Droplet nuclei in non-precipitating clouds: composition and size matter

The composition of cloud droplet nuclei was compared to the composition of ambient aerosol particles in non-precipitating clouds in different regions. Single-particle electron microscope techniques were used to identify particle types. The smallest particles (<0.2 mu m diameter) exhibited a dependence of composition on nucleating ability, with salts (chlorides and sulfates of Na, K, Ca and Mg) being preferred nuclei types, followed by ammonium sulfates and organics. Crustal and industrial metals were less likely to be incorporated into cloud droplets. However, for experiments where only larger particles were sampled, a weaker dependence on composition was found. This suggests that larger particles have sufficient soluble material to nucleate despite their primary type. Both size and composition seem to be important, with composition becoming increasingly important as smaller particles activate at higher supersaturations in the cloudy environment.