title: Molecular hydrogen (H2) emissions from gasoline and diesel vehicles.
abstract: 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 ( approximately 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.
authors.affiliation: Empa, Swiss Federal Laboratories for Materials Science and Technology, Laboratory for Air Pollution/Environmental Technology, Ueberlandstrasse 129, CH-8600 Duebendorf, Switzerland. steven.bond@empa.ch
authors.forename: S W
authors.initials: SW
authors.lastname: Bond
authors.forename: R
authors.initials: R
authors.lastname: Alvarez
authors.forename: M K
authors.initials: MK
authors.lastname: Vollmer
authors.forename: M
authors.initials: M
authors.lastname: Steinbacher
authors.forename: M
authors.initials: M
authors.lastname: Weilenmann
authors.forename: S
authors.initials: S
authors.lastname: Reimann
mesh_terms: D000393:Air Pollutants / Q000032:analysis*; D001336:Automobiles / Q000145:classification / Q000706:statistics & numerical data; D002248:Carbon Monoxide / Q000032:analysis; D005742:Gasoline / Q000032:analysis*; D006859:Hydrogen / Q000032:analysis*; D001335:Vehicle Emissions / Q000032:analysis*
chemical_list: D000393:Air Pollutants; D005742:Gasoline; D001335:Vehicle Emissions; D002248:Carbon Monoxide; D006859:Hydrogen