Characterization of Cloudwater and Precipitation Chemistry and Deposition at Elevated Sites in Central and Southern California

Citation

Collett, Jeffrey Lee, Jr. (1989) Characterization of Cloudwater and Precipitation Chemistry and Deposition at Elevated Sites in Central and Southern California. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/fybt-5982. https://resolver.caltech.edu/CaltechETD:etd-02062007-142411

Abstract

The chemical composition of cloudwater samples collected in the Sierra Nevada Mountains of central California between 1985 and 1988 was dominated by NH⁺₄, NO⁻₃, and SO²⁻₄. The balance between these three species usually was responsible for determining the cloudwater pH, although inputs of formic and acetic acid also are believed to be important in this regard, particularly for relatively unpolluted samples. The pH of cloudwater sampled in Yosemite National Park, which ranged from 3.8 to 5.2, normally was lower than the pH of samples collected in Sequoia National Park (3.9 to 6.5). Cloudwater pH differences between the two Parks appear to be due to small differences in cloudwater concentrations of NH⁺₄, NO⁻₃, and SO²⁻₄.

The composition of cloudwater in Sequoia National Park is determined primarily by the concentrations of precursor aerosol and soluble gases. Park aerosol concentrations were observed to increase during the approach of a cold front, producing higher pollutant concentrations in the clouds associated with the frontal activity than would be expected based on typical regional aerosol and gas concentrations.

Over 265 hours of cloud interception were observed, during a one-year period of continuous monitoring, at 1860 m elevation in Sequoia National Park. Most interception was observed at elevations above 1500 m -- produced during the passage of convective clouds, associated with cold fronts, across the Sierra. Deposition of pollutants, including acids, to the forest canopy at elevations above 1500 m was estimated to be significant, relative to inputs from precipitation and dry deposition. Cloudwater interception may, in fact, be the dominant deposition mechanism for NH⁺₄ and NO⁻₃, particularly for isolated trees or ridgetop canopies where wind speeds are higher and cloudy air parcels can impact directly on foliar surfaces.

A precipitation study, conducted in the South Coast Air Basin surrounding Los Angeles during the winter of 1987, illustrated the rapid scavenging of pollutants from the air column by falling raindrops. Results of a comparison of N(V) and S(VI) deposition during an extended precipitation event with the atmospheric loading of these species over the basin prior to the event suggested that both species were produced in the atmosphere during the rainstorm. Evidence was seen for in-cloud production of SO²⁻₄, but not of NO⁻₃.

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