The role of clouds in the enhancement of cloud condensation nuclei concentrations

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Three exist three possible mechanisms for the enhancement of cloud condensation nuclei (CCN) in the vicinity of and within clouds. Firstly, when clouds form due to mixing air masses, real-time CCN measurements show that CCN concentrations could register an increase around such clouds due to differences in the aerosol content of the two air masses (the mechanism being the transport of new CCN to the measurement site). Secondly, due to sulfate production within clouds themselves, the CCN concentration in the air mass processed by such clouds could be elevated due to a shift in the size distribution and chemical composition of the existing aerosol particles. Thirdly, new aerosol particles could be formed by the process of homogeneous, heteromolecular nucleation in the vicinity of and within clouds resulting from a modification of the supersaturation field. The third mechanism is capable of enhancing simultaneously both condensation nuclei (CN) and CCN concentrations. By analyzing the existing field measurements, examples demonstrating the enhancement in CCN concentration within clouds, close to the cloud top, and in clear but moist air above the cloud tops are provided. It is pointed out that for CCN to play an effective role in climate-regulation (i.e. counteracting the greenhouse warming due to CO 2 etc.), their number concentrations should increase by as much as a factor of four which would cause an increase in the global albedo by ≈ 1.7%. Current evidence shows that such a CCN enhancement may be possible locally but is not observable in general. Although anthropogenically influenced arctic stratus clouds were observed to produce significant CCN enhancement, no such effect was observed in antarctic stratus clouds. A model based on homogeneous, heteromolecular aerosol particle production was used to calculate particle production. Model results of sensitivity studies are presented that explain why the observations of this phenomenon so far have been so few. Enhanced actinic radiation flux, high relative humidity, abundance of gaseous sulfur, and long life span of clouds are found to be common features of observations that indicate CCN enhancement within clouds. Such clouds are very active photochemically. Sampling artifacts encountered in airborne observations could also occur and may explain partially or totally the observed CCN enhancement although this explanation is far from satisfactory and precludes the observed CCN enhancement in clear moist layers above the cloud tops.