Probing air pollution in the Taichung metropolitan area, Taiwan.Part 1: Comprehensive model evaluation and the spatial-temporal evolution of a PM_2.5 pollution event

Taichung City has become the second largest city in Taiwan, where PM_2.5 and O₃ pollution events occur frequently. The causes of these events urgently need a comprehensive and in-depth study. Therefore, a series of well-planned observation experiments were conducted in the city and its surrounding areas. The first experiment supported the current simulation study of the major causes of air quality deterioration in Central-Western Taiwan. First, we utilized the Weather Research and Forecasting (WRF)/Community Multiscale Air Quality (CMAQ) modeling system to simulate the intensive observation period (IOP) from 31 October 2021 to 22 November 2021. The performance of WRF-CMAQ is well justified by comparison with the observations. Next, we selected the PM_2.5 (particles with aerodynamic diameters equal to or <2.5 μm) pollution event occurring on 3–6 November 2021 for detailed analysis. The results reveal that the meteorological conditions of this event were the prevailing easterly/southeasterly winds blocked by the Central Mountain Range (CMR). Central Taiwan is located on the leeward side of the CMR and embedded in a calm wind area, which is especially favorable for the accumulation of PM_2.5 during the nighttime. At noon, the PM_2.5 concentration increased from the surface to the planetary boundary layer height (PBLH), even though the PBLH was overestimated. In the afternoon, the clean sea breeze would dilute the PM_2.5 in the Taichung Basin and the urban PM_2.5 plume would be transported to the mountains. The current simulation suggests that the aloft return PM_2.5 layer would subside along with the sinking PBLH and deteriorate the PM_2.5 concentration at night. At the downtown site of the IOP (UAPRS), from the trend of radicals and nitrogen oxides, ozone (O₃) is closely involved in the formation of nitrate (NO₃⁻) during both daytime and nighttime. If Taichung City is Volatile Organic Compounds (VOC)-limited according to our previous study, then reducing VOC emissions could decrease O₃ and thus the NO₃⁻ concentration in Taichung City.