School of Environment, State Key Joint Laboratory of Environment Simulation and Pollution Control, Tsinghua University, Beijing 100084, China.
Chinese Research Academy of Environmental Sciences, Beijing 100012, China; Collaborative Innovation Center of Atmospheric Environment and Equipment Technology (CICAEET), Nanjing 210044, China.
Sci Total Environ. 2019 Apr 1;659:267-274. doi: 10.1016/j.scitotenv.2018.12.336. Epub 2018 Dec 24.
Mileage of taxi fleets is significantly higher than regular passenger vehicles in China, which might trigger greater tailpipe emissions of air pollutants. To investigate their real-world gaseous emissions, we applied portable emissions measurement systems (PEMSs) to test 44 gasoline and 24 bi-fuel taxis in seven cities. Our real-world measurement results indicated that a major part of the tested China 3 and China 4 gasoline taxis, especially the samples with high mileage (>300,000 km), far exceeded the corresponding emission limits of NO, THC and CO. Only the newest China 5 gasoline taxis with relatively lower mileage had effective emission controls and the gaseous emissions were below the limits. Illegal tampering, malfunction and deterioration of three-way catalytic converters (TWC) are major reasons for high emissions from high-mileage taxis. First, China 4 gasoline taxis without TWC (purposely removed by drivers) increased their gaseous emissions than TWC-equipped counterparts by more than one order of magnitude. Second, bi-fuel taxis when using compress natural gas (CNG) had much higher NO and THC emissions than those when using gasoline, which might be probably attributed to unsophisticated engine calibration and unfavorable TWC working conditions. Furthermore, TWC renewal could bring immediate and substantial emission reductions (up to 70%) for high-mileage taxis. However, such benefits from TWC renewal would become less significant as the mileage levels further increase. We also found a good correlation between CO and THC emissions for gasoline taxis, whose cold start effects were both significant. This study poses significant concerns regarding real-world emissions of high-mileage taxi fleets in China, which could consist of many gross emitters in the urban areas. Stringent in-use compliance programs and in particular frequent TWC renewals for high-mileage taxis should be implemented by policy makers in China.
在中国，出租车的行驶里程明显高于普通乘用车，这可能会导致更多的尾气污染物排放。为了研究其实际气态排放，我们应用便携式排放测量系统（PEMS）在七个城市测试了 44 辆汽油车和 24 辆双燃料出租车。我们的实际测量结果表明，大部分测试的中国 3 号和中国 4 号汽油出租车，尤其是高里程 (>300,000 公里)的样本，远远超过了 NO、THC 和 CO 的相应排放限值。只有相对较低里程的最新中国 5 号汽油出租车具有有效的排放控制，其气态排放低于限值。高里程出租车排放过高的主要原因是三效催化转化器（TWC）的非法篡改、故障和劣化。首先，没有 TWC（被司机故意拆除）的中国 4 号汽油出租车的气态排放比配备 TWC 的出租车高出一个数量级以上。其次，使用压缩天然气（CNG）的双燃料出租车的 NO 和 THC 排放比使用汽油时高得多，这可能是由于发动机标定不完善和 TWC 工作条件不利所致。此外，TWC 更换可立即显著降低（高达 70%）高里程出租车的排放。然而，随着里程水平的进一步增加，TWC 更换带来的好处将变得不那么显著。我们还发现，汽油出租车的 CO 和 THC 排放之间存在良好的相关性，其冷启动效应都很显著。这项研究对中国高里程出租车车队的实际排放提出了重大关注，这些出租车车队可能包含许多在城市地区的大量排放源。中国的决策者应实施严格的在用合规计划，特别是对高里程出租车进行频繁的 TWC 更换。
