Measurements of the scavenging of sulfate and nitrate in clouds

Measurements are presented on the scavenging of sulfate and nitrate by cumulus, stratus and strato-cumulus clouds. Assuming that all of the particulate sulfate was in the size range 0.1–1.0 μm radius and that sulfate was scavenged with the same efficiency as sub-micrometer particles in general, the nucleation scavenging coefficient of sulfate in these clouds was deduced to be 0.7 ±0.2 and evidence for sulfate production (1.0±0.3 μgm⁻³) within cloud water was also obtained. Evidence for nitrate scavenging, by nitrates serving as cloud condensation nuclei or by the absorption of HNO₃ by cloud droplets, is also presented. The data suggest that either gaseous nitrogen compounds in the air other than HNO₃ can dissolve and contribute to the nitrate concentration in cloud water or that nitrate can be produced within cloud droplets.