Pollution Characteristics, Sources And Light Absorption Characteristics Of Atmospheric Brown Carbon In The Northern Suburbs Of Nanjing

The chemical compositions of atmospheric fine particulate matter(PM_2.5)are very complex,including water-soluble inorganic ions and light absorbing organic carbon(mainly brown carbon),which have an important impact on human health and regional climate change.Although there have been many studies on the chemical compositions and sources of PM_2.5,the understanding of the chemical compositions,sources and light absorption properties of brown carbon is still limited.As an important chemical component of brown carbon,nitrated aromatic compounds are still unclear about their sources and light absorption characteristics due to the limitation of measurement technologies.Nanjing is one of the key areas of air pollution control in China,and the high humidity weather condition in winter is one of the key factors to induce the frequent formation of haze pollution.This paper focused on the chemical compositions,sources and optical characteristics of brown carbon aerosols in winter foggy days and nonfoggy days in the atmosphere of the northern suburbs of Nanjing by using the comprehensive off-line analysis technology of multi instrument coupling,including organic carbon/elemental carbon analyzer,total organic carbon/total nitrogen analyzer,ion chromatograph,ultraviolet-visible spectrometer,black carbon aerosol mass spectrometer and liquid chromatography-mass spectrometry.The main conclusions are as follows:(1)The mass concentrations of carbonaceous components and nitrated aromatic compounds were analyzed by organic carbon/elemental carbon analyzer and liquid chromatography-mass spectrometry.Compared with nonfoggy days(12.23?g/m³),the average mass concentration of organic carbon in foggy days(22.86?g/m³)had nearly doubled,indicating that the environmental and meteorological conditions in foggy days(such as high humidity and static weather)may promote the formation of organic aerosols and aggravate pollution.The total concentration of nitrated aromatic compounds in organic aerosols increased with PM_2.5,of which 4-nitrophenol accounted for the highest proportion,with an average of more than 40%,indicating that the environmental and meteorological conditions in foggy days were also conducive to the formation of nitrated aromatic compounds,and its concentration characteristics are similar to PM_2.5(total average concentration:105.20±59.71 ng/m³ in foggy days,51.24±33.32 ng/m³ in nonfoggy days).(2)The off-line high-resolution water-soluble organic aerosol mass spectrometry data were analyzed by positive definite matrix factor analysis,and the sources of five light absorbing organic aerosols(OA)were identified:traffic emissions OA(HOA),biomass combustion OA(BBOA),local organic matter OA(local OA)low oxidation OA(LO-OOA)and high oxidation OA(MO-OOA).Both foggy and nonfoggy days were dominated by primary source contribution.BBOA(49.0%)dominated in nonfoggy days and local OA(30.6%)dominated in foggy days.The greater contribution of LO-OOA in foggy days indicated that the liquid phase process played an important role in the formation of water-soluble low oxidizing secondary organic aerosols,while MO-OOA accounted for a large proportion in nonfoggy days,indicating that the secondary photochemical reaction process had an important impact on the formation of high oxidizing water-soluble secondary organic aerosols.The contribution of primary sources to the mass concentration of nitrated aromatic compounds was higher in foggy days(60.6%),while the contribution of secondary source was higher in nonfoggy days(51.2%).(3)The absorption characteristics of brown carbon was measured by UV-Vis spectrophotometer.It was found that the average light absorption coefficient of brown carbon in foggy days was 16.91±7.47 M/m,which was twice that in nonfoggy days(7.41±5.69 M/m).The average mass absorption efficiency of brown carbon was 1.07±0.27 m²/g.The mass absorption efficiencts in nonfoggy and foggy days were close.The primary sources had a greater effect on the light absorption of brown carbon.Nitrated aromatic compounds had the largest contribution to light absorption near 330 nm and 340nm in foggy and nonfoggy days,respectively.And the average value of the maximum contribution in nonfoggy and foggy days were 9.0%and 4.5%,respectively.Among the detected nitrated aromatic compounds,both in nonfoggy and foggy days,4-nitrophenol contributed the most to the light absorption(51.8%and 56.8%),followed by 4-nitrocatechol and 2-methyl-4-nitrophenol respectively,which reflects the important contribution of urban traffic emission sources to nitrated aromatic compounds in brown carbon.