| 摘要: | 有害空氣污染物 (Hazardous Air Pollutants, HAPs) 隨著工業發展被大量排放威脅著人民的健康。HAPs的監測標準方法，美國以EPA TO-15方法為主，台灣參考此方法開發了NIEA A715.16B方法，均利用採樣筒離線採樣。台灣針對特殊性工業區應用NIEA A715.16B，離線採樣工業區周界空氣，頻率為六天採樣一次，數據僅為24小時的平均，無法提供即時數值且無法反應當地長時間環境污染物濃度變化，因此本研究目的即延續前人研究持續開發線上熱脫附氣相層析質譜技術 (Online Thermal Desorption GC/MS，簡稱Online TD-GC/MS)，除了能持續監測空氣中86種HAPs每小時的數值，更重要的是延長離子源的使用壽命，延長連續監測的時間，使連續監測工作可以更完善。
離子源劣化快速導致離子源維護頻繁，影響數據的連續性為Online TD-GC/MS方法之最大難點，本研究透過探討空氣成分確認了水氣為造成離子源感度下降的主要因素，在不影響MDL的條件下將採樣體積減少並加大熱脫附分流流速使進入GC-MS的樣品體積減少，有效的減緩離子源感度下降的速率，維護頻率下降，在進行每日中濃度標準品查核時目標物種的回收率能維持在± 30%的範圍的時間增加，連續監測時間延長到兩週以上所測得之連續數據增加。
利用調控後的參數，實際在實場連續監測91天，離子源壽命最長可使用20天，因為連續性改善，可以清楚的看到如四氯化碳等污染物具背景值，此背景濃度值和NOAA (National Oceanic and Atmospheric Administration, NOAA) 之背景濃度相近，證實儀器對低濃度監測的準確度，監測期間的數據更利用與離線採樣方法NIEA A715.16B及PTR-MS數據進行平行比對獲得良好的比對性，再一次佐證此系統之數據準確性。
本研究除了內標準品外還透過監測氟氯碳化物 (Chlorofluorocarbon, CFC) 濃度來確認儀器的穩定性，CFC於周界空氣中濃度穩定，可作為環境內標來佐證系統可信度。
監測期間的物種高值稱之為特殊事件，例如氯乙烯伴隨著1,2-二氯乙烷同時出現高值的特殊事件，將測得之連續數據結合當時的氣象資料可以透過後推軌跡的方法對污染物進行溯源，氯乙烯及1,2-二氯乙烷結合後推軌跡後證實兩者來自相同排放源。Online TD-GC/MS即時監測的結果可用於評估當地環境的污染物暴露濃度，每小時一筆數據能即時反應突然的特殊事件，對事件進行溯源尋找排放源，使排放管制更有成效。
;Hazardous air pollutants (HAPs) have been released in large quantities with industrial development, threatening people′s health. The standard method for monitoring HAPs in the United States is mainly based on the EPA TO-15 method, according to which Taiwan developed an analogy, the NIEA A715.16B method. Both use canisters for offline sampling. Taiwan applies NIEA A715.16B for the special industrial district. The frequency of sampling is once every six days and each for 24 hours, which cannot provide real-time values and reflect local concentration variability. Therefore, this research is a continuation of the previous study to further refine the method of online thermal desorption gas chromatography/mass spectrometry (called the Online TD-GC/MS). In addition, by continuously monitoring 86 HAPs in the air with hourly resolution and, more importantly, prolonging the service life of the ion source in MS at the same time we have successfully extended the time length of monitoring, making the online method more applicable.
Due to the rapid decline of the ion source response, the ion source needs to be revived frequently, resulting in the lack of long-term data continuity, which is the greatest obstacle to the online TD-GC/MS method. By investigating the air composition, it is confirmed that moisture is the main factor causing the deterioration of the ion source response. As a result, we reduced the sampling volume and increased the thermal desorption split flow rate without affecting the MDL to reduce the amount of sample entering the GC-MS, effectively slowing down the decline of ion source response and; thus, fewer interruptions in monitoring. As a result, the time spent for the target species to be maintained in the recovery range of ±30% based on the daily performance check increases. In the end, the time length of continued monitoring is extended to more than two weeks before service of the ion source is required.
Using the adjusted parameters, the actual continuous monitoring in the field was operated for 91 days, during which the ion source was able to last for up to 20 days for each segment of measurement. Because of the improvements made in this study, it can be seen in the field data that the atmospheric background level of carbon tetrachloride (CCl4) was clearly revealed at around 0.1 ppb, despite concentration variability. This background concentration value is similar to the National Oceanic and Atmospheric Administration (NOAA) data, confirming the instrument′s low concentration monitoring detection limits.
In this study, the instrument′s stability was confirmed by monitoring the chlorofluorocarbon (CFC) concentrations and normalized internal standards. Because the concentrations of CFCs in the ambient air are stable, they can be used as an environmental internal standard to prove the system′s reliability.
The high values of species during the monitoring period are called special events, such as the occurrence of high values of vinyl chloride and 1,2-dichloroethane simultaneously. Combining the online data with a simple two-dimensional back-trajectory algorithm, the spikes of vinyl chloride and 1,2-dichloroethane were determined to be caused by an emission source from polyvinyl chloride (PVC) factory. As a result, the online TD-GC/MS method not only can be used in the risk assessment of HAPs in a local environment but it also can instantaneously respond to sudden special events, and trace back to emission sources, making emission control more effective. |
