Long-term trends and sensitivities of PM2.5 pH and aerosol liquid water to chemical composition changes and meteorological parameters in Hong Kong, South China: Insights from 10-year records from three urban sites

Abstract

Aerosol pH and aerosol liquid water (ALW) play key roles in regulating many atmospheric processes. In many countries, large reductions in SO2 and NOx emissions as a result of air pollution regulatory policies have led to improved air quality and significant changes in aerosol mass and chemical composition. Understanding the long-term trends and sensitivities of aerosol pH and ALW to changes in meteorological conditions and chemical composition is useful for predicting how atmospheric processes that depend on aerosol acidity and/or ALW will evolve as air pollutant emissions change over time. At present, long-term monitoring records of aerosol pH and ALW are limited in most parts of China and elsewhere in Asia due to the scarcity of long-term aerosol chemical speciation measurements acquired through regular sampling schedules and consistent analysis protocols in this region. We used a 10-year dataset (2011–2020) from three long-term monitoring sites to investigate the long-term trends of PM2.5 pH and ALW concentrations, identify key factors that drive the variations of PM2.5 pH and ALW concentrations, and investigate the sensitivities of PM2.5 pH and ALW concentrations to chemical composition changes during different seasons in Hong Kong, a highly populated urbanized city located in South China that has experienced improved air quality as a result of various effective air pollution regulatory policies. Despite the substantial decreases in PM2.5 concentrations and changes in chemical composition in this 10-year period, PM2.5 pH showed little change. The rates at which PM2.5 pH and ALW concentration changed ranged from −0.007 to −0.001 pH units/year and from −1.45 to −0.53 μg m−3/year, respectively. Clear and consistent seasonal oscillatory cycles were observed for both PM2.5 pH (∼2–∼4) and ALW concentrations (∼5–∼40 μg m−3) at the three sites during this 10-year period. PM2.5 was the most acidic in the fall, and was the least acidic in winter. ALW concentrations were the lowest in summer, and the highest in winter/spring. Sensitivity tests revealed that chemical composition, especially particulate sulfate concentrations, plays a major role in driving the variations in aerosol pH and ALW concentration during each season. However, the magnitudes to which aerosol pH and ALW concentration respond to changes in chemical composition depend strongly on the meteorological conditions in each season. Given that many urbanized cities in South China share similar air quality and meteorological characteristics as Hong Kong, the results presented here are useful for advancing our understanding of how air pollution regulatory policies can alter aerosol pH and ALW concentrations, and consequently aqueous aerosol chemistry, in other South China cities.