Aerosol-cloud chemical fractionation: enrichment factor analysis of cloud water

Supercooled cloud water collected as a function of droplet size at a mountain-top site in Colorado was analyzed for trace element, anion, and cation content. The influence of chemical fractionation of aerosols during cloud droplet formation was determined by evaluation of enrichment factors (EFs) of chloride, potassium, sodium, and sulfate ions relative to aluminum and Na as marine and crustal reference elements. The highest EFs for all ions were calculated for droplets smaller than 10-15 mu m diameter. Potassium and Cl ion EFs in bulk cloud water were in crustal and marine proportions, respectively. Greater enrichments in small droplets is a result of their formation on small aerosol particles; large droplets form on largest aerosol particles.