Hydrogen in the atmosphere: Observations above a forest canopy in a polluted environment

[1] Long-term in situ observations of atmospheric concentrations of molecular hydrogen were monitored every 24 minutes for three years (1996-1998) above Harvard Forest, Massachusetts, in concert with measurements of carbon monoxide and twelve other trace gases. A seasonal cycle with a spring maximum and autumn minimum was observed. The diurnal cycle was characterized by a morning minimum and an afternoon maximum, reflecting the combined effects of uptake by soils and boundary height fluctuations. Enhancements of H-2 concentrations in pollution events, concurrent with winds from the southwest, were typically 100-200 ppb above the background seasonal cycle. The mean molar ratio of H-2 and CO (DeltaH(2)/DeltaCO) in pollution plumes was 0.396 +/- 0.050 ppb/ppb. The results agree with expectations from the water-gas equilibrium (CO + H2O <-> CO2 + H-2) for conditions in automobile engines and with car emission data. These observations suggest that automobiles are the major anthropogenic source of atmospheric hydrogen and that the ratio of DeltaH(2)/DeltaCO has changed little as emission controls have taken effect.