The Chemical Composition of Fogs and Clouds in Southern California

Citation

Munger, James William (1989) The Chemical Composition of Fogs and Clouds in Southern California. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/YFER-M388. https://resolver.caltech.edu/CaltechETD:etd-02132007-152409

Abstract

Fogs and clouds are frequent occurrences in Southern California. Their chemical composition is of interest due to their potential role in the transformation of sulfur and nitrogen oxides to sulfuric and nitric acid and in the subsequent deposition of those acids. In addition, cloud and fog droplets may be involved in the chemistry of low-molecular-weight carboxylic acids and carbonyl compounds.

The major inorganic species in cloud and fogwater samples were NH₄⁺, H⁺, NO₃⁻, and SO₄²⁻. Concentrations in fogwater samples were 1-10 x 10⁻³ M; pH values ranged from ≃2 to 6. Nitrate usually exceeded sulfate. Acidity depended on the availability of NH₃ from agricultural operations. Stratus cloudwater had somewhat lower concentrations; pH values were in the range 3 - 4. The major factors accounting for variation in fog- or cloudwater composition were the preexisting aerosol and gas concentrations and variations in liquid water content. Deposition and entrainment or advection of different air masses were also important during extended cloud or fog episodes.

The droplet size dependence of cloudwater composition was investigated on one occasion in an intercepted coastal stratus clouds. The observations were consistent with the hypothesis that small droplets form on small secondary aerosol composed of H₂SO₄, HNO₃ and their NH₄⁺ salts, while large droplets form on large sea-salt and soil-dust aerosol. Species that can exist in the gas phase, such as HCl and HNO₃, may be found in either droplet-size fraction.

Concentrations of S(IV) and CH₂O in the range 100 - 1000 µM were observed in fogwater from urban sites in Southern California. Lower concentrations were observed in stratus clouds. The high levels of S(IV) and CH₂O were attributed to the formation of hydroxymethanesulfonate (HMSA), the S(IV) adduct of CH₂O. Direct measurement of HMSA in fogwater samples from Bakersfield, CA were made by ion-pairing chromatography. Glyoxal and methyiglyoxal were observed at concentrations comparable to CH₂O in fogwater samples from Riverside, CA and in stratus cloudwater samples from sites along the Santa Barbara Channel.

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