Factors determining the most efficient spray size distribution for marine cloud brightening

Abstract:

We investigate the sensitivity of marine cloud brightening geoengineering to the properties of the added salt particle distribution using a cloud parcel model, with an aim to address the question: what is the most efficient particle size distribution to produce a desired cooling effect?. We investigate this by using a parcel model with different configurations of salt spray size distribution to assess the approximate power required to generate an albedo increase sufficient to approximately offset a doubling of carbon dioxide. Our findings show that this question depends on the spray generation method employed. We find that for all three methods of spray generation investigated (supercritical fluid, Taylor cone, and Rayleigh jet), distributions of salt particles with median dry diameters in the range Dm = 30 to 100 nm are the are the most effective in terms of brightening per power consumed in generating the particles. Size distributions that resemble the present day emitted sea-spray distribution are not particularly efficient for brightening clouds. The Rayleigh jet method is found to be the most energy efficient method when compared to the supercritical fluid and Taylor cone-jet method. We also find that care needs to be taken when using droplet activation parameterizations: for the concentrations considered Aitken particles do not result in a decrease in the total albedo, as was found in a recent study, and such findings are likely a result of the parameterization's inability to simulate the effect of swollen aerosol.