Source And Light Absorption Characteristics Of Water-soluble Organic Carbonaceous Aerosol In Nanjing

Water-soluble organic carbon is the important component of carbonaceous aerosol.The light-absorptive water-soluble organic carbonaceous aerosol,named as water-soluble brown carbon,absorbs solar radiation in the short-wave band and affects the atmospheric radiation transmission process.On a global scale,the radiative forcing caused by water-soluble organic carbon accounts for 25%of the radiative forcing caused by black carbon.However,most climate simulating models fail to take the light-absorption by water-soluble organic carbon into consideration,mainly due to the uncertainty of content,the diversity of sources,and the difference of its light-absorbing properties caused by the diversity of sources.Therefore,the accurate assessment of the water-soluble organic carbonaceous aerosols content,source and its influence on the light-absorbing properties is not only helpful to reduce the uncertainty of radiation forcing assessment,but also able to further direct the policy towards the global climate change.This study was carried out in Nanjing,a typical city in East China.Fine particulate matter and size-segregated particulate matter were collected and analyzed to obtain the seasonal characteristics of its chemical composition and light absorbing parameters.Combined with the obtained chemical composition,PMF and molecular tracer were used to analyze the source apportionment of particulate matter.The relationship between water-soluble organic carbonaceous aerosol sources and light absorption was quantitively determined by multiple linear regression.Among the various emission sources,the effects of biomass burning and primary biogenic emission on water-soluble organic carbon were paid close attention to.Finally,a pollution event was taken as an example to quantitatively explain the influence of biomass burning emissions transported by long-range transport on the composition and light absorption capacity of carbonaceous aerosols in Nanjing.The main conclusions are as follows:(1)The sources of water-soluble organic carbon in fine particulate matter in Nanjing were mainly:biomass burning,secondary+sea-salt source,resuspended soil and primary biogenic emission.Biomass burning was the main contributor,followed by secondary+sea-salt source.The contribution of primary biogenic source was significant in summer and spring.Resuspended soil barely made a contribution in winter.The annual and seasonal pattern of source contribution to absorption coefficient of water-soluble organic carbon was similar to that to the level of water-soluble organic carbon,except that resuspended soil barely released light-absorptive water-soluble organic carbon.The mass absorption efficiency of water-soluble organic carbon from different emission sources was estimated by multiple linear regression.Among all sources,biomass burning released the water-soluble organic carbon with the highest mass absorption efficiency(1.22 m2 g-1),followed by secondary+sea-salt source(0.90 m2 g-1).The mass absorption efficiency of water-soluble organic carbon from primary biogenic source was surprisingly notable(0.71 m2 g-1).While the water-soluble organic carbon from resuspended soil showed a poor light-absorbing capacity(0.33 m2 g-1).(2)Sources like fossil fuel combustion,biomass burning,secondary formation and fungal spores had important effects on the light-absorbing coefficient,mass absorption efficiency,and light-absorbing wavelength dependence of water-soluble organic carbon.Sea-salt source and resuspended soil source barely contributed light-absorptive water-soluble organic carbon.The light-absorptive water-soluble organic carbon emitted from fossil fuel combustion,biomass burning,secondary source and fungul spores was mainly concentrated in fine particulate matter.Anhydrosugars from biomass burning were mainly with the size<2.1 ?m.Sugar alcohol from fungi was mainly distributed in coarse particles in spring and summer.In addition to primary biogenic emissions,other sources such as combustion also released some sugars and sugar alcohols.Therefore,it may cause estimation errors to directly use these saccharides as primary biogenic tracers in research.(3)During a wintertime pollution event in Nanjing,biomass burning contributed approximately 20%organic carbon and water-soluble organic carbon in PM2.5.The biomass burning originated from straw burning activities in southeast China,and atmospheric regional transport brought the emission pollutants to Nanjing.The average mass absorption efficiency of water-soluble organic carbon at 365 nm was 1.0±0.2 m2 g-1.The radiative forcing caused by water-soluble organic carbon was about 25%of that caused by black carbon.The radiative forcing caused by water-soluble organic carbon due to biomass burning was about 0.2 W m-2,reaching up to 0.9 W m-2.This result indicates that the carbonaceous aerosol emitted by biomass burning can absorb solar radiation,especially that in near ultraviolet band,after long distance transport and aging process,accordingly affected the atmospheric radiation balance.The spatial distribution of potential source contribution factor of carbonaceous aerosol concentration and light absorption parameters further shows that in this pollution event,the long-range transported biomass burning emissions greatly affected the chemical composition and light absorbing property of carbonaceous aerosol in Nanjing.