Comparison of tailpipe carbonyls and volatile organic compounds emissions from in-use gasoline/CNG bi-fuel vehicles.

Ground-level ozone contamination has been globally an urban air quality issue, particularly for China, which has recently made significant progress in purifying its sky. Unregulated exhaust emissions from motor vehicles, predominantly carbonyls and volatile organic compounds (VOCs), are among the leading contributors to ozone formation. In this chassis-level study, the unregulated emissions from five China-5 certified gasoline/CNG bi-fuel taxis, along with their ozone forming potential (OFP), were evaluated. It is found that carbonyls and VOCs were mainly emitted during the starting phase no matter the engine was cold or hot. Compared to gasoline, CNG fueling reduced VOCs emissions on a large scale, especially in the starting phase, but had elevated carbonyls. On a fleet average, CNG fueling derived 15% and 46% less OFP than gasoline in cold- and hot-start tests, respectively. VOCs contributed to over 90% of the total OFP of the exhaust. In terms of alleviating ground-level ozone contamination, CNG is a feasible alternative to gasoline on light-duty vehicles.