Quantitative analysis of influencing factors to aerosol pH and its responses to PM2.5 and O3 pollution in a coastal city

Abstract

Aerosol acidity (pH) plays an important role in the multiphase chemical processes of atmospheric particles. In this study, we demonstrated the seasonal trends of aerosol pH calculated with the ISORROPIA-II model in a coastal city of southeast China. We performed quantitative analysis on the various influencing factors on aerosol pH, and explored the responses of aerosol pH to different PM_2.5 and O₃ pollution levels. The results showed that the average aerosol pH was 2.92 ± 0.61, following the order of winter > spring > summer > autumn. Sensitivity tests revealed that SO₄²⁻, NH_x, T and RH triggered the variations of aerosol pH. Quantitative analysis results showed that T (37.9%-51.2%) was the main factors affecting pH variations in four seasons, followed by SO₄²⁻ (6.1%-23.7%), NH_x (7.2%-22.2%) and RH (0–14.2%). Totally, annual mean meteorological factors (52.9%) and chemical compositions (41.3%) commonly contributed the aerosol ΔpH in the coastal city. The concentrations of PM_2.5 was positively correlated with aerosol liquid water content (R² = 0.53) and aerosol pH (R² = 0.26), indicating that the increase in pH was related with the elevated NH₄NO₃ and decreased SO₄²⁻, and also the changes of T and RH. The O_x (O₃ + NO₂) was moderately correlated with aerosol pH (R² = -0.48), attributable to the fact that the proportion of SO₄²⁻ increased under high T and low RH conditions. The study strengthened our understanding of the contributions of influencing factors to aerosol pH, and also provided scientific evidences for chemical processes of atmospheric particles in coastal areas.