Slow water transport in MgSO4 aerosol droplets at gel-forming relative humidities

The effect of gel formation on the mass transfer of water during evaporation or condensation from MgSO4 droplets is studied using aerosol optical tweezers coupled with Raman spectroscopy. In particular, the kinetics of water transport during hydration and dehydration are followed for variable step changes in relative humidity and compared with previous measurements using different methodologies. Slow diffusion of water in the particle bulk is shown to limit water evaporation and condensation from the aerosol. Desorption of water continues over a long time at the very low RH region and this is validated with complementary studies made by FTIR-ATR and measurements of water adsorption isotherms. The observations can be rationalized when considering the possible phase transformation of the gel structure at very low RHs. Finally, the influence of the duration of the drying time (RH <= 10%) on the kinetics of condensation during hydration is investigated. Apparent diffusion coefficients of water molecules in the gel are obtained, showing little dependence on the water activity and droplet composition, and are consistent with the slow removal of water during drying from pores formed at the gel transition RH.