Ej. Carter et Rd. Borys, AEROSOL-CLOUD CHEMICAL FRACTIONATION - ENRICHMENT FACTOR-ANALYSIS OF CLOUD-WATER, Journal of atmospheric chemistry, 17(3), 1993, pp. 277-292
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.