Estimating the type of hygroscopic behavior exhibited by aqueous droplets

A method that allows hygroscopic aerosol behavior to be classified into three types based on the values of the two nondimensional parameters gamma and zeta is presented. Both parameters can be estimated relatively simply, allowing prediction of the type of behavior a hygroscopic aerosol is likely to exhibit. The parameter gamma is the ratio of the mass of droplets per unit volume divided by the mass of vapor per unit volume that must be exchanged between the droplets and the surrounding air in order to reach equilibrium. The parameter zeta is a measure of the importance of hygroscopic size changes if the aerosol were to be treated as a single droplet. Under many circumstances, zeta is large enough that aerosol behavior is determined solely by gamma. In this case, hygroscopic size changes are negligible if gamma is greater than approximately 3. In contrast, if gamma is less than approximately 0.1, hygroscopic size changes are significant but can be treated using a classical one-way coupled approach. For other values of gamma, hygroscopic size changes require a two-way coupled treatment to predict accurately. Values of gamma and zeta are estimated for several hygroscopic aerosols whose behavior is given in the literature, and it is found that these aerosols do classify into the above categories according to gamma and zeta.