Aerosol over the Southern Ocean

In a first attempt to assess a proposed climatic change feedback process involving cloud condensation nuclei (CCN) and cloud albedo, CCN concentrations N as a function of supersaturation S were measured on a. voyage from latitude 43 to 65 degrees S in October November 1988. The usual relationship N= CSk, with k = 0.5 and C a constant was a fair approximation for S in the range 0.3-0.7% implying that CCN concentrations should largely determine cloud drop concentrations and hence albedo for clouds with S in that range. South of latitude 50 degrees S and at smaller S, k was 1 or larger on average, which would lead to reduced dependence of albedo on CCN for the relevant clouds. N varied very widely for separations of the order of 100 km or 6 hours in time, particularly when the sea was partly ice-covered, suggesting strong local influences. During a large increase in N near 60 degrees S, unaccompanied by an increase in condensation nuclei (CN), cloud drops grew more rapidly than usual. In a subsidiary experiment particles were collected and examined by transmission electron microscopy. For particles less than 0.2 mu m diameter, 80-90% appeared to consist of ammonium sulfate, the remainder being sea salt or an unknown substance which was more liquid and heat-resistant. Dialysis showed that the sulfate particles contained a few percent of insoluble material. Particles which formed cloud drops in vapours other than water, were also studied. Comparison of these and water CCN and the rates of droplet growth suggested that the water insoluble portion of the particles was ethanol-soluble and surface-active. CN concentrations decreased by a factor of about 2 between 43 and 65 degrees S, a change closely parallelled by ethanol CCN concentrations.