Temperature dependence of cloud drop activation of insoluble particles

The critical supersaturation of cloud droplet activation by water-soluble aerosols increases at lower temperatures. This is due to the Kelvin effect, with the logarithm of the saturation ratio being inversely proportional to the absolute temperature and linearly proportional to the surface tensions and molecular volume of water. Less is known about the temperature dependence of critical supersaturation when the cloud condensation nuclei (CCN) are water-insoluble.  The FHH activation theory describes the CCN activation of insoluble particles by combining the FHH (Frenkel-Halsey-Hill) adsorption isotherm and the Kelvin equation. The temperature dependence induced by the Kelvin term is inherently similar to that observed in water-soluble particles. However, the influence of the adsorption term on critical supersaturation as a function of temperature remains unclear.  The typical temperature dependence of water vapour adsorption is such that an increase in the adsorption layer thickness is expected with decreasing temperature at a constant saturation ratio. Nevertheless, it is known that some adsorbent materials behave differently, adsorbing water vapour more efficiently at higher temperatures, while a third class of adsorbents shows no temperature dependence at all. In this study, we investigate the temperature dependencies of critical supersaturations for water-insoluble particle types that exhibit diverse temperature responses in adsorption measurements. We interpret the results in terms of the FHH adsorption activation model.