Reduced efficacy of marine cloud brightening geoengineering due to in-plume aerosol coagulation: parameterization and global implications

Abstract

Abstract. The intentional enhancement of cloud albedo via controlled sea-spray injection from ships (Marine Cloud Brightening) 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, amongst other factors, uncertainties in aerosol and cloud microphysics. A major assumption used in recent cloud- and climate-modeling studies is that all sea spray was emitted uniformly into some oceanic grid boxes, and thus these studies did not account for sub-grid aerosol coagulation within the sea-spray plumes. 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 of particles depends on meteorological conditions, including wind speed and boundary-layer stability, as well as the emission rate and size distribution of aerosol emitted. Under previously proposed injection rates and typical marine conditions, we find that the number of aerosol particles is reduced by over 50%, but this reduction varies from under 10% to over 90% depending on the conditions. We provide a computationally efficient parameterization for cloud-resolving and global-scale models to account for sub-grid scale coagulation, and we implement this parameterization in a global-scale aerosol-climate model. We find that accounting for this sub-grid scale coagulation reduces cloud droplet number concentrations by 46% over emission regions, and reduces the global mean radiative flux perturbation from −1.5 W m-2 to −0.8 W m-2.