Abstract
The number of cloud droplets formed from a population of aerosols depends on the aerosol number concentration, NA, the size distribution, and the chemical composition. The cloud albedo effect occurs when increasing NA causes increases to the droplet concentration, ND. We examined the effects of changing aerosol size, composition, and number on ND within the United States. We found that changing the water‐soluble organic carbon (WSOC) fraction from 50% to 0.05% in the fine mode aerosol and from 50% to 95% in the coarse mode aerosol decreased ND by an average of 34%. Our results show that the changes to the aerosol composition cause over a 20% change to ND, a magnitude previously estimated to cause a 1 W m−2 change in radiative forcing. Given the realistic range of aerosol compositions used here, it is not possible for global models to correctly calculate the cloud albedo effect if composition is ignored.
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