| 摘要: | 蘭嶼位於台灣本島的東南方，是我國唯一有長期背景觀測二氧化硫(Sulfur dioxide, SO2)與臭氧(Ozone, O3)的離島，為中央氣象局建立，觀測時間自1995年3月至今。從全球趨勢而言，一次污染物SO2因為脫硫技術進步與各國嚴格規範排放量而使其排放量逐年下降，而二次污染物O3卻因其前趨物排放量的增加導致其濃度逐年上升。本研究分析蘭嶼氣象站(22.04°N, 121.55°E, 324 m a.s.l.)所觀測的SO2及O3長期趨勢，作為太平洋西岸背景大氣中一次與二次污染物變化特徵的代表，也和台灣趨勢比較其差異。
本研究顯示，蘭嶼SO2與O3年成長率皆為負成長，每年分別以0.06 ± 0.03 ppb與0.36 ± 0.07 ppb的速度下降，此外也發現2017年夏季O3濃度因強勁西南氣流而濃度低達9.5 ppb。利用HYSPLIT分析後推氣流軌跡(Backward trajectories)並將氣團分成五個來源(Clusters)：中國、日本、太平洋、南海、當地，其中當地來源包含未分類之剩餘氣團，可見O3季節變化隨氣團來源而改變，北風、東北風所對應的濃度較高，南風、西南風的濃度則相對較低；然而，各來源的SO2濃度差距不大，季節變化較不顯著。隨著觀光發展，蘭嶼遊客量為當地SO2濃度短期變化的主因，位於測站西南方的發電廠於旅遊淡旺季時發電運轉程度的不同，可對應至西風與西南風之SO2濃度變化。從日夜變化亦可觀察到SO2在白天7-16時的濃度較高，較其他時間增加7.3 %；O3的日夜變化則無特定特徵。
本研究並分析兩種污染事件的類型，包括長程傳輸與台灣本島污染擴散事件。前者多伴隨著東北風將中國污染物往南輸送，例如台灣本島亦可監測到SO2污染物隨時間推移的變化;而後者通常發生在蘭嶼盛行西南風時將台灣本島污染往東輸送。搭配MERRA-2的資料，分析兩種事件類型的型態與影響範圍。
;Lanyu is the only remote island of Taiwan with long-term measurements of background atmospheric SO2 and O3. The observation started from March, 1995 until present. Due to the improvement of modern sulfur removal process and strict emission regulations, the concentration of SO2 as a representative of the primary air pollutants has declined in recent years. However, the concentration of O3 as a secondary air pollutant was rising during the same period. This study investigates the long-term variations of SO2 and O3 at the Lanyu CWB station (22.04°N, 121.55°E, 324 m a.s.l.) to understand their characteristics in the western Pacific troposphere.
Our analysis shows that SO2 and O3 showed a decreasing trend with growth rates of -0.06 ± 0.03 ppb/yr and -0.36 ± 0.07 ppb/yr, respectively. In addition, we found a significant decline in O3 with a monthly concentration of 9.5 ppb because of strong southwesterlies in July, 2017. Based on the cluster analysis of HYSPLIT backward trajectories with 5 groups suggested: (1) China, (2) Japan, (3) The Pacific Ocean, (4) South China Sea, and (5) Local, the change of air mass origins was responding for the distinct seasonal O3 variations. Elevated O3 concentrations were mainly corresponding to north/northeast winds, whereas reduced O3 concentrations were mainly corresponding to south/southwest winds. However, no significant SO2 difference was found between the 5 groups. The influences of tourists were suggested responding for the seasonal and diurnal variations of SO2 as a result of emissions by the fire power plant lies to the southwest of the Lanyu station. For instance, more electricity demands during the peak period (April - September) and during the daytime (7:00 – 16:00 local time).
In this study, two typical pollution events were also investigated: (1) Long-range transport of Chinese air pollutants was mainly driven by northeastertlies. (2) Air pollution spread from Taiwan Main Island occurring with southwesterlies. MERRA-2 data were used for analyzing the characteristics of the two types of high concentration events. |
