Chemical composition of coastal stratus clouds: Dependence on droplet size and distance from the coast

Abstract

The aerosol at elevated sites in the South Coast Air Basin in California is a mixture of sea salt and pollution-derived secondary aerosol. The influence of sea salt declines with increasing distance from the coast. Nitric acid appears to react with the NaCl in sea salt aerosol to release HCl_(g) and form NaNO_3 in the aerosol. At inland sites, aerosol concentrations differ during periods of onshore and offshore flow. The highest concentrations were observed during the day when the onshore flow transported pollutants to the sites, while lower concentrations were observed at night when drainage flows from nearby mountains influenced the sites. Variations, in liquid water content are a major influence on cloudwater ion concentrations. Comparisons of the ionic concentrations in two size-segregated fractions of cloudwater collected during several sampling intervals suggest that there is a large difference between the average composition of the smaller droplets and that of the larger droplets. The concentrations of Na^+, Ca^(2+) and Mg^(2+) in the large-droplet fraction were observed to be higher than in the small-droplet fraction, while the concentrations of SO_4^(2−), NO_3^−, NH_4^+ and H^+ were higher in the small-droplet fraction. Chloride concentrations were nearly equal in both fractions. Differences in the composition of size-fractionated cloudwater samples suggest that large droplets are formed on sea salt and soil dust, which are large aerosol, and small droplets are formed on small secondary aerosol composed primarily of (NH_4)_2SO_4 and NH_4NO_3. The concentrations of several components that exist partly in the gas phase (e.g. Cl^−, HCOOH and CH_3COOH) appear to be independent of droplet size.

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