綜觀天氣及不同氣流軌跡影響下的北台灣氣膠特性

沈士翔(Shi-Shung Shen)

指導教授 ： 李崇德

國立中央大學/工學院/環境工程研究所/碩士(2006年)

摘要

近年來中國大陸工業污染排放及沙塵傳輸頗受國際上的重視，這些污染及沙塵氣膠隨著天氣系統的移動，有時候會影響台灣的空氣品質。本研究選定台灣最北端的台北縣石門鄉進行氣膠的觀測，採樣時間為2004年5月至2005年4月，當台灣受大陸性冷高壓南下影響時，石門是位於台灣本島首當其衝的位置，因而可以檢驗長程傳輸氣膠的特性。
在冷高壓事件發生期間氣膠質量濃度和SO2都有增加的現象，在非冷高壓時期則因為污染來源是以當地排放為主，各成分濃度明顯都較低。在冷高壓事件水溶性離子集中在NH4+及SO42-，其含量較冷高壓事件前後更高，可見受大陸污染物影響的程度增加。另外，冷高壓事件期間碳成分的揮發有機碳 (OP) 和低溫元素碳 (EC1) 所佔比例有較顯著的增加。
採樣期間四種氣流軌跡類型中以大陸沿岸傳輸的氣膠質量濃度最高，同時發現SO2 / NOx的比值也是最高，尖峰粒徑集中在0.56~1及3.2~5.6 µm，水溶性離子優勢物種為SO42-及NH4+。高壓迴流質量濃度僅次於大陸沿岸傳輸，尖峰粒徑也是0.56~1及3.2~5.6 µm，最大的不同是硝酸鹽的濃度高於大陸沿岸傳輸，可見其污染來源的差異。本地污染來源與海洋傳輸都屬濃度較低的類型，但海洋傳輸PM2.5 / PM10比僅約0.4，本地污染來源則可達0.6以上，且本地污染來源NOx濃度是所有類型最高者。在碳成分組成方面，本地污染來源次低溫元素碳 (EC2)所佔比例為所有類型最高者，海洋傳輸則是有機碳比例較高，且元素碳含量相當低，大陸沿岸傳輸OP及EC1有較顯著的成長，高壓迴流則是以中高溫度揮發有機碳所佔比例較高。
2005年大陸沙塵到達台灣之前，PM2.5質量濃度往往先有增加的現象，這是人為污染物被黃沙強風氣流帶動的現象。黃沙時期粗微粒的3.2~5.6 µm粒徑區間會有顯著的提升，細微粒則是以0.32~0.56 µm與0.56~1 µm的增加幅度最大。在沙塵影響較明顯的的時段，發現Ca2+、CO32-及PM2.5-10變化趨勢非常相近，表示沙塵是以粗微粒為主，並以CaCO3的型態存在。碳成分方面，黃沙時期有機碳所佔比例下降，但EC1則有增加的現象。微粒低分子量二元酸以oxalic acid的含量最高，且黃沙時期oxalic acid濃度略有增加，說明長程傳輸可能帶來oxalic acid。
當以氣流軌跡加以區分時，發現各氣流軌跡類型除絕對量上的差異外，在化學組成方面也都具有不同的特徵，這說明了以氣流軌跡分類區分氣膠特性的有效性。而針對黃沙事件所進行的分類，也可以很明顯的看出沙塵所帶來的影響，非黃沙時期與黃沙時期在物種組成比例及物種比值特徵皆有顯著的不同。

關鍵字

氣膠化學成份 ； 長程傳輸 ； 綜觀天氣型態 ； 氣流軌跡線

並列摘要

Recently, the China outflow and yellow-dust transport gained a lot of concerns globally. These pollutants and dusts moved by the weather system frequently influenced Taiwan air quality. In this study, aerosol observation was conducted at Shin-Men site, the northern tip of Taiwan, to investigate aerosol properties from long-range transport.
Aerosol concentration and SO2 were found elevated under the influence of cold continental high. In contrast, local emissions were dominant to result in low pollution level during non-cold-high period. The SO42- and NH4+ are dominant aerosol water-soluble ions and are higher than those in the time before and after the cold high event. Meanwhile, aerosol volatile organic carbon (OP) and low-temperature elemental carbon (EC1) were significantly increased in cold high event.
Among the four trajectory types, the transport along the China’s coast is with the highest aerosol level and the highest SO2/NOx ratio. The aerosols were distributed in 0.56-1.0 µm and 3.2-5.6 µm modes with SO42- and NH4+ as dominant aerosol water-soluble ions. The type of anti-cyclonic outflow is only less severe than the transport along the China’s coast in aerosol level and its aerosols are also distribution in 0.56-1.0 µm and 3.2-5.6 µm modes. The main difference between these two types is higher aerosol nitrate in anti-cyclonic outflow than the transport along the China’s coast. The local source contribution and oceanic transport are the two types with lower aerosol level. However, the ratio of PM2.5/PM10 is only 0.4 in oceanic transport as compared to 0.6 in local source contribution. In addition, the NOx concentration is the highest for local source contribution among all types. In aerosol carbon fractions, the second low-temperature elemental carbon (EC2) is the greatest in local source contribution among all types. The oceanic transport is featured with very low EC level but high organic fraction. The OP and EC1 are found significantly in the transport along the China’s coast. The anti-cyclonic outflow is with higher fractions in middle to high temperature OC.
The PM2.5 level in 2005 was found increased before the touch down of yellow dusts due to the contributions of industrial emissions forced by the frontal system. Coarse particles in the size range of 3.2-5.6 µm were significantly increased in contrast to the enhancement of 0.32-0.56µm and 0.56-1.0 µm in fine particles during the dust period. For the time periods when the dusts were significant, Ca2+, CO32-, and PM2.5-10 varied consistently which indicates that dusts are mainly in the coarse fraction and in the form of CaCO3. For aerosol carbons, the fraction of OC was decreased but EC1 was increased during the yellow-dust affected period. The most abundant low molecular-weight dicarboxylic acid was oxalic acid, which was slightly increased in the dust period. It indicates that oxalic acid could be in the air mass of the long-range transport.
For aerosol properties classified by trajectory types, the distinction was found for each type of aerosols both in concentration as well as in mass fraction. This demonstrates the usefulness of classification of aerosol properties by trajectory type. In addition, we can find that the influences of Asian-dust distinctly by classifying Asian-dust events into three periods. The compositions and the characterics of ratio in “non-yellow-dust” are different from “yellow-dust” obveriously.

並列關鍵字

aerosol chemical properties ； Long-range transport ； synoptic weather pattern ； Air trajectory

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