Water activity and activation diameters from hygroscopicity data - Part I: Theory and application to inorganic salts S. M. Kreidenweis1, K. Koehler1, P. J. DeMott1, A. J. Prenni1, C. Carrico1, and B. Ervens2 1Department of Atmospheric Science, Colorado State University, Fort Collins, CO, 80523, USA 2NOAA, Boulder, CO, 80503, USA
Abstract. A method is described that uses particle hygroscopicity measurements, made
with a humidified tandem differential mobility analyzer (HTDMA), to
determine solution water activity as a function of composition. The use of
derived water activity data in computations determining the ability of
aerosols to serve as cloud condensation nuclei (CCN) is explored. Results
for sodium chloride and ammonium sulfate are shown in Part I. The methodology yields solution water activities and critical
dry diameters for ammonium sulfate and sodium chloride in good agreement
with previously published data. The approach avoids the assumptions required
for application of simplified and modified Köhler equations to predict
CCN activity, most importantly, knowledge of the molecular weight and the
degree of dissociation of the soluble species. Predictions of the dependence
of water activity on the mass fraction of aerosol species are sensitive to
the assumed dry density, but predicted critical dry diameters are not.
Citation: Kreidenweis, S. M., Koehler, K., DeMott, P. J., Prenni, A. J., Carrico, C., and Ervens, B.: Water activity and activation diameters from hygroscopicity data - Part I: Theory and application to inorganic salts, Atmos. Chem. Phys., 5, 1357-1370, doi:10.5194/acp-5-1357-2005, 2005.