台北都會區特定揮發性有機化合物污染來源解析及其垂直分布

Abstract

臭氧以及過氧醯基硝酸酯 (Peroxyacetyl nitrate, PAN) 等二次污染物是近年備受關注的議題，都市大氣中的揮發性有機化合物 (Volatile organic compounds, VOCs) 是形成臭氧以及二次有機氣溶膠 (Secondary organic aerosol, SOA) 的關鍵前體，且本身也被認為是有害空氣污染物 (Hazardous air pollutants, HAPs)。探討VOCs垂直變異的研究目前仍相當稀少，且結果也不一致。因此為了探討都市中VOCs (1) 潛在來源及其垂直分佈；以及基於正矩陣因子分解 (Positive matrix factorization, PMF) 所估算的特定來源之 (2) 二次污染物形成潛勢 (3) 吸入性風險優先排序，本研究於2019年11月21日至2020年6月18日在台北市一棟緊鄰交通要道建築物的3個樓層進行VOCs的長期採樣，三個樓層的高度分別為6公尺 (低樓層)、24公尺 (中樓層) 以及44公尺 (高樓層)。採樣介質為填充Carbopack X吸附劑的熱脫附管，透過串聯Nafion乾燥管以避免水蒸氣的干擾，並使用全自動熱脫附氣相層析儀/火焰離子化偵測器 (ATD/GC-FID) 進行9種VOCs的定量。 分析結果顯示苯是監測地點最豐富的物種 (32%)，其次是甲苯 (28%)，總VOCs在中樓層有最高的平均濃度 (74.10 µg/m3)，其次分別是低樓層 (69.06 µg/m3) 以及高樓層 (69.00 µg/m3)，Wilcoxon符號等級檢定得知中樓層以及高樓層之間有顯著差別。透過PMF鑑別出三個主要來源，其分別為因子1：溶劑塗料使用 (28%)、因子2：交通排放 (46%) 以及因子3：與交通相關的老化氣團 (25%)，因子1的貢獻隨著高度增加而減少，因子2以及因子3則是在中樓層有最高的貢獻，且這兩個來源對於中樓層以及高樓層的貢獻有顯著差別，推測這可能是造成總VOCs濃度在樓層間顯著變異的主因。臭氧形成潛勢 (Ozone formation potential, OFP) 的結果表明因子1對於OFP有最大的貢獻 (52%)，而二次有機氣溶膠潛勢 (Secondary organic aerosol potential, SOAP) 的結果表明3個來源間的SOAP差異不大，但是冬季的SOAP顯著大於其他季節。在風險優先排序的方面，中樓層以及交通排放具有較高的風險。本研究顯示台北都會中VOCs的來源及其垂直變異的情形，而OFP、SOAP以及風險優先排序的結果有助於二次污染物管制政策的擬定進而保障國民的健康。

Ozone, peroxyacetyl nitrate (PAN) and other secondary pollutants have received considerable attention in recent years. Volatile organic compounds (VOCs) are the key precursors for the formation of ozone and secondary organic aerosols (SOA) in urban atmosphere, and they are also considered hazardous air pollutants (HAPs). Currently, only few studies focus on the vertical variability of VOCs, and their results are not consistent. In order to explore (1) VOCs potential sources and their vertically distributed; (2) secondary pollutants formation potential as well as (3) the inhalation risk prioritization of the specific sources estimated based on positive matrix factorization (PMF) of the VOCs in urban atmosphere, this study conducted long-term VOCs sampling at three different balconies in a high-rise building near a major traffic road in Taipei Metropolis from November 21, 2019 to June 18, 2020. The heights of the three floors were 6 meters (low-level), 24 meters (mid-level), and 44 meters (high-level). Thermal desorption tubes were used to collect VOCs samples on Carbopack X adsorbents, and a Nafion dryer tube was connected to avoid the interference of water vapor. Meanwhile, an automatic thermal desorption system equipped with a gas chromatography flame ionization detector (ATD/GC-FID) was used to quantify 9 selected compounds of VOCs. The analysis results showed that benzene was the most abundant species at the monitoring site (32%), followed by toluene (28%). The highest average mass concentration of the total VOCs appeared at the mid-level site (74.10 µg/m3), followed by the low-level sites (69.06 µg/m3) and the high-level site (69.00 µg/m3). Wilcoxon signed-rank test revealed that there was a significant difference between the mid-level and high-level sites. Three major sources were identified by PMF, which included factor 1: solvent-use (28%), factor 2: traffic emission (46%), and factor 3: traffic-related aged air (25%). The contribution of factor 1 decreased as the height increased, while factor 2 and factor 3 contributed the most at mid-level site. The contribution of these two sources differed significantly between mid-level and high-level sites. It was speculated that this might be the main cause of the significant difference toward the mass concentration of total VOCs between varied level sites. Ozone formation potential (OFP) results indicated that factor 1 was the largest contributor to OFP (52%), while secondary organic aerosol potential (SOAP) results showed that the SOAP had little difference among the three sources, however, the SOAP in winter was significantly greater than that other seasons. In terms of inhalation risk prioritization, the mid-level site and traffic emission had higher risks. This study showed the potential sources of VOCs and their vertical variability in Taipei Metropolis. The results of OFP, SOAP, and inhalation risk prioritization are helpful for formulating secondary pollutant control strategies to protect the public’s health.