Clouds Kohler curves

There are several basic physical-chemical principles involved in the ability of aerosol particles to act as CCN and hence lead to cloud formation. These are the Kelvin effect (increased vapor pressure over a curved surface) and the lowering of vapor pressure of a solvent by a nonvolatile solute (one of the colligative properties). In Box 14.2, we briefly review these and then apply them to the development of the well-known Kohler curves that determine which particles will grow into cloud droplets by condensation of water vapor and which will not. 

As we have seen in Chapter 9, there are a variety of dissolved solutes in atmospheric particles, which will lower the vapor pressure of droplets compared to that of pure water. As a result, there is great interest in the nature and fraction of water-soluble material in atmospheric particles and their size distribution (e.g., Eichel el al., 1996 Novakov and Corrigan, 1996 Hoffmann et al., 1997). This vapor pressure lowering effect, then, works in the opposite direction to the Kelvin effect, which increases the vapor pressure over the droplet. The two effects are combined in what are known as the Kohler curves, which describe whether an aerosol particle in the atmosphere will grow into a cloud droplet or not under various conditions. 

As we have already seen, the critical supersaturation Sc. corresponding to the peak of the Kohler curve depends on a number of parameters unique to the aerosol particle. Thus, at a given supersaturation some particles will form cloud droplets and some will not. As a result, the total number of CCN will vary with the supersaturation used in the CCN measurement. This is illustrated in Fig. 14.39, which shows the concentration of CCN measured in Antarctica as a function of the percentage supersaturation for CCN that grow into droplets larger than 0.3 and 0.5 gm, respectively (Saxena, 1996). This particular set of measurements... 

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). 

These plots are called Kohler curves after their originator (Kohler, 1936). His assumptions that cloud condensation nuclei (CCN) are water-soluble materials is now widely accepted. In the past, it was often thought that NaCl particles from the ocean were the main CCN however, more recent studies have demonstrated the frequent dominance of sulfate particles with composition between H2SO4 and (NH4)2S04-... 

Junge and McLaren (1971) have studied the effect that the presence of insoluble material has on the capacity of aerosol particles to serve as cloud condensation nuclei. Using Eq. (7-25) they calculated the supersaturation needed for an aerosol particle to grow to the critical radius at the peak of the Kohler curve, from where spontaneous formation of cloud drops becomes feasible. The results are shown in Fig. 7-8. They indicate that the difference is less than a factor of two in radius for particles whose soluble fraction is greater than e =0.1. The majority of particles can be assumed to meet this condition (see Fig. 7-19). By assuming particle size distributions similar to those of Fig. 7-1 for continental and maritime background aerosols, Junge and McLaren also calculated cloud nuclei spectra as a function of critical supersaturation and compared them with observational data. These results are shown in Fig. 7-8b. We shall not discuss the data in detail. The results make clear, however, that the presence of insoluble matter in aerosol particles does not seriously reduce their capacity to act as cloud condensation nuclei. 

Kohler theory describes cloud droplet activation and growth from soluble particles as an equilibrium process [171], The Kohler equation takes into account two competing effects the Raoult or solute, effect which tends to decrease the equilibrium vapor pressure of water over the growing droplet, and the Kelvin, or curvature effect, which serves to increase the equilibrium vapor pressure. The Kohler curve O ig. 3) for a growing droplet describes the equilibrium saturation ratio of water as a function of droplet size and several parameters inherent to the aerosol particle [171, 172] ... 

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