Ten-year trend of PM2.5 sulfate, nitrate, and other inorganic constituents from 2012 to 2021 in urban and suburban sites of Hong Kong, China

Recent successes in reducing PM_2.5 concentrations in the atmosphere have led to significant shifts in the concentrations of various compounds. Sustaining the efficacy of control measures requires continuous monitoring of these changes. This study examined the decadal variations from 2012 to 2022 of concentrations of PM_2.5 and its inorganic components—sulfate, nitrate, ammonium, chloride, and non-volatile cations (NVC)—assessed via offline filters at an urban site (Yuen Long, YL) and a suburban site (HKUST, UST) in Hong Kong. We also cross-referenced the offline filter data with online MARGA results at both sites over a portion of the ten-year period for data validation. Our analysis indicates a consistent decline in PM_2.5 concentrations, registering an average reduction of 1.55 ± 0.16 μg/m³ (p < 0.01) per year in YL and 1.37 ± 0.17 μg/m³ (p < 0.01) per year in UST during the ten-year period. Inorganic compounds constituted 47.4 ± 10.1% and 51.5 ± 10.0% of PM_2.5 by mass in YL and UST, respectively, with sulfate accounting for over half of PM_2.5 at both sites. The decline in inorganic compounds over the years was primarily attributed to sulfate, which decreased at rates of 0.63 ± 0.04 μg/m³ (p < 0.01) per year in YL and 0.67 ± 0.05 μg/m³ (p < 0.01) per year in UST. While nitrate remained relatively steady in its concentrations, it constituted a larger mass fraction of both inorganic compounds and PM_2.5 at both sites. Seasonal variations were explored by comparing summer and winter trends. The rate of sulfate reduction in winter was approximately twice that in summer at both sites, contributing to ~40% of PM_2.5 reduction, as sulfate and PM_2.5 concentrations were significantly higher in winter. In contrast, nitrate concentrations exhibited an upward trend during winter, with notable increases from 1.87 μg/m³ to 7.63 μg/m³ in YL and from 0.75 μg/m³ to 3.47 μg/m³ in UST between 2020 and 2021, elevating its mass fraction in PM_2.5. In comparison, summer nitrate concentrations averaged below 1 μg/m³.

Our data validation indicated that offline filter-based nitrate measurements were underestimated under high-temperature conditions, casting high uncertainty on summer filter measurements. MARGA data revealed nitrate mass fractions as high as 28% in suburban UST in 2021, significantly greater than the 13% estimated from filter data due to this underestimation. This study highlights the escalating significance of nitrate alongside successful sulfate reductions in the PM_2.5 composition of both urban and suburban areas of Hong Kong, particularly during winter. Future air quality improvement policies should prioritize addressing nitrate. Furthermore, caution is warranted when interpreting nitrate concentrations measured by filters under high-temperature conditions due to the associated measurement uncertainty.