本研究以環保署中部空品區1994-2005年之臭氧監測資料，探討受到颱風影響的臭氧事件比例，以及臭氧污染事件與颱風位置、路徑之相關性，進一步並利用MM5模式模擬2005年9月20-23日受到丹瑞颱風(西行颱風未登陸)環流影響之個案，分析高臭氧污染發生之氣象成因。 中部空品區的高臭氧事件自1994至2005年逐年增加，其中約有三成受到颱風影響。本研究針對颱風位置、路徑分類結果顯示：颱風中心位置位於台灣與菲律賓之間與中部空品區的高臭氧污染事件相關性最高，統計中亦發現經過此區域之颱風約有一半機會中部會有高臭氧事件(八小時臭氧濃度超過80ppbv)；影響中部地區高污染事件的颱風路徑分類結果以第五、六類路徑最多。颱風影響的個案中，全台地面測站之總平均風速為3.27m/s，風速遠比其他天氣型態大。地面合成天氣圖顯示在台灣與菲律賓之間有明顯的低壓環流，風向主要受到東北東風影響，此與過去所認知高臭氧事件往往伴隨微弱風場不同，於是進一步透過個案模擬分析第五類路徑(2005年9月20-22日)導致中部地區高臭氧事件之氣象成因。 個案分析發現當颱風距離600-1000公里時，台灣受到颱風外圍環流影響，最為穩定，穩定天氣條件引發的海陸風環流有利於污染物的傳送與累積，此時污染事件往往發生在下風處的內陸地區(南投地區)，當颱風持續西行靠近台灣，颱風環流籠罩全台，但是中部地區因為中央山脈(約2500-3500公尺)屏蔽影響而處於弱風區(地面風速約1-2m/s)，再加上颱風受到中央山脈阻擋而在西側背風區造成沉降運動，此沉降運動導致中部地區的大氣穩定度偏高，污染事件直接發生於都會高臭氧前趨物之排放地區附近(台中地區)，而沒有明顯的傳送現象。 In this study the synoptic patterns conductive to the occurrence of O3 episodes in central Taiwan are categorized by an inspection of the weather charts over the period of 1994-2005. Since the high ozone episodes of the central part of Taiwan are closed related to typhoon and tropical cyclones activities, focus the discussion on the typhoon-related ozone episodes. Furthermore, we used the mesoscale model (Fifth-generation Penn. State/NCAR Mesoscale Model, MM5) to do a case simulation to investigate the influence of typhoon activities on the air quality of the central Taiwan. The statistical result shows that about 31% of total ozone episodes is caused by typhoon or trpical cyclone. When the typhoon-related ozone episodes occur, the center of tropical cyclones is mainly located between Taiwan and the Philippines. Most tracks of such cyclones are classified to the track of 5th and 6th follow the Central Weather Bureau’s classification. The compose weather map of high O3 episodes occurred in the central Taiwan shows that a low-pressure circulation is located in the Philippines. The surface mean wind is 3.27 m/s, larger than others and the synoptic wind direction is ENE in 850hpa. High wind speed is favorable for pollutant transportation. In the further more, we used MM5 model to simulate the typhoon-related ozone episode during 20-22 September 2005 (track 5th). When the center of typhoon Damrey locates 600-1000 km away from central Taiwan, the subsidence effect of its periphery made air very stable. These conditions favor the development of both local thermal circulations and mechanically-induced circulations in the complex terrain area of the Central Taiwan. At this stage, due to the transportation effect of local circulation, the high ozone concentration comes up at the downward city, Nantou. After the typhoon move north-westward, the typhoon circulation covered the whole Taiwan. The low-level prevailing flow is subject to splitting due to the blocking effect of the Central Mountain Range (CMR), a weak wind speed zone is notable in the western central region, which is located in the leeward side of the CMR. The prevailing easterly wind accompany with typhoon Damrey, lead to the lee-side subsidence. The adiabatic warming associated with this leeside subsidence resulted to the appearance of stable atmosphere condition in the Central and western area of Taiwan. These meteorological conditions are unfavorable for pollutant dispersion and cause high ozone day in the city, Taichung. The high ozone episodes just occurred in the vicinity of high emission urban area.
