Effect of In-Plume Aerosol Processing on the Efficacy of Marine Cloud Albedo Enhancement from Controlled Sea-Spray Injections

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 (1); 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 (2,3) 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 which are proposed, in the order of 10^17 1/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. We explore the evolution of these sea-salt plumes 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 distribution of the emitted sea-spray plume and local atmospheric conditions, including wind speed and boundary-layer stability. Under the injection rates reported in (1) 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. Finally, we make suggestions for effective size-resolved emissions for use in climate models. (1) Salter, S. et al., Phil. Trans. R. Soc. A., 2008. (2) Korhonen, H. et al., Atmos. Chem. Phys., 10, 4133-4143, 2010. (3) Partanen, A.-I. et al., J. Geophys. Res., 117, D02203, 2012.