Nucleation in a nonuniform vapor

Abstract

Conventional nucleation theory describes nucleus formation in a uniform vapor as the addition or loss of single molecules from clusters of a given size. However, under nonuniform conditions, clusters can also be removed by diffusing away from the region of interest. Although there are many practical situations in which nucleation does occur under nonuniform conditions, the conventional theory cannot account for the loss of clusters by diffusion. It is therefore the purpose of this paper (1) to formulate a new theory which does include loss by diffusion and (2) to make a quantitative study of the diffusion effect in order to delineate the range of conditions under which it becomes important. The new theory is derived in the context of a typical diffusion cloud chamber by considering the diffusion loss from the narrow slab where nucleation occurs. The supersaturations which give rise to a unit steady rate of nucleation are then compared with and without the diffusion term for model compounds under a variety of conditions. These quantitative studies on typical model compounds show that the diffusion effect first becomes important for vapor pressures as low as 10−8 torr. Thus, in the cloud chamber, nonuniformities do not cause serious errors. More importantly, in polluted atmospheres, even very low concentrations of vapors of involatile organic substances (e.g., sulfinic acids) can nucleate and give rise to the particulate matter commonly found in photochemical smog.