自2000年春季以來，大陸沙塵暴發生頻繁，並對台灣春季空氣品質造成相當影響。本文選取2002年春季個案，說明大陸沙塵暴發生的天氣條件，以及後續會影響台灣地區空氣品質的天氣狀況。這些研究有助於大陸沙塵侵台的預測工作，並可了解到沙塵與本地空氣污染物質對本地空氣品質的影響狀況。
簡言之，西伯利亞冷氣團快速進入大陸北方沙漠區，會因地面強風與垂直大氣不穩定，使得地面沙粒快速被舉升到高層。若此冷氣團能持續南下，並走向東南，則約在2日後攜帶大量沙塵到長江以南地區，並可能影響到台灣地區空氣品質。冷氣團前緣冷鋒面的移動速度，與沿路的降雨狀況，均會影響抵台沙塵的濃度量。同時，由於鋒面逆溫與高壓出海後迴流受中央山脈阻擋等因素，本地污染物質也會在過程中累積，並影響本地空氣品質狀況。了解整個發生過程，與尋找適當的辨認指標，均有助於事前的預測與事後的分析。
本文整理2002年每次大陸沙塵長程傳送及台灣地區的個案，計算發生當日的最大三站小時濃度值，並求其平均值。當該平均值小於150μgm^(-3)時，沙塵影響弱，乃定其等級為0；而當平均值介於150μgm^(-3)與200μgm^(-3)之間時，雖然上游沙塵影響本地空氣品質，但亦不能忽略本地污染物的貢獻，乃定其等級為1。至於平均值大於200μgm^(-3)時，沙塵影響顯著，乃按每增加100μgm^(-3)增定一個等級，並定最高等級為大於500μgm^(-3)以上。所以2002年1~5月間，按中尺度數值模擬應有11個大陸沙塵影響台灣空氣品質的個案，達到等級為1以上的影響案例共有7個，但只有2個案例為大陸沙塵影響顯著者，而對本地PSI有明確影響的日期為：2月9日、2月11日與4月9日等，不過出現PSI大於100的日期僅在：2月9日與11日。
Since spring 2000, frequent mainland dust events have affected Taiwan air quality. In this paper, we have analyzed events in 2002 to understand the weather conditions that are associated with dust storms and the sequential evolution that favorites the transport of dusts to Taiwan. This study is useful for the prediction of dusts affecting Taiwan air quality.
In all, cold air mass coming out of Siberia moves fast into northern China. Surface gust and vertical instability result in a large amount of dust particles being carried upward to higher levels. With the cold air mass moving southward, these dust particles will be transported to the south of Long River and may affect Taiwan air quality. The amount of dust particles arriving at Taiwan region is affected by the movement of cold front and the precipitation condition along the trace. Meanwhile, the frontal inversion and the prevailing wind being blocked by the Central Mountain Range also favor the accumulation of local pollutants.
In this paper, we have calculated the mean value of three highest hourly-mean PM10 level of each dust affecting event. Then, we have set a ranking system based on this mean value. Dung January ~ May, 2002, eleven dust events may have affected Taiwan air quality. Seven events are ranked as 1, i.e. with the mean value being between 150~200 μg m^(-3). Only two events are ranked as 2, i.e. with mean value being between 200~300 μg m^(-3). On February 9 and 11, dust particles have caused the PSI level being larger than 100, i.e. unhealthy condition.
