Seasonal variabilities in chemical compounds and acidity of aerosol particles at urban site in the west Pacific

Highlights: • Obvious seasonal variabilities were found for PM_2.5, PM_2.5-10 and nitrate. • Meteorology and transport played key role in allocation of aerosol phase nitrate in PM_2.5, PM_2.5-10. • Acidity of PM_2.5 was high in summer and low in winter. Mass concentrations of chemical compounds in both PM_2.5 (particle aerodynamic diameter, Dp < 2.5 μm) and PM_2.5-10 (2.5 < Dp < 10 μm), and acidity of aerosol particles were measured at an urban site in western Japan using a continuous dichotomous Aerosol Chemical Speciation Analyzer (ACSA-12) throughout 2014. Mass concentrations of both PM_2.5 and sulfate had distinct seasonal variabilities with maxima in spring and winter, mostly due to long-range transport with the prevailing westerly wind. Mass concentration of nitrate in PM_2.5 (fNO₃) showed an obvious warm-season-low and cold-season-high pattern as a result of both gas-aerosol phase equilibrium processes under high temperature conditions as well as transport. Nitrate in PM_2.5-10 (cNO₃) increased during long-range transport of dust, implying the great importance of heterogeneous processes at the surface of coarse mode particles. In this study, Δ[H⁺] (derived from the difference in pH of extract liquid with/without sampling) was used to indicate the acidity of particles. We found that acidity of particles in PM_2.5 (fΔH) was mostly positive with a maximum in August because of the large fraction of nitrate and sulfate. Acidity of particles in PM_2.5-10 (cΔH) was negative in winter and spring due to presence of alkaline matter from crustal sources. This study highlights the great importance of anthropogenic pollutants on the acidity of particles in the western Pacific Ocean and further impact on the marine environment and climate.