Unveiling secondary inorganic PM_2.5 pollution in northern Taiwan: The role of aerosol acidity and transformation processes

Efforts are being made to manage air pollution in Taiwan. To address the concern of anthropogenic aerosol loadings, it is imperative to comprehensively monitor and analyze the physicochemical and thermodynamic properties of secondary inorganic PM_2.5 constituents. This study focuses on the high-resolution inorganic aerosol chemistry in an urban area of northern Taiwan, using hourly measurements of trace gases and water-soluble inorganic ions (WSIIs), integrated with the ISORROPIA II thermodynamic model. Our findings indicate that aerosol acidity (pH) and aerosol liquid water content (ALWC) are crucial in controlling gas-to-particle phase partitioning of fine aerosols and PM_2.5 mass loadings in the ambient air. The relationship between ALWC and the conversion ratios, including sulfur oxidation ratio (SOR), nitrogen oxidation ratio (NOR), and ammonia conversion ratio (NHR), indicates that secondary ion formation increases with higher ALWC, thereby directly affecting air quality. The diurnal pattern of NOR highlights the significant role of photochemical reactions in NO₃^- formation during the day, while the nighttime increase in SOR levels, coupled with their daytime decrease, supports the occurrence of nighttime aqueous-phase oxidation driven by elevated ALWC. Peak neutralization of secondary anions by NH₄⁺ was observed during winter under NH₃-rich conditions, leading to a hygroscopic increase in PM_2.5 mass concentrations. The pH sensitivity analysis of regional atmospheric secondary PM_2.5 formation identified NO₃^- as the limiting factor. This suggests that reducing HNO₃ and precursor NO_x levels, rather than NH₃, would more effectively and quickly lower PM_2.5 concentrations. Implementing such a strategy could offer a cost-effective and impactful solution for managing high aerosol loadings in northern Taiwan.