Hydrophobic particles can activate at lower relative humidity than slightly hygroscopic ones: a Kohler theory incorporating surface fixed charge

A number of laboratory experiments indicate that hydrophobic carbonaceous particles activate at lower supersaturations than is predicted by Kohler theory. Since a significant fraction of the global energy balance uncertainty is due to the so-called indirect effect, that of clouds, quantifying which particles activate is crucial to accurate prediction of climate. Most material surfaces obtain a fixed charge in water, and this is the case for many materials found in atmospheric aerosols. This fixed charge generates a charge double layer near the material surface which lowers the water activity. In the presence of this surface fixed charge, hydrophobic particles activate at low supersaturations. Since a small number of soluble ions in the particles causes them to only activate at higher supersaturations, surface charge activation is not relevant to activation of particles in the atmosphere. Thus laboratory experiments that measure the growth and activation of hydrophobic particles are measuring an effect, the surface charge effect, that is probably not relevant to the atmosphere.