Measurements of nitric acid vapor and aerosol nitrate, sulfate, and ammonium were made at the Mauna Loa Observatory on Hawaii between October of 1984 and January of 1986 during 10-day periods every 2 months. Nitric acid exhibited both a diurnal cycle (higher concentrations during the upslope wind hours of 0900 to 2100 LT) and a seasonal modulation (highest concentrations during the late summer). In August the daytime/nighttime average HNO3 concentrations were 97/79 parts per trillion by volume (pptv), while in February they were only 18/8 pptv. The ratio of summer/winter average HNO3 concentrations was 8. Dry deposition at the surface may be a major factor causing the diurnal variation, causing the measured surface values to be lower than the concentrations in the free troposphere above the surface. The long-range transport of anthropogenic odd nitrogen from the west coast of North America is a likely cause of the higher summertime nitric acid concentrations. This suggests that there may be regular periods during which apparently ''short-lived'' anthropogenic trace gases may be transported to remote parts of the atmosphere with relatively high efficiency. Aerosol NO3-, SO42-, and NH4 + also showed seasonal variations: summer/winter ratios were 3, 2, and 0.5,respectively. ¿American Geophysical Union 1987