Exploration of the downward transport mechanisms of biomass burning emissions from Indochina at the low boundary layer in East Asia

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.