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Springtime is the biomass burning (BB) season in Indochina. BB emissions are transported to East Asia and Taiwan at elevations of between 2000 and 4000 m and then transported downward to the near-surface layer when weather conditions are favorable. In this study, a numerical model, the Weather Research Forecast with Chemistry (WRF–Chem) model with tracer simulation was employed to explore the downward transport mechanisms and evaluate their potential effects on the planetary boundary layer (PBL) and near-surface layer during the springtime (Feb.–April) in 2018. Ambient backscatter was measured through LiDAR, aerosol optical depth was measured by MODIS and Himawari-8 satellites and by ground observations from the stations of AERONET. The data were applied to identify the transport mechanisms and evaluate the performance of the model. We summarized the downward transport mechanisms surrounding Taiwan, including the thermal effects of mountain downslope circulations (MDC), frontal secondary circulation (FSC) downward transport, and the mixing of MDC and FSC (MMF). Over Taiwan, the contributions of BB emissions for the MDC to the near-surface layer (<500 m), PBL (<1500 m), and free atmosphere (1500–6000 m) were 0.4%, 4.7%, and >95%, respectively, during springtime in 2018. For the FSC (MMF) mechanisms, the BB contributions to the near-surface layer, PBL, and free atmosphere were 2.5% (3.3%), 15.7% (17.1%), and 84.3% (82.9%), respectively. MMF occurred the most frequently among these three mechanisms and had the largest effect on the near-surface layer and PBL. The downward contributions of BB in different regions, including Taiwan, southern China, and the East China Sea (ECS), were also evaluated. Southern China had the highest value (11.0% [2.1%]) in the PBL (near-surface layer), followed by Taiwan and the ECS (8.2% [1.2%] and 4.5% [0.4%]). Overall, this research fills a gap in our understanding of the mechanisms by which BB emissions are transported to the surface and how they affect air quality in East Asia in general and Taiwan in particular. • We analyzed the BB emissions transport from Indochina to East Asia in general. • The downward transport mechanisms of aloft BB emissions have been explored. • The Impacts of BB emission on the PBL over the study regions were discussed. [ABSTRACT FROM AUTHOR]

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]