[Characteristics and Generation Mechanism of Secondary Inorganic Ions in PM2.5 in Changsha-Zhuzhou-Xiangtan City Group]

Abstract

Secondary inorganic ions, the main components of atmospheric PM_2.5, are a dominant contributor to haze formation. The detailed characteristics and main generation mechanism of secondary inorganic ions in PM_2.5 are still unclear in the Changsha-Zhuzhou-Xiangtan City Group, which is suffering from severe haze pollution, particularly in the autumn and winter seasons. For our study, we collected PM_2.5 samples in November 2020 and January 2021 from four urban sites in the Changsha-Zhuzhou-Xiangtan City Group. Secondary inorganic components such as SO₄^2-, NO₃^-, and NH₄⁺ in PM_2.5 were quantified. The average values(μg·m^-3) of ρ(SO₄^2-), ρ(NO₃^-), and ρ(NH₄⁺) in autumn and winter were(5.2±2.5) and(7.9±4.8),(4.1±2.2) and(7.2±4.2), and(17.1±10.5) and(7.8±5.2), respectively. During the heavy haze pollution events in winter, the sum of ρ(SO₄^2-), ρ(NO₃^-), and ρ(NH₄⁺)(SNA) contributed 72.7% to the growth of PM_2.5 mass concentration, and ρ(NO₃^-) accounted for 41.2%. This result suggested that the generation of NO₃^- was the key factor leading to the formation of winter haze pollution. In the polluted stage, high aerosol water content(AWC) promoted the rapid secondary generation of SNA, whereas adverse meteorological conditions also led to the accumulation of pollutants. The values of sulfur oxidation rate(SOR) and nitrogen oxidation rate(NOR) were still high in the dissipation stage. It indicated that the PM_2.5 concentration fell due to the reduction in primary emissions and favorable weather conditions in dissipation, instead of the weakening of secondary generation of SNA. Compared to that in autumn, the higher AWC concentration, pH value, and lower temperature in winter were the main factors for the higher ρ(NO₃^-)/ρ(PM_2.5) and NOR values in the Changsha-Zhuzhou-Xiangtan City Group. At the same time, the heterogeneous reaction was the main generation pathway of NO₃^-, when the AWC concentration was high in winter. Affected by aerosol pH value and generation rate, the liquid-phase oxidation reactions of H₂O₂ and SO₂ were the main generation pathways of SO₄^2- in autumn and winter in the Changsha-Zhuzhou-Xiangtan City Group. Compared to that in autumn, the higher AWC was more conducive to forming SO₄^2-, which led to higher SOR in winter.

Keywords: Changsha-Zhuzhou-Xiangtan City Group; haze pollution; pollution feature; secondary generation; secondary inorganic ions.