Sources,Molecular Composition And Seasonal Variation In Organic Aerosols In Tianjin City,North China

At present,the regional ecology and environment are concerned by the country and society as well as scientist.Air pollution has attracted much attention.The haze phenomenon,with fine particulate matter(PM_2.5)as the primary pollutant,is particularly prominent in the North China Plain.Tianjin,a typical coastal city,the prevailing northwest wind in winter and the air mass from the ocean in summer as well as the unique land and sea breeze circulation make the sources and molecular compositions of organic aerosols（OAs）in Tianjin have obvious seasonal and diurnal variations.However,there are few studies on OAs at the molecular level in Tianjin,and the analysis of seasonal and diurnal variations is quite limited.This study analyzed the winter-and summer-time PM_2.5 samples by using traditional gas chromatography-mass spectrometry（GC/MS）,discussing their abundances,seasonal and diurnal variations.Meanwhile,The possible sources of pollutants were further studied in combination with specific molecular markers and meteorological conditions,such as sea and land breeze circulation.In addition,the contributions of different sources to OC in atmospheric PM_2.5were evaluated based on tracer-method.The main conclusions of this study are as follows:1.In this study,the concentrations of aliphatic lipids（n-alkanes,fatty acids,n-alcohols）,sugars（anhydrosugars,primary saccharides,sugar alcohols）,phthalate esters,bisphenol A,phthalic acids,biogenic（isoprene,monoterpene,sesquiterpenes）and anthropogenic（toluene,naphthalene）secondary organic tracers were dectected.Aliphalic lipids were the most abundant of all the species,followed by saccharides,which accounts for more than 80%of the measured substances.2.Most of the determined organic compounds,such as aliphatic lipids,phthalate esters andβ-caryophyllene had higher concentrations in winter than in summer,and higher at night than in the daytime in winter,while opposite in summer,which may be attributed to the specific sea and land breeze circulation.In addition to local sources,the land breeze could carry anthropogenic sources from fossil fuels and biomass burning over mainland regions,such as the North China Plain（NCP）into Tianjin.In summer,the prevailing marine monsoon containing biological/marine sources during the daytime could arrive in Tianjin by the long-range transport.3.According to the ratios of OC/EC,the concentrations and seasonal variations in WSOC,there were large amounts of secondary organic aerosols（SOAs）in Tianjin PM_2.5.In detail,the PM_2.5 in summer may have more biological/oceanic sources,which may have been experienced the photochemical oxidation,while the products of the fossil fuel combustion were the main precursors of wintertime SOAs.Furthermore,we analyzed the stable carbon isotopic composition of total carbon(δ¹³C_TC)in this study,which suggest that the sources of PM_2.5 in winter mainly came from the combustion of coal,and C₃ plants as well as the formation of secondary photochemical oxidation.In summer,the C₃ plant burning（the burning of wheat straw）,the combustion of a small part of fossil fuels（mainly for the traffic emissions）and aged aerosols during the long-distance transportation were the dominant sources.4.Finally,we calculated the contributions of different sources to OC in PM_2.5 in Tianjin during winter-and summer-time based on the tracer-method.The total contributions of primary（fungal spores,plant debris,biomass burning）and secondary（biological and anthropogenic）sources were 24.8%（day）versus 27.6%（night）in winter,and 38.9%（day）versus 32.5%（night）in summer.There were large amounts of primary anthropogenic sources（the burning of fossil fuels and biomass）in wintertime PM_2.5 in Tianjin,while the SOA and biological/marine sources had high levels in summer.The above series of analyses and comparisons show that the pollution sources of PM_2.5 in Tianjin were more influenced by secondary organic aerosols,and the influences of formation and transmission of secondary organic aerosols on the source assessment should be fully considered in the assessment of atmospheric environment.