Efficient scavenging of aerosol sulfate by liquid-water clouds

Abstract

Continuous aircraft measurements of aerosol sulfate concentration, [SO 4 2−], (flame-photometric detector) and light-scattering coefficient, b scat, (integrating nephelometer) in cloud interstitial air and in associated clear air are reported for several locations in the eastern United States. Measurements were made in situations where, on the basis of other physical and chemical measurements, it can be argued that the cloud air and the clear air have similar history and composition. It is found in cloud interstitial air that [SO 4 2−] approaches zero and b scat approaches the Rayleigh scattering coefficient of air. Sulfate concentration and b scat in successive subsaturated regions along a given flight path at a given altitude were frequently nearly constant or slowly varying, the pattern presented appearing to be that of a fairly uniform concentration of sulfatecontaining, light-scattering aerosol from which segments had been removed in regions corresponding to the presence of cloud. The strong decrease in both [SO 4 2−] and b scat within cloud interstitial air relative to clear air is attributed to nearly complete uptake of sulfate-containing and light-scattering aerosol particles into cloud droplets. From comparison of the in-cloud signal to the non-cloud signal it is possible to set an upper bound to the fraction of unscavenged sulfate aerosol (frequently as low as 0.25) and to b scat (frequently as low as 0.1), although for both quantities the measurements would be consistent with essentially complete scavenging. Additionally, the observation in these situations that no cloud-free regions exhibited the low values of [SO 4 2−] and b scat characteristic of the in-cloud regions is indicative of the return of the dissolved material to the air as clear-air aerosol upon cloud evaporation.