Intensive studies of Sierra Nevada cloudwater chemistry and its relationship to precursor aerosol and gas concentrations

Abstract

Measurements of inorganic aerosol and gas phase species are presented for three sites in central California during a 4 day period in April 1988. The measurement sites were located along an east-west transect at Visalia, Ash Mountain, and Lower Kaweah, with elevations of 90, 550 and 1900 m, respectively. Aerosol compositions were nearly neutral at all locations, however large concentrations of NH 3 at Visalia contributed significant excess alkalinity to the air mass sampled there. Concentrations of all major species were observed to decrease with elevation during most of the sampling periods. Concentrations at the upper two sites exhibited diurnal fluctuations, with peaks in the late afternoon, consistent with the transport of pollutants from San Joaquin Valley sources by daytime upslope winds. Concentrations of most of these species reached a maximum at the elevated sites on 28 April, as a weak cold front approached, reducing the atmospheric stability over the valley floor. Concentrations at Visalia on this day were somewhat lower than those observed earlier in the week. Clouds intercepting the mountain slopes on 28 April were sampled at two locations. The coudwater pH at both sites was observed to fall throughout the event, dropping as low as 4.34. Precursor concentrations of aerosol NO 3 −, SO 4 2- and NH 4 +, and gas phase HNO 3 and NH 3, were sufficient to account for the observed cloudwater loadings of NO 3 −, SO 4 2- and NH 4 +. In-cloud measurements made near the cloud base indicated a considerable S(IV) oxidation potential in the form of H 2O 2, but only low S(IV) concentrations. Cloudwater concentrations of formic acid were approximately three times acetic acid concentrations. Carbonyl concentrations were dominated by formaldehyde and glyoxal.