Experimental study of ion-induced nucleation of volatile organic compounds by radon decay

Abstract

Wilson's classic experiments in the last century gave evidence that the presence of an ion can promote nucleation. Since then, a number of experimental studies of ion-induced nucleation have been performed using cloud chambers. He and Hopke reported an experimental study of ion-induced nucleation by radon decay for methanol, ethanol, 1-propanol, and l-butanol vapors in a thermal diffusion cloud chamber. In that study, the ion tracks became visible at a critical supersaturation below that needed for homogeneous nucleation. The nucleation rates increased substantially with increasing radon at low activity concentrations but leveled off at high concentrations because of vapor depletion preventing the growth of droplets to a measurable size. An imposed external electric field has a significant effect on the observed rates of the nucleation. It examines the results by plotting the relationship between the dipole moments of the alcohols and the measured critical supersaturations of ion-induced nucleation. It determines that the critical supersaturations decrease as the dipole moments increase from 1.66 to 1.70 (Debye), butanol to methanol. There is a relationship between the ion-induced nucleation process for a series of related compounds and their molecular properties. Further investigation is thus needed to provide more information on the role of molecular properties on ion-induced nucleation.