Pollution Properties of Water-Soluble Secondary Inorganic Ions in Atmospheric PM2.5 in the Pearl River Delta Region

ABSTRACT

Based on the online observation of PM_2.5 mass concentration, its water-soluble inorganic ions, and their gaseous precursors during August of 2013 to March of 2014 at the atmospheric supersite in the Pearl River Delta (PRD) region, the inter-action of the secondary compositions and their precursors was discussed, and the pollution properties of the secondary inorganic ions were revealed. During the whole measurement period, the average concentrations of SO₄^2–, NO₃^– and NH₄⁺ were 16.6 µg m^–3, 9.0 µg m^–3 and 10.2 µg m^–3, respectively, with total contribution to PM_2.5 of 55.8%, indicating the significant role of secondary transformation in PM_2.5 pollution. The seasonal average total contributions of SO₄^2–, NO₃^– and NH₄⁺ to PM_2.5 varied from 46.0% to 64.3%, lowest in summer and highest in winter. The contributions of SO₄^2– and NH₄⁺ to PM_2.5 were relatively stable; while those of NO₃^– in different seasons were distinct, even dominating PM_2.5 in some pollution cases in winter. NH₃ was abundant with an annual average concentration of 15.2 µg m^–3, facilitating the neutralization of H₂SO₄ and HNO₃ with the average [NH₄⁺]/(2[SO₄^2–] + [NO₃^–]) equivalent charge ratio of 1.1. The maximum daily peak concentration of HNO₃ was as high as 18.6 µg m^–3, providing an evidence for the strong oxidizing property of the atmosphere in the PRD region. The theoretical equilibrium constant (K_e) of NH₄NO₃ is always lower than the observed concentration product (K_m = [NH₃] × [HNO₃]) in spring and winter with higher HNO₃ concentrations; while in over 60% of the time during summer and autumn, mainly during daytime, K_e was higher. In general, the strong oxidizing property and NH₃ played important roles in the fine particle pollution in the PRD region.