| 摘要: | 本研究為臺灣首次利用微脈衝光達觀測網之連續觀測資料探討臺灣西部低層大氣結構，站點包括位於北部近郊的中大站、位於中部交通繁忙都市的西屯站、位在中部內陸地區的斗六站，及位於南部臨海的左營站，微脈衝光達觀測資料經由垂直觀測驗證後顯示光達所提供的氣膠剖面與大氣邊界層高度反演結果與探空氣球與無人機觀測結果趨勢一致。根據過去研究表示地面空氣品質為大氣擴散條件優劣以及污染源排放污染物濃度的共同影響結果，其中大氣擴散條件包含環境水平通風能力與大氣垂直結構變化，因此為更瞭解大氣垂直結構與空氣品質的關係，本研究利用微脈衝光達觀測大氣氣膠垂直剖面資料，發展一套適用於低緯度地區的大氣邊界層高度反演法，從而建立臺灣大氣邊界層高度資料庫，並針對以下三項主題進行探討：(1)大氣邊界層高度季節及日夜特徵與氣象因子之關係；(2)日間對流邊界層結構特徵與主導發展之因子；(3)大氣垂直結構對地面空氣污染程度之影響。
本研究發展適用於臺灣的大氣邊界層反演法，針對西部大氣邊界層高度(Atmospheric boundary layer heigth, ABLH)特徵歸納，據統計資料指出，北部近郊區、中部都市區域及中部內陸地區的ABL發展結構相似，而南部因選用站點臨海，大氣結構在氣候上呈現較接近於海洋型邊界層的特徵，即發展高度較低且日夜變化不顯著；ABLH發展受風速與氣溫所影響，春季時ABL發展主要為風速及氣溫共同影響；ABLH在夏季時則為氣溫主導，發展高度與氣溫相關係數最高達0.72；秋季時北部與中部都市為氣溫主導大氣結構發展、中部內陸及南部則受風速影響；冬季時東北季風首當其衝的北部其ABL發展受風速主導，較強的風切使北部ABL發展高度最高，中南部三站則為風速及氣溫共同主導。
平均而言，日間邊界層較夜間邊界層高約150公尺，各地夜間邊界層高度低於平均高度佔比超過70%，且高度皆不及500公尺，而在晴朗條件下，由於各地晴朗成因不同，使各地對流邊界層高度(Convective boundary layer height, CBLH)發展分別受不同因素主導，天氣條件在春季和冬季主要受高壓系統移動影響，夏秋兩季晴朗條件多半是受到颱風與低壓移動影響。
為評估大氣擴散程度(Atmospheric Dispersion Level)與空氣污染事件之關係，進一步分析不同地域之大氣垂直與水平的擴散條件，應用於解析大氣邊界層與地面空氣品質之關係，釐清大氣結構對於不同地理環境下空氣品質的重要性。透過大氣通風能力(Ability of Atmospheric Ventilation, V)定義兩大指數：大氣擴散程度指數(Atmospheric Dispersion Level Index, DI)及空氣污染程度指數(Air Pollution Level Index, PI)，定量各地大氣垂直結構在高污染事件中的影響程度，其中以PIV為1代表受到垂直擴散條件不佳造成之空氣污染，PIH為則代表受到水平擴散條件不佳造成之空氣污染。本研究結果表示，北部近郊區域的春季與中部內陸地區的春冬兩季之高污染事件主要是受到垂直擴散條件不佳導致，其中中部內陸地區平均風速僅1.6 m s-1，終年低風速使環境對污染物的容忍度較低，處於容易發生空氣品質惡化的狀態，當ABLH出現變化時，便會明顯影響山麓區域的空氣品質，為臺灣獨特的地理特徵之空污效應，此區域在發生高污染事件時PIV為1時佔比大於35%；北部近郊區域的冬季、中部都市區域春冬兩季與南部沿海區域春冬兩季之高污染事件主要是由於水平擴散條件不良所導致，其中南部沿海地區因ABL結構屬於海洋型邊界層特徵且受東北風背風沉降抑制發展，ABLH變化較小使此區域空氣品質在高污染季節中為受水平擴散條件所主導，在冬季PIH為1時佔比達51%。
大氣擴散條件優劣以及污染源排放污染物濃度共同影響地面空氣品質，而本研究進一步分析臺灣不同地域之大氣垂直與水平的擴散條件，釐清大氣結構對於不同地理環境下空氣品質的重要性，本研究成果有助於未來欲針對缺乏大氣垂直剖面連續觀測的地區，進行空氣品質惡化肇因之評估依據。;This study is the first research that uses the continuous aerosol vertical profiles observation provided by aerosol lidar, micro pulse lidar (MPL) in Taiwan. The Micro Pulse Lidar Network is composed of four stations with different types of geography, including NCU, the Northern suburban site located at tableland; Xitun, the Central crowded-traffic city stie located at a shallow basin; Douliu, the Central inland city located at a foothill region; and Zuoying, the coastal city in the Southern area located at the coastal plain. According to the previous studies, the surface air quality is the result of the joint impact of the atmospheric dispersion condition and the pollutant concentrations. The atmospheric dispersion includes the horizontal ventilation ability and the vertical variety of atmosphere. In order to better understand the relationship between the vertical structure and air quality, this study develops a series of methods to retrieve the atmospheric boundary layer height (ABLH), which is suitable in low-latitude regions, to establish an ABLH database for Taiwan, and discusses the following themes: (1) the relationship between the characteristics of ABLH and meteorological factors within seasonal and diurnal variation; (2) the characteristics of the convective boundary layer and the factors that dominate its development; and (3) the influence of the vertical structure of the atmosphere on the level of surface air quality.
The results show that the development of ABL in the Northern and Central areas is similar to each other, while due to the location of the Southern site near the sea, the average of ABLH is lower than others with less diurnal variation; in other words, the ABL feature of the coastal site is close to the marine atmospheric boundary layers. The development of ABL is affected by temperature or wind speed (WS). In spring, the ABL is affected by both temperature and WS; while the temperature dominates the ABL developed in summertime, the correlation coefficient between summer ABLH and temperature is up to 0.72. In autumn, the temperature dominates the ABL growth of northern and central cities, and the WS affects the ABL development of inland cities and coastal sites. In the wintertime, the northern area bears the NE monsoon, and strong wind shear leads the development of ABLH to dominate by WS as shown in results.
In general, the difference between ABLH in daytime and at night is about 150 meters on average, and over 70% of the nighttime ABLH is lower than the average. Because the causes for clear days are different from site to site, the main factors of convective boundary layer height (CBLH) are different in different locations. The high-pressure systems dominate the condition of clear days in spring and winter, while the low-pressure systems and typhoons dominate the weather condition in summer and autumn.
To evaluate the relationship between the ability of atmospheric ventilation (V) and air pollution events, this study defined two indexes to quantify the affection of the vertical structure of the atmosphere in severe air pollution: the Atmospheric Dispersion Level Index (DI), which represents the dispersion ability of the environment, and the Air Pollution Level Index (PI), which represents the polluted level of dispersion condition. The PIV and PIH represent the air pollution affected by weak vertical and weak horizontal dispersion conditions, respectively. The results show that the wind is not only dominant in the ABL development in the northern area but also the reason for the horizontal dispersion condition that leads to high-polluted cases. In a centrally crowded-traffic city, poor vertical dispersion leads to polluted days in the spring, while weak horizontal ventilation ability leads to the accumulation of pollutants in the winter. For the inland cities, the atmospheric environment is influenced by low wind speeds, which makes them less tolerant to pollution; thus, the changes in vertical structure play an important role in the air quality in this area; the PIV belongs to unhealthy types of inland regions and is up to 35%. For the coastal city, due to the affection of marine and the repression from the leeward side of the NE monsoon making the various ABLH obscure and the horizontal dispersion condition dominating the polluted events as results, the PIH belongs to unhealthy types of coastal regions and is up to 51%.
This study further analyzes the atmospheric dispersion condition and clarifies the atmospheric structure to surface air quality. The main achievements of this study provide a reference for evaluating the cause of air quality deterioration based on ABL structures and the dispersion abilities of the atmosphere with geographical features. |
