Molecular Composition, Size Distributions And Seasonal Variation Of Urban Organic Aerosols In Beijing

The sources and formation processes of organic aerosols in the atmosphere are complicated.The molecular composition of organic aerosols has been well studied,while the analysis of their size distributions is relatively lacking.Studies on the size distributions and sources are important to understand the physical and chemical characteristics of organic aerosols,which plays a guiding role in environmental control and measures.Size-segregated aerosol samples collected in Beijing during 2017 to 2018 were analysed using GC/MS.Information on concentrations,size distributions and seasonal variation in organic aerosols were obtained.The main conclusions are as followed:1.The abundances of primary organic aerosols(POA)were much higher than secondary organic aerosols(SOA).The molecular compositions of organic aerosols varied with season.The levels of aliphatic hydrocarbons,primary sugars,sugar alcohols and biogenic secondary organic aerosols were higher in summer under the influence of vegetation.Aromatic acids and phthalate esters were affected by the burning and volatilization of plastic waste in summer.Anhydrosugars,lignin/resin acids and PAHs were affected by biomass burning and fossil fuel combustion.2.Influenced by biomass/fossil combustion in cold seasons,the relative contributions of POA to OC were higher than those of SOA.SOA were affected by vegetation and plastic waste emissions and contributed significantly in summer.3.Size distributions of different organic components showed differentiation.Aliphatic hydrocarbons showed a bimodal distribution,mainly in the coarse mode.Anhydrosugars showed a unimodal distribution,mainly in fine particles.PAHs were bimodal with fine particles preference.Aromatic acids and lignin/resin acids were bimodal.Phthalate esters showed a three-modal distribution.Except spring,the biogenic oxidantion products mainly existed in the fine mode.4.Air mass of haze and dust storm in each season showed difference in Beijing,leaing to the different molecular composition and size distributions.The springtime dust storms transported a large quantity of coarse particles into the atmosphere,inducing a remarkable size shift towards the coarse mode in aliphatic hydrocarbons,sugars and sugar alcohols.Larger GMDs of most organic compounds in the fine mode were found in haze days.5.POA had a great impact on the haze in Beijing.Aliphatic hydrocarbons were the most contributor.Fossil fuel combustion and biomass burning contributed more to haze in winter.The contributions of SOA were smaller,while had a certain influence on the haze in summer.