| Atmospheric dry and wet deposition is one of the important allochothous sources of marine DOC(dissolved organic carbon),while the atmospheric deposition of organic carbon in coastal cities is affected by both land and ocean,and the sources are complex.Therefore,the investigation of the source,fate,seasonal changes and influencing factors of organic carbon in coastal atmospheric deposition is helpful for understanding the role of atmospheric deposition in the biogeochemical cycles.In this paper,we conducted routine sampling and analysis of dry and wet precipitation from December 2018 to September 2019 and from September 2018 to September 2019 in Xiamen,respectively.Low molecular weight organic acids(LMWOAs)and dissolved black carbon(DBC)are used as labile and refractory molecular markers respectively,and the spectral characteristics of dissolved organic matter(DOM)and the inorganic anion parameters are combined to obtain information on the sources,fluxes,bioavailability,seasonal changes,and influencing factors of dissolved organic matter in Xiamen atmospheric precipitation.Based on observations of rainwater DOC and the molecular markers,we found that DOC,DBC,and LMWAs all have significant seasonal changes,and their flux and concentration changes are almost the same.DOC and DBC are mainly higher in spring and winter than in summer and autumn,with the peak appears in March to May for LMWAs.The high values of DOC and DBC may be.related to the transport of pollutant air masses in the north of china in spring and winter.The peak value of LMWOAs from March to May may be related to stronger release(both direct and indirect)of plants during this period in Xiamen.The main sources of DBC in Xiamen rainwater include soil dust and oxidation of soot;the main sources of LMWOAs include plant releases,automobile fuel combustion and soil dust;the sources of DOC mainly include plant releases,soil dust and automobile exhaust emissions.There are many factors that affect the concentration of DOM in rainwater,including rainfall(the concentration of dissolved substances is negatively related to rainfall(the dilution effect)):the greater the rainfall,the lower the dissolved substance concentration is),the source of the air mass(The concentrations of DOC,DBC,and LMWOAs all have significant differences with different backward trajectories.Generally speaking,they show land type>landsea mixed type>ocean type),local emissions(the higher concentration of pollutants in the northern china),and meteorological factors(high wind speed is conducive to the diffusion of pollutants).However,there were no significant differences in parameters such as LMWOAs/DOC,F/A(formic acid/acetic acid)for events with different backward trajectories.This indicates that the progress of human activities also releases a large amount of active organic carbon into the atmosphere.At the same time,we estimated that the annual fluxes of DOC,DBC,and LMWOAs in Xiamen rainwater to the sea were 1.72,0.013,and 0.33 g C m-2 year-1,respectively.The annual export flux of LMWOAs accounted for 19%of DOC.Through the investigation of microbial incubation experiments in rainwater,we found that the bioavailability of LMWOAs is mostly above 90%,and we quantified the percentage that active LMWOAs occupied 30%of the total active DOC in rainwater for the first time,This means that nearly 1/3 of the bioavailable DOC in rainwater is formic acid and acetic acid.The annual fluxes of BDOC and Bio-LMWOAs in rainwater in Xiamen Bay are 1.13 and 0.27 g C m-2 year-1 respectively,which account for 66%and 81%of the annual fluxes of Xiamen rainwater DOC and LMWOAs.The values of total suspended particulate matter(TSP)and water-soluble organic carbon(WSOC)in Xiamen aerosols are lower than those of inland cities and highr than those of coastal areas and oceans.And the concentrations of TSP,WSOC,WSBC(water-soluble black carbon)and the value of a254(absorbance at 254nm)all have significant seasonal changes,which are higher in spring and winter and lower in summer and autumn.The high values in spring and winter may be related to the transport of pollutant air masses from northern china and calm winds which are not conducive to the spread of pollutants.The low values in summer and autumn may be caused by the transport of clean ocean air masses,frequent scouring of rainfall and high wind speeds.At the same time,there are significant seasonal changes in WSOC/TSP,and this value is also significantly positively correlated with WSBC.Therefore,seasonal changes in WSOC/TSP may be caused by changes in temperature,rainfall frequency,and source(taking combustion activities as an example).The positive correlation between WSBC and B6/B5 in Xiamen aerosol may be caused by different main sources in different seasons.WSBC may come mainly from the burning of fossil fuels with a higher degree of aromaticity in Spring and winter.While in Summer and autumn WSBC mainly come from the resuspension of soils with lower aromaticity.The influencing factors of WSOC in Xiamen aerosol include backward trajectory(land type>land-sea mixed type>ocean type),particle size(WSOC mainly exists in fine particles),rainfall frequency(frequent rainfall is beneficial to erosion of WSOC)and meteorological factors(large wind speed is conducive to the diffusion of pollutants).At the same time,we compared Xiamen rainwater and aerosol.We found that the seasonal differences of aerosol optical parameters were significant,but there was no significant seasonal difference in rainfall SUVA 254 and S275-295.This may be related to the fact that rainwater is an environmental sample dominated by small molecular compounds,and the pH of rainwater will also affect it to some extent.The inter-annual change of optical parameters in Xiamen aerosol(decreased in S275-295)may be related to the intensification of human activities,which also indicates that the air quality is continuously deteriorating.Adding the comparison of the optical parameters and molecular markers of environmental samples from different regions such as rivers,estuaries,offshore and oceans,we find that aerosol contains larger molecular compounds and higher proportions of refractory compounds,while rainwater contains smaller molecular compounds and higher proportions of active compounds.Therefore,rainwater is very important for the bioavailability of the water environment system,and the atmospheric dry precipitation is an important carrier for removing refractory organic carbon from the atmosphere.It is also an important source of refractory organic carbon in the ocean and even has a significant role in the entire marine carbon cycle. |
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