Effect of waiting time on the water transport kinetics of magnesium sulfate aerosol at gel-forming relative humidity using optical tweezers

With the loss of water, the amorphous gel states in aqueous magnesium sulfate (MgSO₄) aerosol forms and results in nonequilibrium dynamics, owing to the extended time scales for diffusive mixing. The mass transfer resistance in MgSO₄ aerosol droplets during evaporation or condensation is investigated using aerosol optical tweezers (AOTs) coupled with Raman spectroscopy. In addition, the kinetics of water transport during hydration and dehydration after different waiting time is studied. With the cyclic change of the relative humidity (RH) below gel-forming, the waiting time is varied to examine the effect of the duration of drying and humidifying on water transport kinetics during subsequent hydration and dehydration process. Apparent diffusion coefficients (D_ap) of water molecules in the gel state after different waiting time are obtained. The results indicate that the duration of drying will affect water transport kinetics for subsequent humidifying process due to the different structure and composition in MgSO₄ aerosol droplet at different ambient humidities. However, the duration of humidifying has little effect on water transport kinetics for subsequent drying process below gel-forming RH.