| Nitrogen oxides are the key factors in the formation of complex air pollution in China in recent years,mainly from fossil fuel combustion,vehicle exhaust,biomass combustion,and microbial nitrogen cycling in agricultural soils.Nitrate is one of the main secondary inorganic components of atmospheric fine particulate matter(PM2.5),and the conversion of NOx to nitrate will cause the accumulation of atmospheric particulate matter and the outbreak of haze.Therefore,the formation mechanism of NOx conversion to nitrate is studied,and quantitative analysis of NOx emission sources,which has important theoretical and practical significance for my country to study the formation and outbreak mechanism of atmospheric fine particles and provide a reference for urban haze pollution control.In this study,the air pollution status and composition characteristics of water-soluble ions in winter in Beijing from 2013 to 2018 and in Nanchang from November 2017 to January 2018 were respectively investigated.Quantitative analysis of the source,the main conclusions are as follows:（1）The order of winter atmospheric PM2.5 concentration in Beijing from 2013 to 2018was 2013>2016>2014>2018>2017.From 2013 to 2018,the atmospheric PM2.5 mass concentration in Beijing dropped by about 69.97%in winter as a whole,indicating that the Beijing government has taken a series of emission reduction measures to play a positive role in air pollution control.（2）From 2013 to 2018,the aerosol nitrate mass concentration in Beijing decreased by66.42%.When PM2.5<75μg·m-3,the OH pathway（gas-phase oxidation）was the main channel to generate nitrate aerosol;when PM2.5 exceeds 75μg·m-3,more than 80%of the nitrate is mainly generated by the N2O5 hydrolysis reaction（heterogeneous process）.Theδ18O-NO3-values in the clean atmosphere period were all lower than those observed in the moderate pollution and haze pollution periods,indicating that the relative contributions of different oxidation pathways in the process of NOx conversion to NO3-were different under different PM2.5 concentrations.（3）The Bayesian isotope model was used to analyze the emission sources of NOx under different air quality,and it was concluded that during the period of severe pollution,the contribution of vehicle tails to nitrate showed an upward trend from 2013 to 2018,while coal combustion and production Substance combustion has gradually decreased,which may be due to the gradual replacement of coal-fired heating by natural gas after Beijing’s implementation of the Clean Air Action Plan,resulting in a lower contribution from coal combustion.（4）From November 2017 to January 2018,the daily average concentration of atmospheric PM2.5 in Nanchang City in winter exceeded the national first-level standard and second-level standard,accounting for 84.62%and 24.18%of the total,respectively.The order of water-soluble ion concentration is NO3->SO42->NH4+>Cl->Ca2+>K+>Mg2+,in which the concentrations of NO3-,SO42-and NH4+account for 68.07%of the total water-soluble ions.The main sources of nitrogen and sulfur in atmospheric PM2.5in Nanchang in winter are mobile sources（such as vehicle exhaust）,supplemented by stationary sources.（5）Through Bayesian model analysis,it is concluded that vehicle exhaust and biomass burning are the main sources of nitrate aerosols in Nanchang in winter.Motor vehicle exhaust has become one of the main sources of nitrates in Nanchang in winter,which may be related to the increase in the number of cars in Nanchang. |
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