Impact of Nitrate Aerosols and Meteorological Dynamics on PM2.5 Pollution During an Urban Haze Event in Central Taiwan

The study investigated a significant haze event on November 4-5, 2021, in central Taiwan's urban areas, where PM2.5 levels peaked at 110 µg/m³, with nitrate as the dominant component, particularly at night.  Nitrate concentrations dramatically rose from 4.4 µg/m³ during the day to 39.0 µg/m³ at night, accounting for 43.5% of the PM2.5 increase. Key meteorological factors included the formation of a lee-side vortex due to the interaction between ambient airflow and the Central Mountain Range. This facilitated air pollutants accumulation over Taichung urban areas in central Taiwan, transporting them northward to the ocean and returning as wind patterns shifted from easterly to southeasterly.

N₂O₅ hydrolysis played a critical role in nighttime nitrate formation over Taichung city. Simulated nitrate production rates at lower elevations ranged from 6-13 µg/m³ during key stages. Nighttime surface cooling resulted in a lower mixing height and higher relative humidity, contributing to pollutant accumulation and enhancing heterogeneous N₂O₅ processes. These findings highlight the complex interplay between meteorological conditions and chemical processes in air quality degradation during haze events. Understanding these complex physical and chemical processes is essential for effective pollution management in urban environments, as they highlight the challenges and intricacies involved in mitigating air quality issues.