Aerosol Instabilities

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Abstract
All aerosols are temporally unstable. Some important aerosol characteristics which can change with the passage of time are total mass concentration, fraction of a contaminant in the particle or vapor phase, and particle size distribution. A wide variety of aerosol instabilities must be considered in air sampling because they may influence sampling strategy, occurrence of artifacts in samples, and extrapolation of measured aerosol characteristics to earlier or later times. Four types of aerosol instabilities are discussed: coagulation, deposition of particles on surfaces, deposition of vapor on surfaces and evaporation of vapor from surfaces, and vapor/particle interactions. Each is described in terms of its characteristic time and the aerosol properties which change. The concept of characteristic time allows comparisons among competing processes and predictions of whether a process which tends to change an aerosol property will go nearly to completion or have no significant effect during the observation time. Emphasis is placed on predicting the partitioning of contaminants between the particle and vapor phases of an aerosol. When the saturation vapor concentration of a material is much smaller than the total airborne concentration, essentially all the material will be found in the particle phase. If the saturation vapor concentration is on the same order as the total airborne concentration, it may be found in both the particle and vapor phases. If the saturation vapor concentration is much larger than the total airborne concentration, the material might be found in the particle phase as well as in the vapor phase depending on the concentration of particles in which the material is soluble.

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