公車於動力計上排放廢氣中固液氣三相 PAHs 特性之研究

Abstract

國內對於柴油車廢氣之檢測大多只含粒狀及氣狀污染物，並無相關 PAHs 之檢測，且過去相關研究柴油車排放 PAHs 也只包含固相及氣相。另外，國內執行柴油車廢氣檢測之採樣設備其體積龐大，操作價格昂貴且複雜。有鑑於此，本研究設計一柴油車廢氣採樣設備，能夠採集廢氣中固、液、氣三相 PAHs，並評估其採樣有效性，及進行相關柴油車廢氣中 PAHs 之探討。 本研究設計出三種不同形式之柴油車廢氣採樣設備，經過評估比較後，選定一種作為最具代表性之採樣設備。顯示採樣設備能夠採集廢氣中固、液、氣三相 PAHs，且其相關性及穩定性為良好，另外體積小方便易攜帶。 於動力計上採集數台市區公車執行廢氣採樣檢測顯示，有加裝觸媒濾煙器之環保公車，排放較低之 PAHs 濃度。而在各相 PAHs 分佈上，固相中以三環之PAHs 比例為最高；液相則在二三環 PAHs 比例為最高，氣相中則幾乎以二環 PAHs 佔所有比例；在總 PAHs 各環數分佈上，以二環 PAHs 比例為最高，其次為三環 PAHs。總 PAHs 三相分佈顯示，低分子量以氣相為主，中高分子量則以固相為主。且隨著引擎負荷增加，固相比例在中高分子量會愈來愈多，液相比例在中分子量會愈來愈少，氣相比例則往低分子量集中，愈來愈多。 利用亨利定律探討公車廢氣中氣液相 PAHs 之傳輸，顯示公車廢氣排出瞬間，低分子量 PAHs 會揮發成氣相，之後迅速溶於液相而後愈來愈慢達平衡；而中分子量 PAHs 則會瞬間溶於水中，而後慢慢達平衡。另外本研究使用採樣所得資料，推估市區公車於不同行駛狀態下之排放係數顯示，公車於等速20公里無負載排放量為最高，其次為等速40公里無負載及惰轉。

Exhaust emission tests of diesel vehicles mostly focus on particulate and gaseous pollutants in Taiwan. Measurements of PAHs from diesel vehicle emissions are not included. Studies on liquid phase PAHs are still lacking. Moreover, application of sampling apparatus is restricted by its large volume, high cost and complexity. A sampling apparatus of diesel vehicle exhaust was designed in this study to achieve the objective of sampling solid, liquid and gaseous phase PAHs in exhaust emissions from diesel buses. Effectiveness assessment and the characteristics of three phases PAHs were investigated as well. One representative apparatus would be culled from the three apparatus designed in this study. The analytical results indicate that the apparatus can collect three phases PAHs in exhaust emissions. Besides, they are portable and their performances are stable. A chassis dynamometer was used in this study to simulate the driving patterns of buses in the sampling procedures. PAHs concentrations in exhaust emissions from buses with catalyst particulate filters were observed to be lower. As discussing the characteristics of three-phase PAHs, 3-ring compounds account for the largest proportion of solid phase PAHs. 2-ring and 3-ring compounds account for the largest proportion of liquid phase PAHs. For gas phase PAHs, 2-ring compounds account for most proportion. Low-molecular-weight PAHs mostly exist as gas phase but high-molecular-weight ones exist as solid phase mainly. The proportion of high-molecular-weight compounds in solid phase PAHs and the proportion of low-molecular-weight compounds in gas phase PAHs exhibit increasing trends with the increase of engine loading. Henry’s Law was applied to investigate the PAHs transfer between liquid and gaseous phases. As bus exhaust just emits, low-molecular-weight PAHs would vaporize into gas phase, then solute in liquid phase and achieve the stabilization finally. However, middle-molecular-weight PAHs would solute in liquid phase immediately and achieve the stabilization slowly. According to emission factors of buses on different driving patterns, they emit the most pollutants at the speed of 20 km/hr with no-loading. Pollutants emit secondly at the speed of 40 km/hr with no-loading and at idle speed.