Retrieving the translational diffusion coefficient of water from experiments on single levitated aerosol droplets

Abstract

The time-dependent growth and shrinkage of aqueous aerosol particles trapped in an electrodynamic balance exposed to changes in relative humidity (RH) depend on the translational diffusion coefficient of water (DH2O). Resonances in the Mie scattering patterns of the illuminated micrometre-sized droplets are used to follow the compositional evolution through stepwise changes in RH. Under conditions where the diffusion of water molecules becomes sufficiently slow, e.g. in the highly viscous or even glassy regime, the concentration and temperature dependent values of DH2O can be determined iteratively by comparing the observed shifts in the Mie resonant wavelengths with predicted shifts from a diffusion model of a multi-layered sphere. It is shown that condensation and evaporation of water vapour from or to highly viscous or glassy droplets follow different kinetic regimes, a result that is consistent with previous studies of adsorption and desorption on glassy surfaces.

Viscous organic aerosol particles in the upper troposphere: diffusivity-controlled water uptake and ice nucleation?

“…The changes in composition and size of the droplets upon varying the conditions in the chamber are compared to predictions from a diffusion model (Zobrist et al, 2011;Lienhard et al, 2014). The numerical model subdivides the particle into up to several thousand individual shells and solves the non-linear diffusion equation in spherical coordinates while accounting for the concentration dependence of the water diffusion coefficient, i.e.…”

“…The method to determine water diffusion coefficients, D w (T , a w ), from such experiments as a function of temperature and water activity, a w , has been described in detail previously (Zobrist et al, 2011;Lienhard et al, 2014;Steimer et al, 2015). In short, we use a double ring electrodynamic balance (EDB) in a three-wall glass chamber with a circulating cooling liquid between the innermost two walls which controls the temperature with a stability better than 0.1 K and accuracy of ±0.5 K and an insulation vacuum between the outer walls.…”

“…The RH in the EDB is controlled by adjusting the flows of dry and humidified nitrogen gas via mass flow controllers. The levitated particle is illuminated by a HeNe laser (633 nm), an LED (560 to 610 nm) and a narrow bandwidth tunable diode laser (765 to 781 nm) which allow tracking of changes in composition and size by analyzing resonances in the Mie scattering spectra (Lienhard et al, 2014;Steimer et al, 2015). Mass and radius growth factors as well as density and refractive index of the particles can be derived from these measurements over the entire concentration range after allowing sufficient time for equilibration (Lienhard et al, 2012a;Steimer et al, 2015) (examples are given in Appendix A3).…”

“…Only a few studies have measured the diffusion of water in glassy model systems for atmospherically relevant relative humidities at room temperature and even fewer studies at lower temperatures (Zobrist et al, 2011;Tong et al, 2011;Bones et al, 2012;Price et al, 2014;Lienhard et al, 2014;Steimer et al, 2015). Estimations of the influence of slow water diffusivity on homogeneous and heterogeneous ice nucleation as well as on multi-phase chemistry have been extrapolated from sucrose properties as a model system towards different types of SOA (Berkemeier et al, 2014;Arangio et al, 2015).…”

Diffusivity measurements of volatile organics in levitated viscous aerosol particles

“…The experiments for this study were conducted in the setup previously described in depth by Lienhard et al (2014) and Krieger et al (2000). An inductively charged single particle is injected into a double-ring EDB (two plane-parallel AC electrode rings between two DC end caps) using an ink jet cartridge.…”