Chemical-composition characteristics of PM1 and PM2.5 and effects on pH and light-extinction coefficients under different pollution levels in Zhengzhou, China

Abstract

Atmospheric particulate matter (PM) is a significant pollutant in the atmosphere. An increase in PM_2.5 concentration notably diminishes atmospheric visibility and leads to haze formation. PM₁, a type of atmospheric PM with smaller particle size, has a more pronounced effect on visibility and aerosol pH compared to PM_2.5. In this study, PM₁ and PM_2.5 samples were collected in August, November, and December of 2020, as well as in April of 2021 in Zhengzhou. The chemical composition, aerosol pH, and light-extinction coefficients (b_ext) of PM₁ and PM_2.5 were analyzed based on the aerosol concentrations in three pollution categories. Throughout the sampling period, the average mass concentrations of PM₁ and PM_2.5 were 56.2 and 71.9 μg m⁻³, respectively, with nitrate constituting 24.3% of PM_2.5. This highlights the severe submicron-level particulate pollution in Zhengzhou. Differences in the composition of PM₁ and PM_2.5 led to variations in aerosol water content, but the combined effect on aerosol pH was minimal. PM₁ displayed a stronger negative correlation with visibility because SNA (NO₃⁻, SO₄²⁻, and NH₄⁺) was primarily concentrated in PM₁. The b_ext of nitrate consistently increased during the pollution period, reaching 58% during the moderate pollution phase. This result suggests that the extinction contribution of nitrate was the main factor contributing to the decreased visibility in Zhengzhou. Reducing nitrogen oxide emissions can lower PM_2.5 concentrations and enhance visibility. Additionally, controlling nitrate production can achieve a secondary reduction in aerosol concentration by decreasing the aerosol water content, which in turn reduces the secondary formation of PM. This study is beneficial for formulating measures to reduce the concentration of PM_2.5 and reducing pollution.