| dc.description.abstract | Air quality in Southeast Asia is heavily impacted by biomass burning aerosols emitted from forest fires and burning crops. Although “Seven South East Asian Studies” (7-SEAS) has used ground sampling, ground and satellite remote sensing, and model simulations to characterize aerosols and long-range pollution transport, due to a lack of high-resolution vertical in-situ measurements, the mechanism for lifting aerosols in the source area and transporting them downwind has not been fully elucidated. Therefore, this study built on the research framework of 7-SEAS, and conducted intensive UAV vertical observation experiments to explore the interaction between planetary boundary layer (PBL) meteorology and biomass burning aerosols in Fang, Chiang Mai, Thailand from March 10th to 30th, 2019. In total, 84 sets of high-resolution meteorological and aerosol vertical profile data were obtained and combined with ground in-situ measurements and MERRA-2 reanalysis data. This study aimed to analyze the typical PBL characteristics in northern Thailand, and the mechanisms of pollutant accumulation and dispersion.
The results showed that, in the morning, there were easterly winds and high PM2.5 concentration below the inversion layer, and this structure was lifted up with the development of the PBL. In the afternoon, strong westerly winds brought turbulent flow into the PBL in the valley. The turbulent flow was transmitted downwards and promoted the upward mixing and dispersion of pollutants. Meanwhile, westerly winds continued to transported the pollutants eastward. The average hourly ground PM2.5 concentration was dropped from 257 μg m-3 at 11 a.m. to 137 μg m-3 at 4 p.m. However, this process was not obvious on high pollution days on 3/15, 3/24, and 3/30, primarily due to a stable vertical structure of the PBL, which led to weak mixing and poor dispersion; pollutants accumulated below about 400 m. The daily average concentrations of ground PM2.5 on high pollution days were 278 μg m-3, 297 μg m-3, and 378 μg m-3, respectively. Through further analysis of three case studies, it was found that high-pollution events can be described by the spatiotemporal evolution between a high-pressure system at 700 hPa, westerly wind at 700 hPa and a frontal system over the region. In Case 1 and 2, a northwest wind behind the trough promoted the accumulation of the pollutants and a southwest wind behind the trough promoted the dispersion of the pollutants, while in Case 3 a northwest wind behind the trough and south wind brought by a high-pressure system above the South China Sea, which to greater pollutant accumulation and seriously degraded local air quality. On the whole, this study used intensive UAV vertical observation data for analysis, which were lacking in previous research efforts in Southeast Asia. This work represents significant research progress for local biomass burning aerosols and PBL issues, and provides a new perspective for Southeast Asia studies. | en_US |