A fast technique for measuring water activity of atmospheric aerosols

A fast technique for measuring the water activity of atmospheric aerosols using the electrodynamic balance (EDB) has been developed. This method is based on the in situ monitoring of evaporating droplets upon a step change in the relative humidity (RH) of the feed to the EDB. By comparing the time scales of the associated processes involved in the evaporation (or growth) of a droplet, the overall evaporation process is found to be limited by the ambient RH change in the EDB. Hence, a quasi-equilibrium is established between the evaporating droplet and the ambient environment. This fast approach can reduce the time of measurement of water activity of aerosols from several hours needed by corresponding conventional measurement in an EDB to within an hour. The measured data are in good agreement with literature data. Application of this technique for measuring the water activities of droplets containing volatile solute is also demonstrated. © 1997 American Association for Aerosol Research.