BISTABILITY OF CCN CONCENTRATIONS AND THERMODYNAMICS IN THE CLOUD-TOPPED BOUNDARY-LAYER

RECENT discussions of the effect of cloud condensation nuclei (CCN) in controlling the albedo of marine stratus cloud and hence global climate1-3have raised questions concerning the factors that control CCN populations and how (or whether) such factors should be included in climate models4. The observed near-constancy5 of CCN concentrations in remote marine air is remarkable in view of the wide variety of possible conditions, the range over which they might be expected to vary, and the considerable difference between marine and continental CCN concentrations. Here we explain this relative lack of variability, thought to be due to a balance of source and sink processes6,7, in a simple model of the marine cloud-topped boundary layer. Solutions to this model give two stable CCN concentration regimes-one corresponding to the low concentration observed over the ocean, the other corresponding to the higher concentration observed over land-each dominated by a different sink mechanism. The two regimes have very different optical properties, and thus may be of climatic significance. © 1990 Nature Publishing Group.