Molecular hydrogen was measured during January--February and June--July 1979 at a monitoring station at the Naval Research Laboratory (NRL), Washington, D.C. Automatic sampling allowed analysis of a sample-standard pair each 9 min throughout the study period. Results show H2 concentrations ranging from near 'clean air' values (0.6 ppmv) to concentrations exceeding 3 ppmv. Maxima appeared and dissipated over periods of a few (3--5) hours, and H2 appeared to correlate well with CO and NOx but poorly with SO2. The bimodal H2 distribution observed in winter follows the traffic patterns near NRL well, but during the summmer only the midmorning peak correlated well with traffic; the second H2 concentration peak occurs after the maximum traffic load. On the other hand, the timing of both winter and summer peaks appears to be related to patterns of atmospheric boundary layer formation and decay. The data suggest that H2 in the urban troposphere is largely controlled by periodic motor vehicle input (twice daily) and diurnal loss owing to a meterological flushing (boundary layer collapse and wind action).