Real-world gaseous emissions of high-mileage taxi fleets in China.

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.