PNNL

Abstract

Under certain conditions growth kinetics may affect the growth of cloud condensation nuclei (CCN) sufficiently to limit significantly the number of activated droplets relative to the number that are predicted based on the assumption that particles remain in equilibrium until activated. This work identifies three such mechanisms, and the effect of kinetic limitations on activated cloud droplet number and cloud albedo is assessed using detailed one-dimensional cloud microphysics parcel models for a variety of aerosol size distributions, updraft velocities, and entrainment rates. In assessing the effect of kinetic limitations, we have assumed cloud droplets to be not only those that are strictly activated (as dictated by classical Kohler theory), but also unactivated drops large enough to contribute significantly to cloud albedo. We find that the equilibrium assumption leads to an overprediction of droplet number by less than 10% for marine aerosol, but this overprediction can exceed 40% for urban type aerosol. The maximum change in cloud albedo as a result of kinetic limitations is less than 0.005 for cases such as marine aerosol; albedo differences can however be large, exceeding 0.1, for more polluted conditions. Kinetic limitations on cloud droplet formation are not expected to be climatically significant, but can have locally a large impact on cloud albedo. Overall, the effect of kinetic limitations on cloud albedo can be considered to become important when equilibrium activation theory consistently overpredicts droplet number exceeding 10%.