Cloud droplets, formation

Fenner (11) has pointed out that short-lifetime constituents of the atmosphere such as nitrogen oxides, carbon monoxide, and nonmethane hydrocarbons may also play roles related to global warming because of their chemical relations to the longer-lived greenhouse gases. Also, SO, with a very short life interacts with ozone and other constituents to be converted to particulate sulfate, which has effects on cloud droplet formation. 

As expected from the earlier discussion of the Kohler curves, not all particles act as CCN. For example, only about 15-20% of the Aiken nuclei (see Chapter 9.A.2) in a marine air mass off the coast of Washington state acted as CCN at 1% supersaturation (Hegg et al., 1991b). Similarly, in a marine air mass in Puerto Rico, between 24 and 70% of the particles measured at 0.5% supersaturation before cloud formation led to cloud droplet formation (Novakov et al., 1994). 

Gillani, Leaitch, and co-workers (1995) carried out a detailed study of the fraction of accumulation mode particles (diameters from 0.17 to 2.07 /Am) that led to cloud droplet formation in continental stratiform clouds near Syracuse, New York. When the air mass was relatively clean, essentially all of the particles were activated to form cloud droplets in the cloud interior and the number concentration of cloud droplets increased linearly with the particle concentration. However, when the air mass was more polluted, the fraction of particles that were activated in the cloud interior was significantly smaller than one. This is illustrated by Fig. 14.40, which shows the variation of this fraction (F) as a function of the total particle concentration, Nun. In the most polluted air masses (as measured by large values of Nun), the fraction of particles activated was 0.28 + 0.08, whereas in the least polluted, it was as high as 0.96 + 0.05. The reason for this is likely that in the more polluted air masses, the higher number of particles provided a larger sink for water vapor, decreasing the extent of supersaturation. 

However, this relationship did not hold true for continental air masses. The fraction of aerosol particles that lead to cloud droplet formation is clearly less than one, in agreement with the studies of Gillani et al. (1995) discussed earlier. In addition, the relationship is much more scattered, indicating that the chemical... 

The interaction between molecules in the gas phase and the surface of particles and droplets in the atmosphere is central to important phenomena such as ozone depletion in polar areas, hygroscopic growth of particles, aging, acid rain, particle growth and cloud droplet formation. 

Both the number concentrations and sizes of aerosol particles directly affect many of their properties and effects. For example, the ability of particles to serve as nuclei for cloud droplet formation depends on their composition as a function of size, although their effectiveness in any given situation depends also on the number of particles present. Knowledge of these aerosol properties is required to evaluate the indirect effects (Section 4.04.7.3) of aerosol particles on climate, i.e., the effect of aerosol particles on cloud reflectivity and persistence. Therefore much attention has been and continues to be focused on determining particle number concentrations and size distributions. 

It is important to note that few simplifications have been considered in application of Henry s law (sometime also called Henry-Dalton s law) the validity of the ideal gas equation, ideal diluted solution and that the partial molar volume of the dissolve gas is negligible compared with that in the gas phase. However, the range of its validity in the climate system is appreciable. It is clear that during heterogeneous nucleation (CCN to cloud droplet formation) Henry s law is not valid. Absolute values of solubility cannot be found from thermodynamic considerations. Nevertheless, general rules are valid for all gases ... 

Stratmann, F., Kiselev, A., Wendisch, M., Heintzenberg, J., Charlson, R. J., Diehl, K., Wex, H., and Schmidt, S. Laboratory studies and numerical simulations of cloud droplet formation under realistic supersaturation conditions, J. Atmos. Ocean. Tech., 21, 876-887, 2004. 

Padro et al. developed a method called Kohler theory analysis (KTA) which uses Kohler theory coupled with measurements of surface tension, chemical composition, and CCN activity to infer molar volume and solubility [178]. This is a powerful tool for the characterization of the cloud droplet formation potential of ambient particles containing water-soluble organic compounds (WSOC). 

Nenes A, Ghan S, Abdul-Razzak H, Chuang PY, Seinfeld JH (2001) Kinetic limitations on cloud droplet formation and impact on cloud albedo. Tellus B 53 133-149... 

Asa-Awuku A, Nenes A (2007) Effect of solute dissolution kinetics on cloud droplet formation extended Kohler theory. J Geophys Res Atmos 112 D22201. doi 10.1029/ 2005JD006934... 

---