Probing air pollution in the Taichung metropolitan area, Taiwan.Part 1: Comprehensive model evaluation and the spatial-temporal evolution of a PM2.5 pollution event.

Taichung City has become the second largest city in Taiwan, where PM 2.5 and O 3 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 (O 3) is closely involved in the formation of nitrate (NO 3 −) 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 O 3 and thus the NO 3 − concentration in Taichung City. [Display omitted] • The modeling of an intensive observation campaign is comprehensively evaluated. • The modeling performed well and reproduced meteorological and chemical fields. • Aloft return PM 2.5 layer subsidence could enhance pollution at night. • NO 3 − is a major species during nighttime PM 2.5 pollution events. • Reducing VOCemissions could decrease NO 3 − in the Taichung Basin. [ABSTRACT FROM AUTHOR]