Molecular hydrogen (H2) emissions from gasoline and diesel vehicles

This study assesses individual-vehicle molecular hydrogen (H2) emissions in exhaust gas from current gasoline and diesel vehicles measured on a chassis dynamometer. Absolute H2 emissions were found to be highest for motorcycles and scooters (141 ± 38.6 mg km− 1), approximately 5 times higher than for gasoline-powered automobiles (26.5 ± 12.1 mg km− 1). All diesel-powered vehicles emitted marginal amounts of H2 (∼ 0.1 mg km− 1). For automobiles, the highest emission factors were observed for sub-cycles subject to a cold-start (mean of 53.1 ± 17.0 mg km− 1). High speeds also caused elevated H2 emission factors for sub-cycles reaching at least 150 km h− 1 (mean of 40.4 ± 7.1 mg km− 1). We show that H2/CO ratios (mol mol− 1) from gasoline-powered vehicles are variable (sub-cycle means of 0.44–5.69) and are typically higher (mean for automobiles 1.02, for 2-wheelers 0.59) than previous atmospheric ratios characteristic of traffic-influenced measurements. The lowest mean individual sub-cycle ratios, which correspond to high absolute emissions of both H2 and CO, were observed during cold starts (for automobiles 0.48, for 2-wheelers 0.44) and at high vehicle speeds (for automobiles 0.73, for 2-wheelers 0.45). This finding illustrates the importance of these conditions to observed H2/CO ratios in ambient air. Overall, 2-wheelers displayed lower H2/CO ratios (0.48–0.69) than those from gasoline-powered automobiles (0.75–3.18). This observation, along with the lower H2/CO ratios observed through studies without catalytic converters, suggests that less developed (e.g. 2-wheelers) and older vehicle technologies are largely responsible for the atmospheric H2/CO ratios reported in past literature.