CLOUD PROCESSING OF THE CLOUD CONDENSATION NUCLEUS SPECTRUM AND ITS CLIMATOLOGICAL CONSEQUENCES

A modelling study of the effects of aqueous phase cloud chemistry in changing the activated cloud condensation nucleus (CCN) spectrum is presented. The chemistry model is contained within a model of the development of a hill cap cloud. The aqueous phase oxidation of sulphur (IV) to sulphur (VI) by the oxidants ozone and hydrogen peroxide is considered for each of the explicit cloud-droplet categories. All gases enter cloud droplets at a finite rate which is calculated in the model. Significant modification of the CCN spectrum emerging downwind of the processing cloud is observed. This leads to the formation of strongly bimodal aerosol size distributions. The degree of modification is strongly correlated to the concentration of sulphur dioxide, and in oxidant limited situations to the concentration of hydrogen peroxide. The cloud-droplet chemistry is seen to have the largest effect on the smallest activated CCNs. This enables such particles to activate much more readily after cloud processing at critical supersaturations up to 20 times lower than originally required. A simple cloud model is used to investigate the sensitivity of this secondary activation. With updraughts typical of a stratocumulus-type cloud, droplet effective radii are seen to be lowered by up to 3 mum, 500 m above cloud base. This result is insensitive to the concentration of sulphur dioxide present in the processing cloud down to very low concentrations. If reproduced on a global scale this result could have important climatological consequences, as recently indicated by Slingo (1990). However, the reduction in effective radius is expected to be important only on local scales close to the sources of new aerosol, where the processing of the CCN spectrum by clouds may be occurring for the first time. Additional cloud processing will lead to changes in the direct radiative properties of the aerosols but will have little effect upon the radiative properties of clouds subsequently forming on them.