Interannual Variation Of Chemical Composition Of Atmospheric Precipitation And Source Of Nitrate In Nanchang

In 2019,Jiangxi Province put forward the"strategy of strengthening the provincial capital"and promoted the construction of Nanchang metropolitan area,which indicates that the economic development of Jiangxi Province and Nanchang will usher in new changes and have an important impact on the atmospheric environment of Nanchang.The shutdown caused by the outbreak of COVID-19 in early 2020 has further changed the atmospheric environment of Nanchang.Therefore,in the context of the combination of epidemic prevention and control and economic strategies,atmospheric environment issues may take a new turn.Therefore,in this study,the chemical composition of atmospheric precipitation in Nanchang city from November 2020 to November 2021 and nitrate nitrogen and oxygen isotope in rainwater samples collected by our research group from2016 to 2017 were determined.The seasonal and interannual variation characteristics of atmospheric precipitation and the influencing factors of chemical composition change in Nanchang city were analyzed.The seasonal and interannual variation characteristics of atmospheric precipitation and the influencing factors of chemical composition change in Nanchang city were analyzed.Meanwhile,the enrichment factor method,nitrogen and oxygen isotope and Bayesian model were used to analyze the chemical composition and the source of nitrate in precipitation respectively.The results showed that:（1）NH₄⁺、SO₄^2-、NO₃^-and Ca²⁺are the most important four ions in atmospheric precipitation in Nanchang city,accounting for more than 80%of the total ion concentration.SO₄^2-and NO₃^-are the most important anions,accounting for 46.1%and 36.6%of the total concentration of anions,respectively.NH₄⁺and Ca²⁺are the most important cations,accounting for 56.3%and 27.8%of the total cation concentration,respectively.（2）During the sampling period,the concentration of total inorganic ions in precipitation in Nanchang was the highest in winter,close to that in spring and autumn,and the lowest in summer.The concentration of total inorganic ions is winter>spring>autumn>summer and there are obvious seasonal changes.（3）Compared with 2016-2017,precipitation pollution in Nanchang showed significant changes.Moderate and above polluted precipitation types decreased by 34.1%,while clean precipitation types increased by 23.3%.Precipitation pollution types in Nanchang gradually changed to clean and mildly polluted precipitation.（4）From 2007 to 2021,there is a significant difference in the change of acid ions in precipitation in Nanchang.SO₄^2-decreases by 38.8-61.6%,while NO₃^-does not change significantly,which may be related to the implementation of emission reduction policies and the increase of motor vehicle emissions in China in recent years.（5）According to the calculation results of enrichment factor method and source contribution method,the contribution of all ions to total ion concentration in precipitation in Nanchang city mainly comes from human activities,accounting for 75.02%,crustal sources accounting for about 20%,and Marine sources accounting for only 4.92%.The contribution of Cl^-ions was 57.01%and 41.87%,respectively.SO₄^2-and NO₃^-ions mainly come from anthropogenic sources,accounting for more than 97%,while Marine and crustal sources contribute less than 2%,which can be ignored.K⁺ions are mainly terrestrial sources,and their contribution is 98.51%.Mg²⁺ions mainly came from soil sources,accounting for 87.14%and 12.86%,respectively.（6）The averageδ¹⁵N values of precipitation are positive in spring and autumn,and negative in summer and winter,showing obvious seasonal variation.This change may be caused by the dual effects of higher vehicle exhaust emissions in summer and winter precipitation and higher coal combustion sources in spring and autumn precipitation.（7）In summer and autumn,the conversion of NO_xto nitrate is mainly through reaction with OH.In winter,NO_xis mainly oxidized with O₃to form N₂O₅.（8）The results of isotopic analysis and Bayesian model showed that nitrate in autumn and winter precipitation in Nanchang mainly came from biomass combustion,motor vehicle exhaust emissions and coal combustion,which contributed more than 86%.Vehicle exhaust emissions and biomass combustion are the most important sources,both contributing more than 30%.Although coal combustion is also an important source,it accounts for about 23%of emissions compared with biomass combustion and vehicle exhaust emissions.