Quantifying sensitivity of PM2.5 mass to ammonia and nitrate availability in Hong Kong based on four-year hourly measurements

Abstract

Ammonium and nitrate are major components of PM_2.5, and their fractional contributions to urban PM_2.5 in China have increased in recent years, largely due to successful sulfate reductions. Owing to their semi-volatile nature, ammonium and nitrate in PM_2.5 are strongly affected by gas–particle partitioning, which depends on temperature, relative humidity, and the ionic composition of PM_2.5. Quantifying the sensitivity of PM_2.5 mass to precursor availability under locale-specific atmospheric conditions is therefore essential for air quality management. In this study, we analyzed four years (2013–2017) of hourly concentrations of inorganic ions and their gaseous precursors at an urban site in Hong Kong. We estimated aerosol water content (AWC) and aerosol pH and conducted a detailed seasonal analysis. Our results revealed distinct sensitivities of PM_2.5 to ammonia and nitrate availability, modulated by aerosol pH and AWC. In fall, sulfate-dominated PM_2.5 exhibited low sensitivity to HNO₃, while in winter, increased partitioning ratios led to higher PM_2.5 sensitivity to HNO₃. Quantitative assessment showed that reductions in water-soluble inorganic PM were proportional to decreases in total nitrate (TNO₃, including gaseous HNO₃ and particulate nitrate) and sulfate, but exhibited a parabolic relationship with reductions in total ammonia (TNH₃, including gaseous NH₃ and particulate ammonium). As TNH₃ levels are further reduced, the sensitivity of PM_2.5 to NH₃ would increase. Our findings highlight the importance of synergistic reduction of NH₃ and NO_x emissions in effectively mitigating PM_2.5 pollution. This analysis provides valuable insights to inform the development of targeted, integrated air quality management strategies for urban environments.