Chemical characteristics and formation mechanism of secondary inorganic aerosols: The decisive role of aerosol acidity and meteorological conditions

In recent years, there has been a growing concern about air pollution and its impact on the air quality and human health, especially for fine particulate matter (PM_2.5) and its associated secondary aerosols in urban areas. This study conducted a year-long field campaign to collect PM_2.5 samples day and night in an urban area of central Taiwan. Higher PM_2.5 mass concentrations were observed in winter (27.7 ± 9.7 μg/m³), followed by autumn (22.5 ± 8.3 μg/m³), spring (19.2 ± 6.4 μg/m³), and summer (11.0 ± 3.1 μg/m³). The dominant formation mechanism of secondary inorganic aerosols was heterogeneous reactions of NO₃^- at night and homogeneous reactions of SO₄^2- during the day. Additionally, significant correlations were observed between aerosol liquid water content (ALWC) and NO₃^- during nighttime, indicating the importance of aqueous-phase NO₃^- formation. The role of aerosol acidity was explored and a unique alkaline condition was found in spring and summer, which showed lower PM_2.5 concentrations than the neutralized condition. Under the neutralized condition, higher PM_2.5 concentrations were commonly found when combining the ammonium-rich regime with molar ratios of [NO₃^-]/[SO₄^2-] exceeding 1.6, suggesting the importance of reducing both NH₃ and NO_x. Furthermore, the results showed that reducing NH₃ should be prioritized under high temperature conditions, while reducing NO_x became important under low temperature conditions. Clustering of backward trajectories showed that long-range transport could enhance the formation of secondary aerosols, but local emissions emerged as the main factor driving high PM_2.5 concentrations. This study provides insights for policymakers to improve air quality, suggesting that different mitigation strategies should be formulated based on meteorological variables and that using clean energy for vehicles and electricity generation is important to alleviate air pollution.