| 摘要: | 摘要 本文利用地面觀測及模式模擬針對大台北地區氣膠高濃度事件(以下簡稱氣膠事件)形成的原因進行全面性探討。從大台北地區2002年至2005年發生的事件日當中，本文根據綜觀天氣型態將事件日分為七類，其中亞洲大陸高壓回流(HPR)、亞洲大陸高壓冷鋒前暖區(PCF)、颱風(TYP)、太平洋高壓西伸(PHP)以及微弱西南氣流(WSW)五種類型受到了地形的影響，其他兩種類型分別為微弱高壓籠罩型(WHP)及亞州大陸高壓推擠型(MHPP)，後者與亞洲大陸長程傳輸污染有關。 為探討天氣類型與地形對大台北地區氣膠特性的影響，本文在2004年4月14日至23日於都市、郊區、背景三個測站同時進行大氣氣膠觀測，觀測期間受到WHP、HPR及PCF天氣類型影響，導致發生氣膠事件；PM2.5 (氣膠氣體動力直徑小於或等於2.5 微米者)硫酸鹽在都市及背景測站的濃度相當穩定，並佔PM2.5質量濃度約30%左右，顯示PM2.5硫酸鹽是來自分布廣泛污染源，另外，PM2.5硝酸鹽則是來自本地污染源的影響。當發生HPR類型時, 台北盆地在盛行氣流的壟罩下是位於雪山山脈的背風處；發生 PCF類型時, 台北盆地發生微弱南風或西南風是位於雪山山脈與大屯山脈的迎風處，這兩種天氣類型都使得台北盆地受到地形阻擋效應而處於微弱風速區，因而發生氣膠累積高濃度。本文也使用中尺度氣象模式第五版(MM5)及社區多尺度空氣品質模式(CMAQ)探討發生大台北地區氣膠事件的原因，確認綜觀天氣與地形效應的交互作用是主要的成因。 針對亞洲大陸污染的長程傳輸，本文分別對黃沙事件及PM2.5氣膠傳輸兩方面進行研究。關於黃沙事件，本文發現都是發生在MHPP類型，分析2002年環保署北部氣膠超級測站的監測數據，顯示在｢黃沙期間｣PM10(氣膠氣體動力直徑小於或等於10 微米者)與PM2.5-10 (PM10-PM2.5)最大濃度值遠比｢黃沙前期｣為高；而且PM10與PM2.5-10最大濃度值與黃沙影響時間的長短成高度相關。氣膠粒徑分布顯示｢黃沙期間｣的超微米模態較高，｢黃沙前期｣及｢黃沙後期｣則是次微米模態較高。此外，探討超級測站氣膠化學成分的監測數據，本文發現PM2.5硫酸鹽時間序列與亞洲大陸長程傳輸氣流有關，其他成分則較與本地車輛排放有關。 在PM2.5氣膠傳輸方面，本文以模式模擬亞洲大陸高壓向南移動至台灣的過程，發現PM2.5有機碳佔PM2.5的比例從上海地區的22-24%減至台灣附近的21%；半揮發性PM2.5硝酸鹽則從16-25%減至1%；相對地，PM2.5硫酸鹽卻由10%提升至35%，值得一提的是，PM2.5 銨根離子及PM2.5元素碳的比例幾乎沒有減少。 整體而言，本文先從天氣類型及地形效應的觀點彙整出大台北地區氣膠事件的七種天氣型態，然後利用觀測資料及模式模擬深入地探討發生頻率最高的HPR、PCF及MHPP三種型態下，氣膠成分的來源及物化特性。 Abstract This thesis utilized both ground-level observation and model simulation to investigate comprehensively the causes of high aerosol-concentration events (aerosol events) occurred in the Greater Taipei area. Based on aerosol events compiled in the Greater Taipei area from 2002 to 2005, the associated synoptic weather patterns can be classified into 7 types. Among the identified weather patterns, High-Pressure Reflux, Prior Cold Front, TYPhoon, Pacific High Pressure system stretching westerly, and Weak Southern Wind are related to terrain blocking. The remaining patterns are Weak High Pressure system and Migratory High-Pressure system Pushing, respectively, with the latter one being related to long-range transport. To study the variation of aerosol properties under the mixed influence of terrain and weather pattern, a field observation was conducted at the urban, suburban, and background sites simultaneously from 14th to 23rd in April 2004. During this field observation, aerosol events occurred under High-Pressure Reflux, Prior Cold Front, and Weak High Pressure system weather patterns. PM2.5 (aerosol aerodynamic diameter less than or equal to 2.5 ?m) sulfate level was quite stable with a fraction of 30% in PM2.5 both at the urban and background sites. It exemplified that PM2.5 sulfate was originated from broad sources. In contrast, PM2.5 nitrate was contributed from local sources. Apparently aerosol accumulates in the stagnant environment when the Taipei basin is on the lee side of Snow Mountains Chain (SMC) under the prevailing wind in High-Pressure Reflux pattern or upwind of Snow Mountains Chain under Prior Cold Front pattern. Both weather patterns aggravated aerosol accumulation in the Taipei basin due to the formation of weak wind area from terrain blocking. In this thesis, the fifth-generation Mesoscale Model (MM5) and Community Multiscale Air Quality Modeling System (CMAQ) were also adopted to investigate the formation mechanism of an aerosol event in the Greater Taipei area. From the modeling, the mix of synoptic weather patterns and terrain blocking is verified to be the cause of an aerosol event in the Greater Taipei area. For the study of long-range transport pollution from Asian continent, this thesis investigates the transport of yellow dust and PM2.5 components, respectively. In the study of yellow dust event, the weather pattern was found all under Migratory High-Pressure system Pushing pattern. Based on the analysis of monitoring data from North Aerosol Supersite at the Taiwan Environmental Protection Administration, the averages of the hourly PM10 (aerosol aerodynamic diameter less than or equal to 10 ?m) and PM2.5-10 (PM10 - PM2.5) were much higher in the during period as compared to those in the before period. It is interesting to note that the time lapse in the during period was well correlated with the maximum level of both PM10 and PM2.5-10. The aerosol size distribution verified that supermicron particles dominated in the during period, and that submicron particles were predominant in the before and after periods. For the chemical properties of the aerosols, time series results indicated that aerosol sulfate was mostly contributed by the dust transport, and the other components were more related to vehicle exhausts. In the study of aerosol transport, the model simulation was applied to the movement of a southward high-pressure system to Taiwan. As the simulation revealed, during the transport process, that the fraction of PM2.5 organic carbon in PM2.5 plume only slightly decreased from 22－24% in Shanghai to 21% near Taiwan. However, the fraction of semi-volatile PM2.5 nitrate in PM2.5 decreased from 16－25% to 1%. In contrast, the fraction of PM2.5 sulfate in PM2.5 increased from 16－19% to 35%. It is interesting to note that the fraction of PM2.5 ammonium and PM2.5 elemental carbon in PM2.5 remained nearly constant. In summary, this thesis began with classifying 7 weather patterns from the study of aerosol events under the influences of synoptic weather patterns and terrain blocking in the Greater Taipei area. Field observation and model simulation of aerosols were subsequently applied to investigate thoroughly the aerosol sources and chemical properties for the most frequently occurred High-Pressure Reflux, Prior Cold Front, and Migratory High-Pressure system Pushing patterns. |
