Chemical Compositions And Optical Properties Of Atmospheric Particulate Matter In A Typical Coastal City

Fine particulate matter(PM2.5)is one of the current most important air pollutants,which affects human health,radiation balance and global climate.Coastal areas are located in a special zone between land and sea,and the chemical compositions and optical properties of atmospheric particulate matter in these areas are unique and complex due to the interaction of multiple pollution sources.However,the limited research on PM2.5 in coastal areas hinders the comprehensive understanding of aerosol components,characteristics and source/sink mechanisms,resulting in the inability to accurately evaluate the environmental and climate effects of atmospheric aerosols.Thus,it is necessary to conduct field observation studies on PM2.5 in typical coastal areas in order to deeply explore its chemical components as well as its properties,which are of great significance to improve the atmospheric research data in coastal areas,optimize the global climate model and formulate air pollution prevention and control measures.In this study,Qingdao,China was selected as the target area to represent coastal areas,and PM2.5 samples were collected during different periods.The concentration level and chemical characteristics of water-soluble ions(WSI)and water-soluble organic carbon(WSOC)were investigated.Ultraviolet and visible(UV-vis)absorption spectroscopy and excitation emission matrix(EEM)fluorescence spectroscopy were used to analyze the optical properties of watersoluble brown carbon(WS-BrC).At the same time,the impacts of marine and terrestrial emissions to WS-BrC were explored by combining field observation and laboratory simulation data,and expanded the application of EEM fluorescence spectroscopy combined with parallel factor(PARAFAC)analysis in explaining atmospheric chromophores and aerosol possible sources.The results are as follows:(1)The concentration of WSI accounted for 43.2%of winter PM2.5 in Qingdao,of which NO3-,NH4+ and SO42-contributed by 86.8%of the total ion mass concentration,and mainly in the form of(NH4)2SO4 and NH4NO3.Compared with previous studies,the mass concentration of NO3-exceeded that of SO42-.NO3-became the most important ion in WSI and dominated the formation of secondary inorganic aerosols,revealing that mobile sources have gradually become the main sources of air pollution.The concentration of Na+was significantly higher than that in inland areas due to the nearby marine emissions,however terrigenous sources were still the dominant sources of WSI in coastal areas.The contribution of WSOC to winter PM2.5 concentration in Qingdao was about 9.9%.Its sources were complex and included biomass combustion,marine source emissions and so on.At the same time,the high humidity in coastal areas also provided necessary conditions for the formation of WSOC via liquid-phase reaction.(2)The light absorption properties of WS-BrC presented obvious seasonal variations,but basically no diurnal differences.WS-BrC in winter had stronger light absorption capacity and wavelength dependence,which may be attributed to the different sources of WS-BrC in different seasons.The results of EEM-PARAFAC showed that the fluorescent components in WS-BrC were mainly related to humic-like chromophores with high unsaturation.And,due to the influences of atmospheric aging on primary marine aerosols in summer,the marine chromophore.may be transformed into other more oxygenated chromophores,making its contribution to the total fluorescence intensity in summer lower than that in winter.(3)Marine aerosols were successfully generated by the sea spray aerosol generation tank and the surface seawater of the Yellow Sea of China.It was found that the fluorescence peak position of marine aerosols was in good agreement with a chromophore in WS-BrC in this study,confirming that marine emissions do have a certain impact on WS-BrC in coastal areas.The results of source analysis further showed that WS-BrC originated from different terrestrial sources in different seasons in Qingdao.In winter,WS-BrC was significantly influenced by biomass combustion emissions.However,in summer when biomass burning was limited,WSBrC was mainly derived from secondary organic aerosols,and aging might reduce its light absorption capacity.