Organic Molecular Compositions And Source Apportionments Of Marine Aerosols Over The South China Sea

Marine aerosols consist of a great variety of organic compounds,whose compositions are very complicated.Marine organic aerosols are not only derived from oceanic emissions,but also influenced by continental outflow through long-range transport.The South China Sea?SCS?is the biggest mariginal sea of China.The chemical characteristics of marine aerosols over the SCS are greatly influenced by the emissions from its surrounding regions.However,to date,field measurements for the organic molecular compositions of marine aerosols over the SCS remain limited.Here,we collected day-and nighttime PM_2.5 samples at Weizhou Island,which is located in the northwest of the SCS.In addition,filter samples of total suspended particulate?TSP?were also collected during two cruise campaigns over the northern?2013?and western?2014?SCS.Organic compounds including n-alkanes,fatty acids,fatty alcohols,saccharides,lignin/resin products,sterols,hopanes,PAHs,aromatic acids,hydroxyl-/polyacids and biogenic SOA tracers were quantified by gas chromatography-mass spectrometry?GC-MS?.Other chemical components?OC,EC,water-soluble ions and stable carbon isotopes?as well as fire spots and air mass back trajectory analysis,tracer-based method and principle components analysis?PCA?were performed to discuss the possible sources and source apportionments of marine organic aerosols over the SCS.The major results are shown as follows:?1?Springtime biomass-burning activities in the northern Southeast Asia?SEA?significantly influenced the emissions and molecular compositions of marine organic aerosols in Weizhou Island.Saccharides and fatty acids were the major organic compound classes while levoglucosan was the most abundant organic species,which ranged from 4.33–517 ng m^-3(114±120 ng m^-3)and 2.31–955 ng m^-3(170±204 ng m^-3)in daytime and nighttime,respectively.?2?OC and EC showed two major peaks during the whole sampling period at Weizhou Island as marked as E1 and E2.Aerosols in episodes E1 and E2,which originated from the northern SEA,were observed with high concentrations of levoglucosan,suggesting the significant influence of biomass burning.However,the type of biomass burning as well as the sources and compositions of organic aerosols in E1 and E2 were different.Hardwood and/or C₄ plants were the major burning materials in E1,while burning of softwood and/or C₃ plants played important role in E2.Furthermore,organic aerosols in E1 were significantly influenced by the biomass burning,while more complex sources and enhanced secondary contribution were found to play a part in organic aerosols in E2.?3?The organic components in marine aerosols over the northern and western SCS were mainly influenced by the continental outflows from its sourrounding regions.Biogenic emissions?including higher plant waxes,biomass burning and terrestrial primary biological origin?and atmospheric secondary oxidation were the major sources of marine aerosols over the SCS.The concentrations of organic compounds in the samples which were greatly influenced by continental emissions were much higher than those in other samples.?4?The mixed continental sources including different terrestrial biogenic emissions,significantly contributed to marine organic aerosols over the northern SCS during 2013.The organic compounds correlated well with each other,suggeating their source regions and/or transport pathyways were similar.Besides,marine aerosols over the western SCS during 2014were mainly influenced by biomass burning?C₃ plants?from the SEA.