Remote Free Troposphere Research Articles

Seasonal and diurnal variability of nitric acid vapor and ionic aerosol species in the remote free troposphere at Mauna Loa, Hawaii

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.