Cloud properties inferred from bimodal aerosol number distributions

Nonprecipitating clouds leave a distinctive fingerprint on the aerosol part icles that cycle through them by segregating aerosol particles into two pop ulations, those which are incorporated into cloud droplets and those which are not. This leads to a bimodal or double-peaked character in the aerosol number distribution. If some reasonable assumptions are made, cloud microph ysical properties can be inferred from the bimodal aerosol distributions. W e have collected over 1700 bimodal distributions from five Stations in Nort h America and inferred cloud droplet concentration and maximum supersaturat ion in the clouds which processed the aerosol particles. Average cloud drop let concentrations are 100-200 cm(-3) at "background" stations, while at th e polluted site, cloud droplet concentrations were as high as 3000 cm(-3). Inferred values of maximum supersaturation ranged from 0.3% at clean sites to 0.1% at the polluted site. Cloud droplet concentration and maximum super saturation were usually inversely correlated. Cloud droplet concentration a nd geometric mean diameter of the cloud-processed mode in the aerosol numbe r distribution were also inversely correlated. These two relationships can be understood by comparison with a simple model of cloud activation.