整合無人機與光達觀測解析斗六地區空污事件之演變過程;Dissecting critical factors of PM2.5 deterioration in Douliu area, Taiwan using UAV and lidar observations

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Title:	整合無人機與光達觀測解析斗六地區空污事件之演變過程;Dissecting critical factors of PM2.5 deterioration in Douliu area, Taiwan using UAV and lidar observations
Authors:	陳誼;Chen, Yi
Contributors:	大氣科學學系
Keywords:	無人機;微脈衝光達;空氣污染;大氣邊界層;海陸風環流;Unmanned aerial vehicle;Micro pulse lidar;Air pollution;Atmospheric boundary layer;Land-sea breeze circulation
Date:	2021-08-03
Issue Date:	2021-12-07 11:07:11 (UTC+8)
Publisher:	國立中央大學
Abstract:	空氣污染問題不僅受污染源排放、大氣化學反應影響，氣象條件以及地形效應的影響也是造成高污染事件發生的重要因素。過去模式研究指出，臺灣在高壓迴流或是弱綜觀的天氣型態時，會有利於高PM2.5濃度事件發生，雖然位於雲林縣山麓地帶的斗六地區並非臺灣中部PM2.5濃度排放量最高的地區，但斗六地區空氣品質卻經常性不佳。為解析斗六地區空污事件之演變過程，本研究利用新建構的無人機觀測系統與氣膠光達於2020年11月16日至20日，在斗六地區進行高時間解析度的氣象場與氣膠剖面觀測，以提供過去少有的高時間頻率大氣垂直剖面資訊，並透過一系列的資料處理流程與驗證程序，來確保無人機的觀測品質，同時搭配地面空氣品質測站資料和WRF模式模擬的再分析場，整合分析空污事件之演變過程。研究結果顯示，弱綜觀天氣條件下，斗六地區污染物的傳輸與擴散特徵與海陸風環流、背風渦旋及逆溫結構有關。個案期間夜晚風速皆小於1 m s-1，整體水平擴散條件類似，但在17、18日晚間高污染事件發生時，伴隨雙層逆溫結構，分別為近地面的厚度約200公尺的輻射逆溫與800-1000公尺左右的沉降逆溫，沉降作用同時導致高空環境乾燥無雲，有利加劇夜間地表的輻射冷卻，使地面逆溫強度增強，相較於較低污染之 16、19日夜晚的2.6 ℃ km-1與4.5 ℃ km-1，17、18日逆溫強度分別高達8.0 ℃ km-1與12.1 ℃ km-1，此惡劣的垂直擴散條件使斗六空品站於17日和18日晚間分別測得最高PM2.5濃度達70 μg m-3和99 μg m-3。海風環流約於上午9時建立，將潮濕氣團與沿岸污染物往內陸傳輸，18日下午1時可觀測到明顯的海風鋒面抵達，造成斗六比濕與PM2.5濃度同時上升，而光達觀測顯示1300公尺內NRB數值明顯增加，表示海風鋒面結構超過1000公尺。夜間隨著陸風環流發展，將內陸污染物帶離，斗六高污染濃度在17、18日分別於18時與21時之後下降。模式結果顯示個案期間臺灣海峽區域共出現三個背風渦旋，其中兩個為氣旋式環流，一個為反氣旋式環流，渦旋發展主要受大環境風場影響，而有不同的移動路徑與結構變化，其中氣旋式的背風渦旋會導致近地面污染物北傳，而反氣旋式環流的北風則伴隨沉降所造成的乾空氣移入斗六地區，呼應前述18日夜間高污染發生的條件。整體而言，本研究透過無人機與光達高時間解析的垂直剖面觀測進行分析，從污染物與氣象場空間分布演變探討高污染事件成因，並確立此觀測技術能應用於將來大氣剖面研究。 ;Douliu is one of the most air-polluted areas in Taiwan. Factors that caused the deterioration of air quality in this area might attribute to emissions, chemical reactions, meteorology, and geographical distribution, etc. Recent modeling studies point out that high PM2.5 concentration events occurred are usually associated with favorite weather conditions (i.e., high-pressure peripheral circulation and weak synoptic weather). To make a statement on the deterioration of PM2.5 events, we employed a new technology, Unmanned Aerial Vehicle (UAV), and an aerosol lidar to observe the high resolution of meteorological and aerosol profiles at Douliu during November 16th-20th, 2020. A series of data validation procedures had applied to ensure the UAV data quality. As a result, the UAV measurements (i.e., meteorological parameters and PM2.5 concentration) are reliable and show in good agreement with vertical structure obtained from lidar observation. In addition to observations (i.e., UAV, lidar, surface air quality, and meteorology data), we also use a WRF model to run a reanalysis for this case study. According to our results, the roles of land-sea breeze, temperature inversion layer, and leeside vortex in aerosol transportation and dispersion over Douliu during the weak synoptic weather were discovered. The horizontal dispersion condition did not change much during the night of the observation, which was accompanied by the wind speed smaller than 1 m s-1. On November 17th and 18th when the high pollution event occurred, there were two temperature inversion layers in vertical. One was the surface radiative inversion whose depth was about 200 m, and another was the elevated subsidence inversion which was located at around 800-1000 m. The subsidence of the air also led to the dry condition in the sky and was beneficial for radiative cooling at the surface at night. Therefore, the strength of the temperature inversion near the surface increased, compared with the strength of 2.6 ℃ km-1 on the night of 16th and 4.5 ℃ km-1 on the night of 19th, the inversion strength on 17th and 18th were up to 8.0 ℃ km-1 and 12.1 ℃ km-1, respectively. Therefore, these worse vertical dispersion conditions caused the maximum of hourly PM2.5 concentration to be up to 70 μg m-3 in the evening of 17th and 99 μg m-3 in the evening of 18th. The sea breeze circulation developed at about 9 a.m., and the specific humidity and PM2.5 concentration increased at the same time at noon, because the sea breeze transported the pollutant and water vapor from the coastal area to Douliu. The structure of sea breeze front was distinct on November 18th at 1 p.m., and the leading edge of the sea breeze front reached a height greater than 1000 m which was demonstrated from the lidar NRB data. When the land breeze gradually developed, the air quality improved after 18 p.m. on 17th and 21 p.m. on 18th. The result of the numerical simulation indicated there were three leeside vortexes during this case, two were associated with the cyclonic circulation and one was associated with the anticyclonic circulation. The location, structure, as well as direction of rotation with respect to the leeside vortex showed differently is due to the variation of the synoptic winds and how they interacted with the terrain. The cyclonic vortex led northward of the pollutant near the surface, and the anticyclonic vortex brought the dry air of subsidence with the northerly wind over Douliu. Overall, this study used the high temporal resolution vertical observation data from UAV and aerosol lidar to dissect critical factors of PM2.5 deterioration in Douliu area. This work also highlights that the high temporal resolution of UAV and lidar observations can serve as a powerful tool to improve our understanding of the relationship between atmospheric vertical profile and air quality.
Appears in Collections:	[Department of Atmospheric Sciences and Graduate Institute of Atmospheric Physics ] Department of Earth Sciences

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