Characteristic Time for Atmospheric Aerosol Equilibrium

Aerosol particles in the atmosphere contain a variety of volatile compounds (ammonium, nitrate, chloride, volatile organic compounds) that can exist either in the particulate or in the gas phase. We estimate in this section the timescales for achieving thermodynamic equilibrium between these two phases and apply them to typical atmospheric conditions. The problem is rather different compared to the equilibration between the gas and aqueous phases in a cloud discussed in the previous section. Aerosol particles are solid or concentrated aqueous solutions (cloud droplets are dilute aqueous solutions), they are relatively small, and aqueous-phase reactions in the aerosol phase can be neglected to a first approximation because of the small liquid water content. 

If the air surrounding an aerosol population is supersaturated with a compound A, the compound will start condensing on the surface of the particles in an effort to establish thermodynamic equilibrium. The gas-phase concentration of A will decrease and its particulate-phase concentration will increase until equilibrium is achieved. The characteristic time for the two phases to equilibrate due to the depletion of A in the gas phase will be inversely proportional to the total flux of A to the aerosol phase. 

For solid-phase aerosol particles the equilibrium concentration of A is constant and does not change as A is transferred to the aerosol phase. However, if the aerosol contains water and A is water soluble, the equilibrium concentration of A will increase as condensation proceeds, and equilibration between the two phases will be accelerated. This change in equilibrium concentration is a result of changes in the chemical composition of the particle as A is transferred to the particulate phase. The cases of solid and aqueous phases will be examined separately in sub.sequent sections, based on the analysis of Wexler and Seinfeld (1990). 

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