Exploration of complex physical and chemical processes of a severe urban pollution episode over central Taiwan

Nitrate is a major inorganic aerosol and a dominant component during air quality events in central Taiwan. This study analyzes a haze episode with record-high PM2.5 levels, peaking at 110 µg/m³ in central Taiwan’s urban areas (UAPRS) on 4–5 November 2021. During this event, PM2.5 at UAPRS averaged 29.0 µg/m³ in the daytime and 89.7 µg/m³ at night. Notably, nitrate rose sharply from 4.4 to 39.0 µg/m³, accounting for 43.5% of the nighttime PM2.5 increase in central Taiwan on the event day.Simulation results indicated that the lee-side vortex, driven by the interaction between the ambient flow and the Central Mountain Range (CMR), facilitated the accumulation of pollutants, transporting them northward to the ocean and then returning as the ambient wind direction changed from easterly to southeasterly. Additionally, the swept-back plume in the afternoon, driven by the lee-side northwesterly flow and overlaid with urban pollution, was a key contributor to the first PM2.5 peak at 20:00-22:00 LST on November 4. The mechanisms study revealed that nitrate aerosol was dominant, with N₂O₅ hydrolysis playing a critical role in its formation in the nocturnal atmospheric chemistry. Furthermore, the convergence of the lee-side northwesterly flow with the mountain downslope wind at midnight, combined with the reduction in planetary boundary layer height, enhanced the second PM2.5 peak, which occurred between 02:00 and 03:00 LST on November 5. The findings of this study can be applied to other regions with similar complex topography, pollution environments, and comparable relief.