[Changes in Secondary Inorganic Ions in PM_2.5 at Different Pollution Stages Before and After COVID-19 Control]

To investigate the change characteristics of secondary inorganic ions in PM_2.5 at different pollution stages before and after COVID-19, the online monitoring of winter meteorological and atmospheric pollutant concentrations in Zhengzhou from December 15, 2019 to February 15, 2020 was conducted using a high-resolution (1 h) online instrument. This study analyzed the causes of the haze process of COVID-19, the diurnal variation characteristics of air pollutants, and the distribution characteristics of air pollutants at different stages of haze.The results showed that Zhengzhou was mainly controlled by the high-pressure ridge during the haze process, and the weather situation was stable, which was conducive to the accumulation of air pollutants. SNA was the main component of water-soluble ions, accounting for more than 90%. Home isolation measures during COVID-19 had different impacts on the distribution characteristics of air pollutants in different haze stages. After COVID-19, the concentration of PM_2.5 in the clean, occurrence, and dissipation stages increased compared with that before COVID-19 but significantly decreased in the development stage. The home isolation policy significantly reduced the high value of PM_2.5. The concentrations of NO₂, SO₂, NH₃, and CO were the highest in the haze development stage, showing a trend of first increasing and then decreasing. The concentration of O₃ was lowest in the pre-COVID-19 development stage but highest in the post-COVID-19 development stage. The linear correlation between[NH₄⁺]/[SO₄^2-] and[NO₃^-]/[SO₄^2-] at different time periods before and after COVID-19 was strong, indicating that the home isolation policy of COVID-19 did not change the generation mode of NO₃^-, and the corresponding reaction was always the main generation mode of NO₃^-. The correlation between[excess-NH₄⁺] and[NO₃^-] was high in different periods before COVID-19, and NO₃^- generation was related to the increase in NH₃ or NH₄⁺ in the process of PM_2.5 pollution in Zhengzhou.