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

Abstract

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.

The Wavelength‐Dependent Complex Refractive Index of Hygroscopic Aerosol Particles and Other Aqueous Media: An Effective Oscillator Model

“…Agreement between the measurements and the oscillator model is overall good, although at low water activities the measured values of k for aqueous MgSO 4 (Figure d) appear to increase much more rapidly than is predicted by the oscillator model. This rise takes place near a water activity region where gel formation is known to occur upon drying aqueous MgSO 4 droplets (Cai et al, ; Davies & Wilson, ). Therefore, it is likely the case that the refractive index calculation becomes inaccurate when the aqueous solution is not in a liquid phase state.…”

Rebounding hygroscopic inorganic aerosol particles: Liquids, gels, and hydrates

“…The gel arises from chains of hydrated contaction pairs. Water diffusion in MgSO 4 particles at RH <40% is slow (Cai et al 2015;Davies and Wilson 2016). Above 40% RH, bidentate contact-ion pairs break and monodentate contact-ion pairs start to form, and rates of water diffusion correspondingly increase.…”

Advanced aerosol optical tweezers chamber design to facilitate phase-separation and equilibration timescale experiments on complex droplets

“…This water uptake response indicates that the aqueous NaCl core still responds promptly to gas-phase humidity changes even though it is coated by a hydrophobic shell, and the squalane has no measurable effect on the equilibration timescale. Prior AOT experiments have used an increase in the RH equilibration timescale to assess diffusion limitations of water through viscous droplet phases (Tong et al 2011;Cai et al 2015b).…”

“…The WGMs in the AOT can be used to distinguish between the three predominant morphologies: phase-separated core-shell or partial-engulfed structures, and homogeneous single-phase droplets (Kwamena et al 2010;Reid et al 2011;Dennis-Smither et al 2012;Cai et al 2015b;Stewart et al 2015). Furthermore any changes in the droplet's morphology as its composition or the surrounding relative humidity is changed are directly assessed (Rickards et al 2013;Cai et al 2014).…”