Distribution Characteristics And Source Apportionment Of Carbon Aerosols In Qianhu Area,Nanchang

Carbon aerosols have adverse effects on the environment,climate and health.Mastering the distribution characteristics and source contribution of regional carbon aerosols is helpful to scientifically evaluate and prevent the impact and harm of carbon aerosols.In this paper,the concentration of black carbon in the atmosphere of Qianhu Lake in Nanchang in 2021 was monitored in real time by black carbon instrument,and the samples of PM_2.5 in the atmosphere was collected in the same area from March to December.The distribution characteristics and influencing factors of carbon components and water-soluble ions in PM_2.5 are analyzed,and the sources and contributions of regional carbon aerosols are quantitatively analyzed based on multiple source analysis methods/models.The main research conclusions are as follows:1.Distribution characteristics of carbonaceous aerosol（1）Distribution characteristics of BC:The average mass concentration of BC was1.96±1.13 ug·m^-3.The mass concentration of BC was the highest in January,while the lowest concentration of BC appeared in June,BC mass concentration were the highest in winter,and the daily variation trend of BC in each season and month was bimodal,the peak appears at 07:00-09:00 and 22:00-23:00 respectively.The average absorption coefficient of BC was 18.42±1.95 Mm^-1,the average extinction coefficient was137.67±2.74 Mm^-1,and the average contribution of black carbon absorption to atmospheric extinction was 13.38%.The weekend effect of BC was that the morning peak of working days in winter and summer was later than the morning peak of weekends,the evening peak of working days in spring was later than the evening peak of weekends,and the peak time of working days in autumn was basically the same as that of weekends.（2）Distribution characteristics of PM_2.5and OC,EC in PM_2.5:The average concentrations of PM_2.5,OC and EC were 73.69±31.15 ug·m^-3,12.60±6.45 ug·m^-3 and1.56±0.90 ug·m^-3.The seasonal characteristics of the concentrations of OC and EC were winter>autumn>spring>summer.The concentration of OC was the highest in winter,which was 5.29 ug·m^-3,but SOC/OC was the highest in summer,which was38%.Compared with the literature of different cities/regions inside and outside China,the concentration of carbon aerosol in Qianhu lake of Nanchang is medium and high,which needs more attention.（3）By comparing the BC concentration measured by the black carbon instrument with the EC concentration in the particulate matter sampled by the filter membrane,it can be seen that although there was a difference in the black carbon concentration measured by these two methods,in general,the monthly distribution characteristics are basically the same,reflecting the comparability and difference of black carbon/elemental carbon measured by optical method and thermal method.（4）Distribution characteristics of water-soluble ions in PM_2.5:The average mass concentration of total ions of 5 cations and 4 anions was 45.01±25.28 ug·m^-3,and the concentration order of water-soluble ions was NO₃^->SO₄^2->K⁺>Na⁺>NH₄⁺>Cl^->NO₂^->Ca²⁺>Mg²⁺.The sum of the average mass concentrations of NO₃^-、SO₄^2-、K⁺and Na⁺was 33.7±7.91 ug/m³,accounting for 74.23%of the total ions,which was the most important part of PM_2.5.2.Related factors affecting the concentration change of carbonaceous aerosolBC had high correlation with PM_2.5 and NO₂,indicating that PM_2.5,NO₂ and BC had high homology.BC was moderately correlated with SO₂ and CO,indicating that these two gaseous pollutants had the same contribution source as BC.O₃ was almost irrelevant to BC,which might be due to their different sources of production,O₃ comes from photochemical reaction,while BC mainly comes from primary emission.BC concentration was negatively correlated with temperature,relative humidity and wind speed,which was consistent with the situation that BC concentration is high in winter and low in summer.Higher wind speed is conducive to the diffusion of BC,and higher relative humidity is conducive to the removal of BC.OC and EC concentrations were negatively correlated with temperature,relative humidity and wind secular,OC/EC was positively correlated with temperature and wind speed and negatively correlated with relative humidity,and OC/EC was most significantly correlated with wind speed.3.Source analysis of carbon aerosol（1）Correlation analysisThe correlation coefficient between OC and EC was the highest in winter（R=0.70）,which indicates that the sources of OC and EC were relatively simple in winter.Meanwhile,the correlation coefficient between OC and EC became the lowest in spring,indicating the relatively complex sources of OC and EC in spring.The correlation between char EC and root EC was higher in autumn and winter,but not in spring and summer.There was a high correlation between Ca⁺and Mg²⁺,indicating that Ca⁺and Mg²⁺had high homology.Na⁺had good correlation with NO₃^-,SO₄^2-,and NH₄⁺had good correlation with NO₃^-and SO₄^2-,indicating that most of them exist in the form of Na NO₃、Na SO₄、NH₄NO₃and NH₄SO₄ in PM_2.5;The correlation between NO₃^-and SO₄^2-was significant,indicating that they came from the same source,reflecting the formation of secondary inorganic salts.（2）Characteristic ratio analysisAccording to the OC/EC（8.80）and char-EC/root-EC（1.09）ratios,the main contribution sources of carbonaceous aerosols in Nanchang Qianhu area were coal-fired emission,biomass combustion and motor vehicle exhaust emission.According to the ratio of NO₃^-/SO₄^2-（1.25）,the analysis shows that the mobile source has a significant impact on the atmospheric carbon aerosol in the Qianhu Lake area of Nanchang,which is closely related to the linear distance of the sampling point from the Qianhu section of Fengsheng Expressway about 100m.Compared with the NO₃^-/SO₄^2-ratio reported in other regions,the contribution of mobile sources to carbonaceous aerosols in Nanchang Qianhu region might be higher than that in Shenyang,Nanjing and Shijiazhuan,but lower than that of cities such as Suzhou and Mangshi,Yunnan.（3）Source analysis of other methodsBased on principal component analysis,the main contribution sources of carbonaceous aerosols in Nanchang were combustion source,vehicle tail gas source and soil source.Based on PMF method,combustion source,motor vehicle source,dust source and secondary source,and pollution source was the main contribution source;According to the backward trajectory cluster analysis,the carbonaceous aerosols in Nanchang were mainly affected by local sources in spring and summer,and less affected by foreign sources.In autumn and winter,they were mainly from the continuous accumulation of local sources,but also affected by foreign sources.