AEROSOL-CLOUD CHEMICAL FRACTIONATION - ENRICHMENT FACTOR-ANALYSIS OF CLOUD-WATER

A field study was conducted at a mountain-top site in northwestern Col orado. Supercooled cloud water, collected as a function of droplet siz e, was analyzed for anions, cations and trace elements. Enrichment fac tors (EF) of SO42-, K+, Na+ and Cl- relative to crustal and marine ref erence elements (Al and Na) were calculated to determine whether chemi cal fractionation of the aerosol occurs during cloud droplet formation . The largest EF's for all ions were found for droplets less than 10-1 5 mu m diameter. Ratios of the small to large droplet mean EF's ranged from 1 to 2, for SO42- relative to both Al and Na+, to 10 to 12 for N a+, Cl- and K+, relative to Al. EF's of Kf and Cl- in the bulk cloud w ater were in crustal and marine proportions, respectively. It was conc luded that although bulk cloud chemistry may indicate a lack of enrich ment of a species, this may not be true throughout the droplet size di stribution. The higher enrichments in small droplets is likely a resul t of their formation on small aerosol particles whereas the large drop lets form on the largest aerosol particles. This may suppress EF's in precipitation relative to the total aerosol.