PBL 發展和山谷風環流對泰北生質燃燒霾害日夜變化的影響;Effects of PBL Development and Mountain-Valley Circulation on Diurnal Variation of Biomass Burning Haze in Northern Thailand

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Title:	PBL 發展和山谷風環流對泰北生質燃燒霾害日夜變化的影響;Effects of PBL Development and Mountain-Valley Circulation on Diurnal Variation of Biomass Burning Haze in Northern Thailand
Authors:	阿彌陀;Bhujel, Amit
Contributors:	大氣科學學系
Keywords:	邊界層(PBL);山谷風環流;微型微脈衝光達;低成本微型感測器;氣膠光學厚度;Aerosol optical depth;Low-cost PM2.5 measurement;PBL;Mountain-valley breeze;Diurnal pattern
Date:	2023-06-08
Issue Date:	2024-09-19 14:10:53 (UTC+8)
Publisher:	國立中央大學
Abstract:	春季生質燃燒為眾所周知的東南亞細懸浮微粒(PM2.5)主要污染源。為了解泰國北部(清邁省)生質燃燒好發期間 (2019 年春季) ，生質燃燒氣膠於不同地形、時間與空間的分布情形，本研究利用低成本 PM2.5 微型感器系統 (Aerobox)、氣膠遙測觀測網 (AERONET) 網絡進行監測，部署於泰北山區的三個地點：山頂測站 (Doi Ang Khang) 、山谷小鎮 (Fang)和山腳大城市 (Chiang Mai) ，並配合方縣 (Fang)部屬光達(mini-MPL) 監測氣膠垂直分布，再透過 (1) 線性回歸、 (2) 多元線性相關和 (3) 隨機森林機器學習算法，校正 Aerobox 的觀測數據。結果顯示三個測站PM2.5日平均值趨勢相似，而PM2.5與火點日平均並不一致。因地面PM2.5日變化受邊界層 (PBL)發展與地形影響，使PM2.5於日出後隨PBL逐漸增加而減少，而午後 PBL 高度下降後PM2.5隨之上升，直到入夜後 PM2.5濃度將恢復為夜間值，此變化發生的確切時間和規模因地點而異。地表 PM2.5 的日均值與 AOD 具有中等到高度的相關性，而小時值相關性受日夜變化影響，相關性較低。各站PM2.5與AERONET AOD小時相關係數分別為0.59 (Chiang Mai) 、0.51 (Fang)、0.45 (Doi Ang Khang) ，各站PM2.5與衛星 AOD小時相關係數分別為在 0.39 (Chiang Mai) 、 0.46 (Fang)、 0.27 (Doi Ang Khang) 。 AERONET AOD與衛星AOD具高度相關性。光達觀測結果表示，山谷PBL累積氣膠導致 AOD 達到峰值，當PBL在早晨上升時，周圍山脈高度以下的污染物傳送，伴隨殘留污染物被夾帶到PBL中。結合氣象條件對氣膠空間分布的分析表示，山谷環流對氣膠在夜間山谷積累和日間擴散扮演重要角色，氣膠在夜間流入導致埃指數 (AE)增加，而日間氣膠的粒徑大小取決於氣膠吸濕增長效應，相對濕度的晝夜循環由空氣溫度決定。總結本研究分析結果，復雜地形區域地面 PM2.5 和垂直柱狀氣膠變化差異可能很大，而低成本微型感測器可為複雜地形內空品提供可靠的參考依據。;Emission from biomass burning has been known to be a major source of particulate matter (including PM2.5; cut sizes ≤ 2.5 μm) in southeast Asia that peaks during the spring season. This study aims to understand the diurnal pattern of temporal and vertical dispersion of aerosols across varying terrain of Chiang Mai province of Thailand during the peak biomass burning period – spring season, 2019. For this purpose, the low-cost PM2.5 sensor system, Aerobox, and ambient weather stations were deployed at three sites: Doi Ang Khang located at the hilltop, Fang valley with a medium-sized town, and Chiang Mai valley containing a large city. Each of these sites were co-located with Cimel sunphotometer of NASA’s AERONET network. The vertical profile of aerosol loading was obtained by the mini-MPL and UAV-mounted Aeromount deployed at Fang valley. Three methods – simple linear regression, multiple linear correlation, and random forest machine learning algorithm – were tested to correct data from Aerobox. This study shows that widespread haze is formed that travels back and forth throughout the Chiang Mai province. The meteorological parameters showed clear diurnal variation throughout most of the period and the PM2.5 and AOD followed the pattern. The daily average of surface PM2.5 had a moderate to high correlation with AOD while the correlation of hourly averaged data was lower and was biased by the time of day. The diurnal variation of biomass-burning aerosols in the valleys of northern Thailand was observed to be governed by a combination of factors like the complex terrain, metrological condition, PBL development and mountain-valley circulation. The nocturnal air quality in the valley was observed to be poor with a secondary peak in PM2.5 during the midnight as a combined result of low PBL height, stable atmosphere, closed terrain and katabatic flow. The influx of aerosols at night increased quantity of finer-sized aerosols despite high humidity while at day, the size of aerosols was governed by the diurnal cycle of air temperature. The vertical profile of NRB showed that the nocturnal boundary layer height was shallow with residual layer on top of it which splits into two parts. The upper residual layer was observed to get dispersed and transported away while the lower layer got mixed back into the morning-time boundary layer. A combination of shallow mixing height, re-mixing of residual layer into the valley and day-time emission of aerosols seem to have contributed to the peak PM2.5 concentration during the morning in the valleys. In the afternoon, a sudden rise in mixing layer height was observed due to the vertical coupling between mountain atmospheric boundary layer which allows aerosols to get mixed up to higher altitude where it can be carried away by prevalent wind. Along with that, the day-time circulation also aided the reduction of aerosols in the valley. This study showcases how a network of low-cost instruments can be integrated with conventional measurement approaches to understand air-quality variation in complex terrains.
Appears in Collections:	[Department of Atmospheric Sciences and Graduate Institute of Atmospheric Physics ] Department of Earth Sciences

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