Chemical Composition of Cloud Droplets

For sufficiently large droplets the growth rate is approximately proportional to the inverse of the droplet diameter (see (15.74)) and 

FIGURE 15.17 Predicted dependence of the total solute concentration on droplet diameter during the lifetime of a cloud (Pandis et. al., 1990a). 

The above predictions agree with measured concentration/size dependencies measured in clouds that are not heavily influenced by anthropogenic sources. Noone et al. (1988) sampled droplets from a marine stratus cloud and calculated that the volumetric mean solute concentration of the 9 to 18 ixm droplets was a factor of 2.7 smaller than in the 18 to 23 ixm droplets. Ogren et al. (1989) reported similar results for a cloud in Sweden. On the other hand, similar measurements for cloud and fog droplets in heavily polluted environments suggest that solute concentrations decrease with increasing droplet size (Munger et al., 1989 Ogren et al., 1992). No satisfactory explanation exists for such behavior. 

FIGURE 15.18 Measured composition of the small and large cloud droplets collected in coastal stratus clouds at La Jolla Peak, California, in July 1993 (Collett et al., 1994). 

Measurements of bulk cloudwater concentrations have been presented by a number of investigators. These concentrations vary significantly because of both the aerosol loading (degree of anthropogenic influence) and the liquid water content of the cloud. 

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