Nucleation cloud seeding

Of course, nucleation may also be accomplished by seeding or adding externally formed nuclei. The monodisperse gold sols described in Chapter 5, Section 5.7b, are prepared by this method. The use of Agl crystals and other materials as nuclei in cloud seeding has also been studied extensively. This is especially interesting in view of possible applications to weather modification. 

The higher vapor pressure of very small droplets allows air to achieve a considerable supersaturation (partial pressure of water in air greater than the vapor pressure of water) before liquid droplets begin to form. In fact, most cloud droplets form on nucleation centers, which may be dust particles or minute droplets of sulfuric or nitric acid. Cloud seeding involves adding nucleation centers (usually iodide salts), in an attempt to encourage precipitation from supersaturated air. 

In addition to an importance in explaining a wide variety of geophysical and geochemical phenomena—including crystal formation— the principles of nucleation were used in cloud seeding weather modification experiments where nuclei of inert materials were dispersed into clouds with the hopes of inducing condensation and rainfall. 

Particulate chemicals also may be removed from the atmosphere through wet deposition processes. The major mechanism by which particles are incorporated into precipitation is by serving as nucleation sites for condensation at the onset of water droplet or ice crystal formation. [If nucleation sites were absent, water droplets would not form unless the air temperature were significantly below the dew point. In the rainmaking practice of cloud seeding, condensation nuclei in the form of silver iodide (Agl) crystals are dispersed into air masses to increase the formation of ice crystals, which in turn accumulate additional water from the cloud and thus promote precipitation ] Particles also can be incorporated into already-formed water droplets within a cloud by collision. 

Figure 12.6. Possible applications of falling paper principle, (a) Dynamic kite made from expanded polystyrene bead material with S-shaped rotor wing, (b) Radar target consisting of pieces of aluminum foil rotating and translating in random directions, (c) Means for economic use of nucleating agent in a cloud seeding operation, (d) Auxiliary power driven Flettner rotor ship, (e) Axial view of Savonius wind driven rotor, (f) Sailboat with two collapsible nylon sails.

Fig. 9.2. The excellent crystallographic matching between silver iodide and ice makes silver iodide a very potent nucleating agent for ice crystals. When clouds at sub-zero temperatures are seeded with Agl dust, spectacular rainfall occurs.

In the nucleation step, there must be sites upon which the crystals can form. This is similar to seeding the clouds to cause water to precipitate (rain). There are two sources for these nucleating particles homogeneous and heterogeneous agents. 

Schaefer (1966) reported the activation of large numbers of ice nuclei on the addition of trace levels of iodine vapor to car exhaust (containing lead oxide nanoparticles) at temperatures from —3 to —20°C in the laboratory. The formation of lead iodide was concluded to have a seeding effect similar to that of silver iodide particles (Vonnegut, 1947), which had been used in an attempt to artificially modify cloud properties and enhance precipitation. Consequendy this method was proposed as a means to remove harmful aerosol formed in polluted urban areas, and also in artificial weather modification. However, the development of unleaded fuels, for which no similar ice nucleating ability was shown to occur in the presence of iodine (Hogan, 1967), provided a better long-term solution to this problem. 

As previously noted, most zeolite syntheses of commercial value occur in systems clouded with an amorphous gel phase due to higher product yields, admitting to the possibility of homogeneous nucleation due to solubility differences, or to heterogeneous nucleation due to the abundance of foreign surface in the mediiun. Seeding these mixtures, or agitating the solutions, could induce nucleation by any of the secondary nucleation mechanisms. However, zeolite syntheses also have been conducted successfully in dilute clear alumino-silicate media, i.e., in the absence of any amorphous gel phase [14-26]. In fact, one of the early papers by Kerr [1] reported on a technique whereby dried gel was... 

---