Hegg Because the precipitation of clouds is low, sulfate particles go through several cloud condensation-evaporation events before they are removed. This processing within the cloud, involving the absorption of gaseous SO2 into cloud droplets and subsequent oxidation to SO4, has two important effects. First, initially small particles gain mass during cloud condensation-evaporation cycles and are thus able to reach activatable size at lower cloud supersaturation. This enables them to impact a much wider range of cloud type, e.g. the low supersaturation marine stratus clouds that are so important climatically. Second, an increase in mass occurs even for larger particles (r 0.1 um) that have served as cloud condensation nuclei (CCN) during several could cycles. These particles grow into the Mie scattering regime and become more effective light scatters. In summary, the aqueous phase production of sulfate in clouds (followed by evaporation) potentially produces aerosols that are better CCN and more efficient light scatterers. In this SGER study, an attempt is to be made to quantify in- cloud sulfate production in marine stratiform clouds at the Cheeka Peak Research Station on the Olympic Peninsula in Washington state. The consequences of the amount of sulfate production observed for direct and indirect radiative forcing of climate will then be examined. This will be done both through diagnostic modeling studies to relate the observed sulfate production to changes in CCN activity and light-scattering, and by comparing the results with those of colleagues who will be simultaneously measuring the particle size distribution, and light-scattering properties of the sulfate aerosols.
