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\ninjection from ships (marine cloud brightening) has been proposed as a\npossible method to control anthropogenic global warming; however, there\nremains significant uncertainty in the efficacy of this method due to,\namongst other factors, uncertainties in aerosol and cloud microphysics.\nA major assumption used in recent cloud-and climate-modeling studies is\nthat all sea spray was emitted uniformly into some oceanic grid boxes,\nand thus these studies did not account for subgrid aerosol coagulation\nwithin the sea-spray plumes. We explore the evolution of these sea-salt\nplumes using a multi-shelled Gaussian plume model with size-resolved\naerosol coagulation. We determine how the final number of particles\ndepends on meteorological conditions, including wind speed and\nboundary-layer stability, as well as the emission rate and size\ndistribution of aerosol emitted. Under previously proposed injection\nrates and typical marine conditions, we find that the number of aerosol\nparticles is reduced by over 50 %, but this reduction varies from under\n10% to over 90% depending on the conditions. We provide a\ncomputationally efficient parameterization for cloud-resolving and\nglobal-scale models to account for subgrid-scale coagulation, and we\nimplement this parameterization in a global-scale aerosol-climate model.\nWhile designed to address subgrid-scale coagulation of sea-salt\nparticles, the parameterization is generally applicable for coagulation\nof subgrid-scale aerosol from point sources. We find that accounting for\nthis subgrid-scale coagulation reduces cloud droplet number\nconcentrations by 46% over emission regions, and reduces the global\nmean radiative flux perturbation from -1.5 W m(-2) to -0.8 W m(-2).