Molecular And Spatial Distributions Of Dicarboxylic Acids And Related Compounds In Marine Aerosols

Land-ocean-atmosphere interaction is one of the most important issues in the earth system science.Current knowledge on the biogeochemical cycles of organic matter in the tropical marine atmosphere is very limited.Our observation,for the first time,provides detailed information on the atmospheric abundance and spatial distribution of dicarboxylic acids,oxoacids and?-dicarbonyls in marine aerosols collected from the South China Sea(SCS)to the Eastern Indian Ocean(EIO).Such information significantly contributes to the communities of atmospheric chemistry and chemical and climate models.In the present study,marine aerosol samples collected from the South China Sea to East Indian Ocean during a cruise from 10 March to 26 April 2015 were studied for dicarboxylic acids,oxocarboxylic acids,?-dicarbonyls,organic carbon(OC),elemental carbon(EC),water-soluble inorganic ions,water-soluble organic carbon(WSOC)concentrations as well as their excitation-emission matrix fluorescence properties.In view of the air mass backward trajectories and source regions of geographical features,the cruise area is segregated into the SCS,the East Indian Ocean off the coast of western Indonesia(EIO-WI),the EIO off the coast of Sri Lanka(EIO-SL),Malacca and the Sri Lanka docking point(SLDP).Total concentrations of diacids,oxoacids and?-dicarbonyls were much higher at SLDP followed by the SCS,Malacca,and the lowest at the EIO-WI.Oxalic acid(C₂)was found to be the dominant diacid during the cruise,followed by malonic acid(C₃)in the SCS,EIO-WI,EIO-SL and Malacca,whereas succinic acid(C₄)diacid was relatively more abundant than C₃ diacid in SLDP.Except for SLDP,C₃/C₄ mass concentration ratios were always greater than 1,and no significant difference was observed among the cruise.The C₂/C₄ and C₂/total diacids ratios also showed the similar trends.Average mass ratios of adipic acid(C₆)to azelaic acid(C₉)were less than unity except for at EIO-WI;the mass ratios of phthalic acid(Ph)to azelaic acid(C₉)were less than 2 except for at SCS.The concentrations of diacids were higher when the air masses originated from the terrestrial regions than those from the remote oceanic regions.The total carbon contents of diacids account for 12.0±5.75%(range 2.00–32.4%)of WSOC over the EIO.Whilst,the fractions of oxoacids and?-dicarbonyls in WSOC ranged from 0.03 to 1.22%(average 0.34±0.20%)and 0.02–3.12%(0.60±0.68%),respectively.In addition,a strong linear correlation was found between WSOC and C₂diacid,which suggests that the WSOC was mainly generated by secondary formation,rather than biomass-burning emissions associated with the Asian outflow.Based on the molecular distributions of organic acids,the mass ratios and linear correlations of selected compounds in each area,we found that the oxidation of biogenic volatile organic compounds(BVOCs)released from the ocean surface and subsequent photochemical oxidations were the main contributors to diacids,oxocarboxylic acids and?-dicarbonyls over the SCS to EIO.In addition,the continental outflow that enriched with the anthropogenic VOCs and their aging processes influenced the organic aerosol loading,particularly over the SCS.The emissions from local terrestrial vegetations as well as fossil fuel combustion and subsequent in-situ photochemical oxidation also played a prominent role in controlling the organic aerosols loading and molecular distributions of diacids and related compounds at SLDP.