On the Role of Sulphate in Aerosol Indirect Radiative Forcing

W. RICHARD LEAITCH (1), Nicole Shantz (1), Lynn Russell (2), Jeffrey Pierce (3), Ulrike Lohmann (4),

(1) Science and Technology Branch, Environment Canada, (2) Scripps Institution of Oceanography, University of California San Diego, (3) Dalhousie University, (4) ETH, Zurich

     Abstract Number: 558
     Last modified: May 13, 2010

Abstract
One concern for geoengineering is that our global models are not equipped to adequately simulate the effects of changing constituents in the atmosphere. Another concern is that our knowledge of the roles of some constituents in climate forcing is incomplete. One example where these two worries are highlighted is the indirect effect of aerosol particles on climate, in particular the role of sulfate in indirect radiative forcing. The addition of a small amount of sulfate to an aerosol particle enhances its ability to serve as a cloud condensation nucleus (CCN) and the traditional view is that increasing CCN will increase cloud droplet number concentrations (CDNC) thereby cooling the atmosphere. This is often turned around to say that reductions in sulfur will lead to a more rapid warming of the atmosphere. This simple view does not take into account the effects of droplet growth kinetics on the CDNC. Large amounts of sulfate added to a particle enhance the rate at which that particle absorbs water resulting in a reduction in the water supersaturation at cloud base that may reduce the CDNC or offset the effect of an increase in CCN. This diminishes the cooling potential of the aerosol, and it is one reason that empirical relationships of CDNC to sulfate, which have been used in global models to represent the aerosol indirect effect, do not increase linearly with increasing sulfate. Here, we consider the balance of the major processes by which anthropogenic sulfur affects the CDNC, including nucleation of sulfate particles, condensation of sulfate and in-cloud production of sulfate to test the hypothesis that reductions in sulfate will not be catastrophic to climate as we might think.