[Role of Ammonia in Aerosol Liquid Water, pH, and Secondary Inorganic Aerosols Formation at an Ammonia-rich City in Changzhou].

Abstract

Ammonia （NH₃） is an important alkaline reactive nitrogen, which, as a precursor of fine particulate matter, raises public health issues. In this study, online NH₃, SO₂, NO₂, PM_2.5, and its water-soluble inorganic ions were detected to deduce the influence of NH₃ on aerosol liquid water content （AWC） and aerosol pH, including the formation of water-soluble secondary ions in PM_2.5 in winter in Changzhou, an ammonia-rich city in the Yangtze River Delta area in winter. The results showed that NH₄⁺ mainly existed in the form of NH₄NO₃ and （NH₄）₂SO₄, and the remaining NH₄⁺ existed as NH₄Cl. Owing to the NH₃-NH₄⁺ buffer system, the aerosol pH values were found at 4.2 ± 0.4, which was positively correlated with the NH₃ content. The aerosol pH value variation narrowed with the increase in PM_2.5 concentration and tended to be between 4 to 5. AWC increased exponentially with the increase in humidity and SNA content, among which NH₄NO₃, （NH₄）₂SO₄, and NH₄Cl contributed 58.5%, 18.4%, and 8.3%, respectively, due to their hygroscopicity. Aerosol pH, AWC, and NH₃-NH₄⁺ conversion promoted the gas-to-particle conversion of SO₂ and NO₂. In Changzhou, rich NH₃-NH₄⁺ were found to maintain relatively high pH values, push up AWC, and promote the heterogeneous reaction of SO₂, whereas NO₃^- generation was dominated by a homogeneous reaction, which was accelerated by NH₃. According to the simulation results, relatively noticeable changes in aerosol pH and AWC could be found by the reduction of up to 30% of NH₃.