Cloud droplets, formation nuclei

Supersaturations of several hundred percent are necessary for the formation of water droplets in particle free air (see Chapter 11). The need for such high supersaturations indicates the necessity of particles for cloud formation in the ambient atmosphere. The ability of a given particle to serve as a nucleus for water droplet formation, as we have seen in the previous sections, will depend on its size, chemical composition, and the local supersaturation. 

In a cloud chamber detector, ions at atmospheric pressure are electrically focused into a cloud chamber filled with cold water or octane vapors. The presence of the ion serves as the nucleus for the formation of small droplets that can scatter light from a laser beam passing through the cloud chamber. When mobility-separated ions entered the cloud chamber, perturbation in the laser light due to the formation of ion-nucleated particles was detected by a PMT. When the chamber was supersaturated with water, the scattered light intensity increased in the presence of ions, but when the chamber was supersaturated with octane, the intensity decreased in the presence of ions. Mobility spectra of difluorodibromomethane have been reported using cloud chamber detection." ... 

This is in an unscrubbed plume. Now, concerning ammonia, you ae not really talking about ammonia as a catalyst for sulfate formation. The actual process is SO2 to sulfuric acid, followed by ammonia neutralization. The theory about SO2, water, ammonia catalysis is being questioned. The real process seems to be SO2 to sulfuric acid, then reaction with ammonia to form ammonium sulfate or ammonium bisulfate. In fact, in scavenging the sulfate, as the particle size increases, the scavenging efficiency also increases. Sulfuric acid aerosols are submicron particulates for which scavenging is very inefficient (Marsh, Atmos. Environ. 12 401-406, 1978). So what you are finding in a rain droplet is perhaps what is below the cloud or before the condensation nucleus stabilizes. 

When the temperatures in-cloud are near or below —40°C and the ambient relative humidity is close to 100% with respect to water saturation—conducive to cirrus cloud formation—homogeneous ice nucleation is likely to be an important mechanism for ice production. The theory of homogeneous nucleation of ice has shown that when molecules of liquid water comprising a pure droplet a few microns in diameter bind to form a stable ice structure at an observable rate only when temperatures near —40°C is reached. This theory has been confirmed in the laboratory. What has been confirmed in studies in orographic wave clouds, and in cirrus cloud as well, is that when water condenses on a cloud condensation nucleus at temperatures near and below —40°C and grow to a sizes of 1 /xm, the droplets freeze homogeneously. This mechanism is thought to be important in the formation of cirrus clouds. 

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