Generation and delivery of particles to brighten marine clouds: constraints on the size distribution

Assessing marine cloud brightening (MCB) as a potential climate intervention strategy requires knowledge about whether it is possible to predictably and reliably increase planetary albedo through brightening low marine clouds at a scale sufficient to affect climate. Here, we address one of the "checkpoints" that must be resolved in order to know if MCB is a physically viable climate intervention strategy. In order to show "that generators can be designed to produce aerosol particles of an appropriate size and number that can be delivered to the cloud base and activate" (Diamond et al., 2022), this "appropriate" size must first be determined. We assess what modeling and observational studies indicate about the range of injected aerosol size and concentration that could lead to brightening and, conversely, what aerosol sizes and concentrations would likely lead to dimming.

The key behavior underlying MCB is Twomey brightening, which results from increased cloud droplet concentration and reduced droplet size (assuming cloud condensate and cover remain unchanged). However, cloud macrophysical adjustments to changes in droplet size can also strongly contribute to changes in cloud albedo. These adjustments depend in part upon the presence and amount of precipitation in the unperturbed clouds, and the degree to which precipitation is altered by aerosol injection. Smaller cloud droplets reduce the propensity for precipitation formation, so suppression of precipitation in perturbed clouds may increase cloud condensate and cover, enhancing brightening over that from the Twomey effect alone. Smaller droplets can also enhance entrainment mixing at cloud top, which may reduce cloud condensate. If giant (supermicron) aerosol particles are introduced into the cloud in sufficient quantity, then these may encourage precipitation formation, or at least reduce the suppression of precipitation that occurs by increasing droplet concentration. Large aerosol particles might also reduce the peak supersaturation in updrafts and limit the activation of the smaller injected particles. This study reviews existing literature and uses explicit microphysics modeling to provide constraints on the sizes and concentrations of aerosols that will be most effective for MCB.