Distribution Characteristics And Light Absorption Characteristics Of Carbon Aerosols In Autumn And Winter In Qianhu Area Of Nanchang

Carbon aerosols can cause serious harm to global climate change,atmospheric visibility,air quality,radiative forcing and human health.It has become one of the research hotspots in the field of atmospheric environment in recent years.This paper aims to analyze the distribution characteristics and light absorption characteristics of carbon aerosols in the Qianhu Lake area of Nanchang.Atmospheric PM_2.5samples were collected in the Qianhu area of Nanchang from October 1,2022 to October 31,2022 and from December 3,2022 to December 16,2022.The mass concentrations of carbon components,water-soluble inorganic ions（WSII）and other chemical components in PM_2.5were measured.The distribution characteristics of carbon aerosols were analyzed,and their sources were analyzed based on the chemical components of PM_2.5.The light absorption characteristics of carbonaceous aerosols were quantitatively discussed by thermo-optical method and solvent extraction method.The main conclusions are as follows:（1）The mass concentrations of organic carbon（OC）and elemental carbon（EC）in winter(11.40,1.50μg·m^-3)were higher than those in autumn(10.01,0.72μg·m^-3),and those in daytime were higher than those in nighttime,reflecting the differences in emission source intensity or secondary carbon component formation at different times.OC₂,OC₃and OC₄accounted for a large proportion of TC in autumn and winter（79.3%,73.8%）.The mass concentrations of water-soluble organic carbon（WSOC）and methanol-soluble organic carbon（MSOC）in winter(6.79,8.28μg·m^-3)were higher than those in autumn(6.15,7.67μg·m^-3).In autumn and winter,MSOC/OC（73.37%,73.06%）was higher than WSOC/OC（58.50%,59.75%）,indicating that the extraction rate of brown carbon by methanol was higher than that by water.The sum proportion of NO₃^-,SO₄^2-,NH₄⁺and Na⁺in WSII（86%in autumn and 87%in winter）reflects that these four ion components are characteristic water-soluble ions.NO₃^-/WSII>SO₄^2-/WSII reflects the contribution of mobile sources in the region.（2）The results of OC-EC correlation and OC/EC ratio show that biomass combustion sources have a greater impact in autumn,and coal combustion sources have a greater impact in winter.The ratio of secondary organic carbon to OC（SOC/OC）in autumn and winter（0.62,0.67）showed that there were serious secondary pollution in autumn and winter.SOC/OC in daytime（0.69,0.61）was less than that in night（0.70）,indicating that the degree of secondary pollution at night was greater than that in daytime.The results of 8 carbon components show that the regional carbon aerosol is mainly derived from coal combustion,motor vehicle exhaust and biomass combustion.The results of Char-EC/Soot-EC ratio（0.92 in autumn and 3.11 in winter）showed that vehicle exhaust had a greater impact in autumn,and coal and biomass combustion had a greater impact in winter.The results of PMF source analysis showed that the main sources of PM_2.5in autumn and winter were biomass burning source,secondary source,motor vehicle exhaust and coal burning source.（3）The light absorption coefficient(b_abs)of particulate brown carbon（BrC）is different in autumn and winter.The b_abs,BrC,405(3.17 Mm^-1)in autumn was smaller than those in winter(6.88 Mm^-1).In autumn,b_abs,WSOC,365,b_abs,MSOC,365(3.14,4.65Mm^-1)were smaller than winter(5.12,8.23 Mm^-1).b_abs,MSOC,365were larger than b_abs,WSOC,365,indicating that there are a large number of non-polar brown carbons showing stronger absorption characteristics in MSOC.The light absorption contribution(R_BrC/BC)of brown carbon relative to black carbon was smaller in winter than in autumn,and increased with the decrease of wavelength.（4）In autumn and winter,the difference of particulate brown carbon Absorption?ngstrom Exponent（AAE）was small(AAE_405-980,BrCwere 8.01 and 7.93 in autumn and winter,respectively),while the difference of dissolved brown carbon AAE was significant(AAE_300-500,WSOCand AAE_300-500,MSOCwere 9.72 and 10.13 in autumn,AAE_300-500,WSOCand AAE_300-500,MSOCwere 7.13 and 6.22 in winter;high in autumn and low in winter),indicating that dissolved brown carbon in autumn is more wavelength-dependent;In autumn,AAE_300-500,MSOCis mostly higher than AAE_300-500,WSOC,indicating that MSOC may have stronger wavelength dependence than WSOC in autumn.（5）The mass absorption efficiency（MAE）of carbon aerosols in autumn and winter was also different.The MAE_BrC,405in autumn and winter were 0.32 and 0.64m²·g^-1,respectively.In autumn and winter,MAE_WSOC,365were 0.49 and 0.83 m²·g^-1,respectively,and MAE_MSOC,365were 0.59 and 1.09 m²·g^-1,respectively.The seasonal difference of brown carbon mass absorption efficiency（high in winter and low in autumn）reflects the difference of brown carbon source and formation mechanism in autumn and winter.In autumn,MAE_BC,405(18.75 m²·g^-1)was higher than that in winter(17.17 m²·g^-1).In general,the MAE of black carbon was higher than that of brown carbon.（6）Compared with the absorption characteristics of BrC obtained by different methods,the optical parameters of BrC measured by the thermo-optical method are greater than the results of the solvent extraction method,and the absorption characteristics measured by the thermo-optical method and the solvent extraction method in the longer wavelength band are closer.The results of radiative forcing show that although BC is still the main light absorption component,the light absorption effect of BrC in the atmosphere cannot be ignored.