Long-term in situ observations of atmospheric concentrations of molecular hydrogen were monitored over three years (1996-1998) every 24 minutes 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 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 ratio of H2:CO in pollution plumes was 0.403 ± 0.049 ppb/ppb, in agreement with car emission data, indicating that automobiles are the major anthropogenic source of atmospheric hydrogen.