM_2.5during the light to moderate pollution episodes(observation in spring and summer)and heavy pollution episodes(observation in autumn and winter),respectively.The heavier the PM_2.5 pollution,the higher the deposition flux,and the greater the impact on the sediment.Photochemical reaction has a great influence on the formation of SOA in spring and summer,while atmospheric oxidation capacity,precursors emissions and boundary layer height have a great influence on the formation of SOA in autumn and winter.SOA pollution control is an important direction for Beijing to control PM_2.5 pollution in the future.2.The contribution of OH radical oxidation of VOCs to the formation of SOA is11%–58%,and toluene and undecane are the key VOCs species for the formation of SOAThe mass concentration of SOAvoc accounted for 12%and 58%of SOA in light to moderate pollution episodes(observation in spring and summer)and heavy pollution episodes(observation in autumn and winter),respectively.Aromatic hydrocarbons and alkanes were important precursors of SOA.Toluene in aromatic hydrocarbon and undecane in alkane were the key species for SOA generation,which contributed about 50%to SOAvoc.It is recommended that not only the total amount but also the key VOC species should be controlled in order to reduce SOA concentrations and control PM_2.5 pollution in Beijing.3.Oxidation of?-pinene,isoprene,toluene and?-caryophyllene contribute14%–26%to SOA formationThe mass concentration of SOAvoc'accounted for 21%and 15%of SOA during the light to moderate pollution episodes(observation in spring and summer)and heavy pollution episodes(observation in autumn and winter),respectively.Compared with?-pinene and isoprene,toluene and?-caryophyllene were key species for SOA formation.The aging degree of?-pinene SOA in Beijing was relatively low.Isoprene SOA were generated under low nitrogen oxide conditions in spring and summer,and under high nitrogen oxide conditions in autumn and winter.The analysis results of SOAvoc'further indicate that toluene is a key species for the formation of SOA.4.The concentration of SOA generated by the same VOC estimated by different methods varies greatlyThe calculation results of SOAvoc and SOAvoc'were far lower than the SOA concentrations in PM_2.5,and the calculation results of SOA generated by the same VOC were quite different.The complexity of the way of SOA formation and the uncertainty in the calculation together cause these differences.It is recommended that the oxidation of oxygen VOCs and the heterogeneous reaction of VOCs should all be taken into consideration in order to improve the consistency of SOA estimation by various methods.How to use experiments or simulation methods to reduce the estimation differences between the methods by adjusting the parameters in the calculation is a direction that needs further research.5.It is necessary to focus on controlling the emissions of mobile sources,especially the emissions of long-chain alkanes from diesel vehicles,in order to precisely reduce the concentration of SOA in PM_2.5During the PM_2.5 pollution episodes in different seasons,the contributions of mobile sources to key VOCs were about 50%,and increased on PM_2.5pollution days.Undecane mainly comes from the emissions of diesel vehicles in mobile sources and needs to be focused on management and control.Mobile sources and fossil fuel combustion made a greater contribution to key VOCs in spring and winter,while mobile sources and industrial process and solvent use made a greater contribution to VOCs in summer.In order to control the SOA,it is recommended that strict polices for control VOCs emissions should be implemented in Beijing,and more attention should be paid to controlling emissions from mobile sources and fossil fuel combustion sources in spring and winter,while more attention should be paid to controlling emissions from mobile sources and solvent use and industrial process in summer.