The sulfate-CCN-cloud albedo effect: a sensitivity study

Aerosol particles, such as sulfate aerosols, can act as cloud condensation nuclei (CCN). The number of CCN that are available in the atmosphere determines the cloud droplet number concentration and can therefore influence other cloud properties including cloud droplet size distribution, cloud albedo and development of precipitation. Consequently, the increase in anthropogenic emissions of sulfur compounds into the atmosphere may alter such cloud properties and therefore influence the Earths radiative budget. We have used the aerosol sulfate distribution from a chemical model (MOGUNTIA) as input in a general circulation model (GCM) with explicit, but rudimentary, cloud micro physics to study the sensitivity on the global radiative balance of the so-called indirect effect of anthropogenic sulfate aerosols. The chemical model provides us with calculated three-dimensional fields of sulfate aerosol mass. We use several sets of simultaneous aerosol mass and CCN number concentration measurements to establish a hypothetical range of dependencies between the sulfate aerosol mass and cloud droplet number concentration. Three different hypothetical relationships result in an anthropogenic forcing, averaged over the northern hemisphere, ranging from -1.3 to -2.5 Wm{sup -2}. A comparison with observations of cloud droplet radius and cloud albedo indicated that the magnitude of the forcing may have been overestimated. Although these forcing estimates are thus highly uncertain, they do indicate the potential importance of an indirect radiative forcing due to anthropogenic sulfate aerosols. Our study has also shown that the radiative forcing in specific regions of the world is very sensitive to changes in the modelling assumptions. 59 refs, 5 figs, 4 tabs