Long-term Evolution Of Pollution Characteristics Of Ionic Components In Atmospheric PM_2.5 In The Urban Area Of Jiaozuo

Our country’s air pollution problem has improved since the implementation of the“Air Pollution Prevention and Control Action Plan”in 2013,but the PM_2.5 pollution problem persists.In order to explore the seasonal differences and interannual evolution of the characteristics of water-soluble ions in PM_2.5,this study took Jiaozuo City as an example,and 188 samples were collected seasonally from 2020 to 2021 on the 5th floor of College of Resources and Environment,Henan Polytechnic University,and used 171historical test data of water-soluble ion concentrations collected at the same sampling site during winter from 2015 to 2019.Seasonal variations and interannual evolution of water-soluble ion pollution characteristics in Jiaozuo were studied in four dimensions:ion concentration,eigenratio,existence form of secondary ions,and source analysis.The main conclusions are as follows:（1）During the sampling period from 2020 to 2021,the annual average concentration of PM_2.5was（74.27±41.32）μg·m^-3,and the concentration of PM_2.5 was significantly（P<0.05）higher in winter than in other seasons.The concentration of PM_2.5 was higher in winter than in summer 1.9 times.The annual average concentration of water-soluble ions from was（39.78±27.61）μg·m^-3,and the ion concentrations from high to low was NO₃^->SO₄^2->NH₄⁺>Ca²⁺>Cl^->Na⁺>K⁺>F^->Mg²⁺.Except for Na⁺,the concentrations of other ions were the highest in winter,and the highest season was 1.9-7.6 times that of the lowest season.The sulfur oxidation rate was only less than 0.25 in autumn,and the nitrogen oxidation rate was greater than 0.10 in all seasons,indicating that there were high secondary reactions in spring,summer and winter in Jiaozuo.The mass concentration ratio of nitrate to sulfur were significantly higher in autumn（P<0.05）than in other seasons.The contribution of fixed pollution sources was dominant in summer,and the contribution of mobile pollution sources was dominant in other seasons.The annual average value of the anion to cation charge ratio was 1.16 and the atmospheric particulate matter was weakly alkaline.NH₄⁺was mainly existed as（NH₄）₂SO₄ and NH₄NO₃ in spring and summer,and as（NH₄）₂SO₄,NH₄NO₃ and NH₄Cl in autumn and winter.（2）During the sampling period from 2020 to 2021,the concentration of PM_2.5showed a downward trend but did not reach a significant level,with an average annual decreasing speed of 20.10μg·m^-3·a^-1.The concentrations of SO₄^2-,Cl^-,K⁺,and F^-showed a significant（P<0.05）decreasing trend,and the average annual decreasing speed was 5.36,0.81,0.23,0.11μg·m^-3·a^-1,respectively.The mass concentration ratio of nitrate to sulfur showed a very significant（P<0.01）upward trend,and the average annual upward speed was 0.34 a^-1,less than 1 from 2016 to 2017,and greater than 1 from 2017 to 2020,indicating that mobile sources surpassed stationary sources to become the main sources of sulfur and nitrogen in the atmosphere,and the contribution had continued to increase since 2017.During the pollution period,the proportion of SO₄^2-continued to decrease,and the proportion of NO₃^-continued to increase.The proportion of NO₃^-exceeded SO₄^2-and became the largest proportion of water-soluble ions since 2017.（3）Pollution sources of PM_2.5 were identified by clustering analysis and factor analysis from 2015 to 2020 as follow:natural dust source（20.1%）>motor vehicle exhaust source（17.8%）>coal burning source（16.1%）>industrial emission source（15.1%）>biomass combustion（12.7%）.In terms of the relative importance of pollution sources,industrial emission sources showed a decreasing trend,and motor vehicle exhaust sources showed an increasing trend.Jiaozuo changed from coal-fired pollution dominated by stationary pollution sources to exhaust-based pollution features dominated by mobile pollution sources.The proportion of wind coming from the northwest increased greatly from 2017 to 2018.According to the seasonal differences and interannual evolution of water-soluble ion pollution characteristics,recommendations for refined prevention and control of air pollutants are put forward.