| 摘要: | 實驗室曾於2016年底於富貴角量測乙烷與乙烯濃度與其比值，發現可作為偵測境外長程移入污染氣團之有效指標。然而由於台灣為潮濕的海島型氣候，於監測過程中，濃縮捕捉步驟易因多水氣結冰干擾使監測頻繁中斷，故本研究將原先之前濃縮儀進行改良。
改良加裝線上除水機制以解決水氣干擾問題的方法即是在致冷模組中的捕捉管前加入一段玻璃空管，利用致冷模組在-30ºC，進行樣品捕捉時同時先將樣品中的水氣預先凝結在上游的玻璃空管中，以避免大量水氣進入後端捕捉管內，因結冰再熱脫附汽化時造成吸附材料崩解。本研究所開發的致冷除水模組與Nafion dryer相較之下，因不需使用乾燥氣體，且可循環使用，無耗材需求及設備維護上之成本。
本研究在2017年12月5日至2018年1月7日期間，將改良後之監測設備再次放置於富貴角測站內進行乙烷、乙烯的濃度監測。系統對各物種的檢量線之線性關係R2值高於0.99，RSD值介於0.66~0.94%之間，其MDL值介於0.16~0.38 ppb之間，具有良好的精準度。些微的乙烷與乙烯濃度變化皆能夠被偵測，故能清楚指示污染空氣。
當污染物來自於境外時，乙烷與乙烯濃度偏低，但其比值會呈現相對的高值，在與環保署在富貴角測站所監測的PM2.5、PM10、O3、NOX、
CO、SO2及風速風向等數據比對後，證實在12/24的PM2.5高值為本地排放所造成，而在12/25的PM2.5高值則是長程傳輸事件所造成。然而在12/8及12/11的PM2.5高值事件則較為特殊，觀察SO2、NOX濃度變化發現在同一時間點有突然上升的趨勢，因此推測是在海上運作的船舶排放出來的空氣物染物會造成本研究監測系統的干擾，造成乙烷與乙烯濃度偏高，其比值會呈現相對的低值，出現本地排放事件的特徵，使長程傳輸事件的特徵被遮蔽，進而造數據判讀上的錯誤。;At the end of 2016, our laboratory set up a gas chromatographic (GC) system at the northern tip of Taiwan, Fuguijiaoto, to measure real-time mixing ratios of ethane, propane, ethylene and propylene. We found that the ratio of ethane to ethylene can be used as an effective indicator of the long-range transported (LRT) polluted air masses. However, due to high air humidity, the pre-concentration using the sorbent trap at sub-ambient temperatures was frequently clogged due to icing, resulting in insufficient trapping or even damage of the sorbent packing. As a result, the objective of this study is to solve the problem of ice clogging during trapping to facilitate continuous monitoring without interruption.
The solution is to add an empty glass tube in front of the sorbent trap in the same cooling module to be cooled at -30℃ when trapping. Excess water vapor will condense onto the glass tube, while the air sample that has been mostly dried can pass through to the trap. Compared with the Nafion dryer, our solution is more rugged and involves no consumables, and is almost maintenance free.
Laboratory test showed that the linearity of the four target species is higher than 0.99 (R2). Precision (RSD) is between 0.66 and 0.94%, and the measurement detection limits (MDL) is between 0.16 and 0.38 ppb. From December 5, 2017 to January 7, 2018, the improved system was placed in the Fuguijiao station again with 15 min GC cycle time, and no interruption due to ice clogging of the trap had occurred, suggesting the water removal solution was successful.
Continuous data of PM2.5, PM10, O3, NOX, CO, SO2 and wind parameters were used to support the interpretation of the observed ethane/ethylene ratios at the Fuguijiao station. We found that the high PM2.5 peak of 12/24 was a local event as suggested by the relatively low values of ethane/ethylene, while the PM2.5 peak of 12/25 was a LRT event as indicated by the relatively high values of ethane/ethylene. This finding was consistent with our presumption of the ethane/ethylene ratio as an indicator of LRT. However, the PM2.5 peaks at 12/8 and 12/11 are more intriguing. The ethane/ethylene ratios showed relatively low values which should have been indicative of local events. However, both the wind field and model simulation suggested otherwise. We found that the SO2 and NOX levels during these two time periods also elevated, consistent with the relatively low ethane/ethylene ratios. It turns out that the contradiction was most likely caused by the ship emissions off-shore, which explains the elevated values of NOx, SO2, ethane and ethylene, as well as the relatively low ratios of ethane/ethylene. |
