Seasonal variations in PM2.5 levels in Hong Kong induced by eastern and western tropical cyclones

Air pollution, particularly PM2.5 (particles smaller than 2.5 μm), is a major environmental issue in densely populated cities like Hong Kong, as these particles can deeply penetrate the respiratory system. It has been known that tropical cyclones (TCs) can cause a significant increase in PM2.5 concentrations when positioned to the east of Hong Kong. However, the seasonal variations in how the proximity of TCs influences PM2.5 levels and aerosol optical properties in Hong Kong remain poorly understood. Using observations from 1999 to 2020, this study demonstrates that the impact of TCs on air pollution in Hong Kong is more pronounced during the summer (June to August) compared to the autumn season (September to November), primarily due to strong subsidence, surface warming, and the transport of fine-mode aerosols by northerly wind driven by eastern TCs. Conversely, western TCs efficiently remove pollutants in summer due to the influx of clean air from southerly winds, but this effect diminishes in autumn when background PM2.5 levels are high and TC-induced subsidence is absent. Further analysis of aerosol properties reveals that eastern (western) TCs tend to increase (decrease) the fine-mode fraction and decrease (increase) the fine-mode Angström exponent during the JJA season, whereas these differences are negligible during the SON season.