Road vehicle emissions of molecular hydrogen (H2) from a tunnel study

Motor vehicle combustion emissions of molecular hydrogen (H2), carbon monoxide (CO), and carbon dioxide (CO2) were measured during a 6-week period from November 2004 to January 2005 in Gubrist Tunnel, Switzerland, to determine vehicle emission factors for these trace gases and the ratios of the concentration growths ΔH2/ΔCO and ΔH2/ΔCO2 in the tunnel under real-world highway driving conditions. For H2, molar mixing ratios at the tunnel exit were found to be 7–10 ppm (parts-per-million, 10-6) during rush hours. Mean emission factors of , , and were calculated. EH2 was largest during weekday rush-hour traffic, a consequence of the more frequent accelerations in congested traffic when fuel combustion is not optimal. EH2 was smaller for heavy-duty vehicles (HDV) compared to light-duty vehicles (LDV), a finding which was attributed to the diesel vs. gasoline engine technology. The mean ΔH2/ΔCO molecular ratio was 0.48±0.12. This ratio increased to 0.6 during rush hours, suggesting that H2 yield is favored relative to CO under fuel-rich conditions, presumably a consequence of an increasing contribution of the water–gas-shift reaction. The mean ΔH2/ΔCO2 molecular ratio was 4.4×10-3 but reduced to 2.5×10-3 when the relative HDV abundance was at maximum. Using three different approaches, road traffic H2 emissions were estimated for 2004 for Switzerland at 5.0–6.6 Gg and globally at 4.2–8.1 Tg. Despite projections of increasing traffic, Swiss H2 emissions are not expected to change significantly in the near future, and global emissions are likely to decrease due to improved exhaust gas clean-up technologies.