Geochemistry Study On Tropical Marine Carbonaceous Aerosols Over The South China Sea And East Indian Ocean

Tropical marine aerosols play an important role in ecosystems,biogeochemistry,meteorology,climate,and atmospheric chemistry.Although the ocean covers?70.9%of the Earth surface,the atmospheric outflow from land is an important source of marine aerosols.The input of land aerosols hence affects the loadings,compositions,and physicochemical properties of marine carbonaceous aerosols,and is related to marine aerosol radiative forcing,regional climate,extreme events,and the oceanic carbon cycle.Therefore,the research on the sources,carbon compositions,and optical properties of tropical marine aerosols would deepen the understanding of the biogeochemical process of marine carbonaceous aerosols,and benefit the assessment of global climate change.Given that Asia is the major source region of global carbonaceous aerosols,and that the South China Sea and the East Indian Ocean,being bounded by Asia to the north in the tropics,are the potential receptors of air mass outflow from Asia,the South China Sea and the East Indian Ocean are the ideal regions for geochemical research of tropical marine carbonaceous aerosols.To study the influence of aerosols outflow from Asia on the compositions and optical properties of tropical marine carbonaceous aerosols,and to explore the biogeochemical significance of aerosol black carbon(BC)deposition,this study carried out one aerosol cruise sampling campaign in the South China Sea and the East Indian Ocean during March to May 2016,and carried out two long-time aerosol sampling campaigns in the northern South China Sea(Xieyang Island,Jun 2015-May 2016)and the western South China Sea(Bachok Station,Malaysia,January 2016-February 2017),respectively.(1)To study the influence of land atmospheric outflow on the carbonaceous compositions and optical properties of tropical marine aerosols,and to study the differences in the source types/regions of different carbon components in marine aerosols,source apportionments of total organic carbon(TOC,the sum of organic carbon(OC)and BC),water-soluble organic carbon(WSOC)and its light absorption coefficient at 365 nm(Abs₃₆₅),condensed aromatic organic carbon(CAOC)and controlled polycyclic aromatic hydrocarbons(PAHs)were conducted in the present study based on organic molecular markers,Positive Matrix Factorization(PMF),potential source contribution function(PSCF),and multiple linear regression analysis.The results show that carbonaceous aerosols over the northern South China Sea were mainly from:(i)waste incineration/urban secondary organic aerosols(SOA),mainly originating from southern East Asia,followed by northern Southeast Asia;(ii)biogenic SOA,originating from both land and ocean were its source regions;(iii)microbial/plankton primary organic aerosol(POA),mainly originating from the South China Sea,followed by Southeast Asia;(iv)biomass burning POA/SOA,originating from southern East Asia and northern Southeast Asia;(v)fossil fuel combustion POA,originating from the South China Sea and southern East Asia.Different carbonaceous compositions have different source characteristics.TOC was mainly from microbial/plankton POA,followed by fossil fuel combustion and waste incineration/urban SOA.WSOC was mainly from waste incineration/urban SOA,followed by microbial/plankton POA.CAOC was mainly from waste incineration/urban SOA,followed by fossil fuel combustion.PAHs was mainly from biomass burning.It is noted that the continental outflow of biomass burning aerosols might be underestimated in our study due to the low altitude of the sampling site and the uplift and high-altitude transport of biomass burning aerosols from Southeast Asia.The influence of different sources on optical properties of water-soluble brown carbon(WSBr C)is different.WSBr C from waste incineration/urban SOA has a high light absorption efficiency at 365 nm(MAE₃₆₅=2.3±0.2 m²g^-1),and contributes to38.8%of Abs₃₆₅.WSBr C from microbial/plankton POA has a relatively higher MAE₃₆₅(1.7±0.2 m²g^-1),and is one of the main sources of Abs₃₆₅(24.6%).WSBr C from biomass burning(2.5±0.5 m²g^-1)and fossil fuel combustion(1.7±0.5 m²g^-1)have higher MAE₃₆₅values,but make low contributions to Abs₃₆₅(18.6%and 16.6%for biomass burning and fossil fuel combustion,respectively)as these two sources make low contributions to WSBr C.The contribution of biogenic SOA to light absorption of WSBr C was negligible.(2)To study the influence of source types on WSBr C in tropical marine aerosols,and to study the contributions of aerosol dissolved black carbon(DBC)to marine DBC,WSBr C and DBC in the aerosol samples collected at the Bachok Station of Malaysia were analyzed.The results of WSBr C show that the concentrations and light absorption of WSBr C were both predominated by biomass burning.HULIS was the main light-absorbing component of WSOC,and the proportion of HULIS in WSOC may be related to the relative contribution of biomass burning to carbonaceous aerosols.However,the increase in the relative contribution of biomass burning to the carbonaceous aerosols may reduce the conjugation degree of WSBr C,while the effect of fossil fuel combustion on the conjugation degree of WSBr C was opposite.The results of DBC show that DBC was mainly derived from biomass burning.Also,the ratios of DBC to,as well as the correlations of DBC with other carbonaceous components(including anhydrosugars,WSOC,OC,and BC)were source dependent,which implies an uncertainty of the current method for estimating the atmospheric deposition flux of DBC to the global oceans(F_DBC),based on a simple linear relationship beween DBC and WSOC.Considering that DBC and BC are exclusively from combustion sources,and that DBC and BC show significant correlations and different ratios in aerosols where biomass burning and fossil fuel combustion are dominant combustion sources,respectively,this study suggests a novel approach to estimate F_DBC based on the ratio of DBC/BC.The F_DBC estimated in this study was twice as that of the previous report,and DBC in aerosol and seawater show similar conjugation degrees.Consequently,the atmospheric deposition is likely to be a more important source of the marine DBC.In addition,given that biomass burning contributed up to 94%of F_DBC,marine carbonaceous aerosols may be a younger carbon pool of DBC in surface seawater.(3)To study the geographical distribution and sources of tropical marine aerosol BC,and to explore the evidences of aerosol BC as the source of marine DBC,BC and its stable carbon isotope signature(?¹³C)in aerosols over the South China Sea and East Indian Ocean were quantified by hydropyrolysis(Hy Py)technique.The concentrations and?¹³C of aerosol BC show large spatial heterogeneity,indicating that sources of aerosol BC vary between different sea areas.During the sampling period,aerosol BC over northern coastal of the South China Sea was mainly from fossil fuel combustion in South China.While aerosol BC over the central waters of the South China Sea,originating from northwest Pacific or the Philippines,was affected by the burning of coal and C₄ plants.The aerosol BC over Sunda Strait mainly came from the South China Sea.The aerosol BC over the Malacca Strait was strongly influenced by combustion of C₃ plants and peatlands in Southeast Asia.The aerosol BC over East Indian Ocean received fewer land aerosols and reflected the background BC levels of marine aerosols.The similar?¹³C between BC in aerosols and DBC in seawater indicates that BC deposition of marine aerosols may be an important source of marine DBC.