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 H2 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 H2 and CO (ΔH2/ΔCO) in pollution plumes was 0.396 ¿ 0.050 ppb/ppb. The results agree with expectations from the water-gas equilibrium (CO + H2O ↔ CO2 + H2) 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 ΔH2/ΔCO has changed little as emission controls have taken effect.