AAAR 32nd Annual Conference
September 30 - October 4, 2013
Oregon Convention Center
Portland, Oregon, USA
Abstract View
Parameterization of In-Plume Aerosol Processing Effects on the Efficacy of Marine Cloud Albedo Enhancement from Controlled Sea-Spray Injections
Geoff Stuart, ROBIN STEVENS, Dominick Spracklen, Hannele Korhonen, Jeffrey Pierce, Dalhousie University
Abstract Number: 186 Working Group: Aerosols, Clouds, and Climate
Abstract The intentional enhancement of cloud albedo via controlled sea-spray injection from ships has been proposed as a possible method to control anthropogenic global warming; however, there remains significant uncertainty in the efficacy of this method due to uncertainties in aerosol and cloud microphysics. A major assumption used in multiple recent studies is that all sea-spray was emitted uniformly into some oceanic grid boxes, and thus did not account for sub-grid aerosol microphysics within the sea-spray plumes. However, as a consequence of the fast sea-spray injection rates that are proposed, in the order of 10$^(17) s$^(-1), particle concentrations in these plumes may be quite high and particle coagulation may significantly reduce the number of emitted particles and increase their average size. Therefore, it is possible that the emissions necessary to reach a desired cooling may be even larger than currently assumed.
Using a multi-shelled Gaussian plume model with size-resolved aerosol coagulation, we determine how the final number and size of particles depends on the emission rate and size of the emitted sea-spray plume and local atmospheric conditions, including wind speed and boundary-layer stability. Under the injection rates reported in Salter (2008) and typical marine conditions, we find that the number of aerosol particles is reduced by about 40%. This fraction decreases for decreasing emission rates or increasing wind speeds due to lower particle concentrations in the plume.
Based on the relations for expanding plumes from Turco and Yu (1997) and the results of our model, we develop a parameterization for the fraction of particles remaining based on the emissions rate, size distribution of emissions, wind speed, and boundary-layer stability. This parameterization should allow more accurate predictions of the aerosol number concentrations that would be produced by sea-spray geoengineering, and thus more accurate assessments of the efficacy and side-effects of this method.