Solid Aerosols Dust and Smoke

Conceptually, one can think of the nucleation process in the following terms. If the pressure or activity of the vapor, P, is small relative to Po, then AG for a given cluster of molecules will increase with each added molecule. In other words, the tendency will be for clusters smaller than to return to the vapor phase. 

Statistically, one might expect to encounter clusters of all sizes due to random fluctuation processes however, all except the smallest would be very uncommon. There would therefore be httle likelihood of obtaining the critical radius necessary for drop formation to occur. However, as the degree of supersaturation increases, decreases and random fluctuations begin to result in more clusters with that radius. Once that point is reached, the clusters begin to grow spontaneously to form drops. When a specific supersaturation pressure is exceeded, there will develop a steady parade of clusters of the required critical dimensions, resulting in the formation of a visible mist or fog. 

Condensation aerosols can also be prepared by chemical reactions involving two gaseous reactants 

In such a process the particle size will be controlled primarily by the concentrations of the reactants in the gas phase. 

Aerosols composed of solid particles suspended in a gas are commonly referred to as dust or smoke, the exact terminology usually depending on the size and sedimentation rate of the particles, or the method of aerosol formation. In some situations, aerosols formed through dispersion processes are termed dusts while those arising from condensation processes are called smokes. Alternatively, some prefer to label as dusts aerosols of sufficient particle size to have relatively rapid (e.g., noticeable over a short time span) sedimentation rates in air, while smokes would be of smaller, lighter particles. Regardless of the terminology employed, it is clear that solid aerosols constitute a very important, and usually undesirable, component of many modern processes. 

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