KINETIC LIMITATIONS ON DROPLET FORMATION IN CLOUDS

The 'indirect' radiative cooling of climate due to the role of anthrop ogenic aerosols in cloud droplet formation processes (which affect clo ud albedo) is potentially large, up to -1.5 W m(-2) (ref. 1). It is im portant to be able to determine the number concentration of cloud drop lets to within a few per cent, as radiative forcing as a result of clo uds is very sensitive to changes in this quantity(2), but empirical ap proaches are problematic(3-5). The initial growth of a subset of parti cles known as cloud condensation nuclei and their subsequent 'activati on' to form droplets are generally calculated with the assumption that cloud droplet activation occurs as an equilibrium process described b y classical Kahler theory(6,7). Here we show that this assumption can be invalid under certain realistic conditions. We conclude that the po or empirical correlation between cloud droplet and cloud condensation nuclei concentrations is partly a result of kinetically limited growth before droplet activation occurs. Ignoring these considerations in ca lculations of total cloud radiative forcing based on cloud condensatio n nuclei concentrations could lead to errors that are of the same orde r of magnitude as the total anthropogenic greenhouse-gas radiative for cing(1).