| 摘要: | 泰國北部在春季因人為砍伐、森林大火和燃燒農作物，造成大量的生質燃燒氣膠排放，影響此區域的空氣品質，雖然近年來國際「七海研究計畫」 (The Seven SouthEast Asian Studies，7-SEAS)透過地面採樣、地面遙測、衛星遙測和模式模擬等技術，對泰北生質燃燒氣膠特徵與長程傳送特性有一定的掌握，但由於缺少高解析度的垂直現地量測，尚無法完整建構生質燃燒源區之氣膠在當地排放後如何向上傳送，並傳送至下游的機制。因此本研究為架構在7-SEAS的研究框架下，於2019年3月10-30日在泰國北部清邁府芳縣(Fang)進行密集無人機垂直觀測實驗，取得84組有效的高解析度氣象與氣膠垂直剖面資料，並搭配地面現地量測以及MERRA-2再分析資料，探討邊界層氣象與生質燃燒氣膠的交互作用，本研究運用密集的無人機垂直觀測資料進行分析，以補足前人對中南半島研究缺乏的垂直高解析度觀測，嘗試解析出泰國北部邊界層典型特徵，以及污染物從累積到消散的機制，對當地生質燃燒氣膠與邊界層的研究進展有極大幫助。
研究結果顯示，晨間於逆溫層下方約200-400公尺處出現東風伴隨高PM2.5濃度，此結構隨邊界層發展而向上抬升，午後較強的西風將亂流動量帶進山谷內的邊界層，此時西風與邊界層的交界面產生亂流並往下傳送，促使山谷內的污染物向上混和，有利PM2.5污染物垂直擴散，並藉由較高風速的西風持續向東傳送，使地面PM2.5濃度小時平均由11時的257 μg m-3下降至16時的137 μg m-3。然而上述過程在高污染日3/15、3/24、3/30並不明顯，主要是由於邊界層穩定的垂直結構使得混和作用不強，導致污染物無法向上混和而累積於約距地400公尺以下處，高污染日地面PM2.5濃度日平均分別為278 μg m-3、297 μg m-3、378 μg m-3。進一步藉由三個個案分析發現，主導高污染事件的綜觀天氣系統含700 hPa上空高壓環流、700 hPa西風帶和鋒面系統之間的時空演變，其中個案一與個案二的綜觀天氣系統特徵較為相似，皆是槽前的穩定西北氣流促使污染物的累積，以及槽後的西南風促使污染物的消散，而個案三則是受到位於槽前的西北氣流以及南海上方高壓環流所帶來的南風影響，使得污染物累積快速，嚴重影響當地空氣品質。整體而言，本研究運用密集的無人機垂直觀測資料進行分析，補足前人對中南半島研究缺乏的垂直高解析度觀測，對當地生質燃燒氣膠與邊界層的研究進展有極大幫助，為7-SEAS一系列研究提供嶄新的觀點。
;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. |
