Reduced efficacy of marine cloud brightening geoengineering due to in-plume aerosol coagulation: Parameterization and global implications
The intentional enhancement of cloud albedo via controlled sea-spray injection from ships (marine cloud brightening) has been proposed as a possible method to con-trol anthropogenic global warming; however, there remains significant uncertainty in the efficacy of this method due to, amongst other factors, uncertainties in aerosol and cloud mi-crophysics. 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 stud-ies did not account for subgrid 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 fi-nal number of particles depends on meteorological condi-tions, 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 typ-ical 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 condi-tions. We provide a computationally efficient parameteriza-tion for cloud-resolving and global-scale models to account for subgrid-scale coagulation, and we implement this param-eterization in a global-scale aerosol-climate model. While designed to address subgrid-scale coagulation of sea-salt par-ticles, the parameterization is generally applicable for coag-ulation of subgrid-scale aerosol from point sources. We find that accounting for this subgrid-scale coagulation reduces cloud droplet number concentrations by 46 % over emission regions, and reduces the global mean radiative flux perturba-tion from −1.5 W m −2 to −0.8 W m −2 .