Exploring the hygroscopicity, water diffusivity, and viscosity of organic–inorganic aerosols – a case study on internally-mixed citric acid and ammonium sulfate particles

Abstract

Internally mixed aerosol particles containing organic molecules and inorganic salts are prevalent in the atmosphere, arising from direct emission (e.g. from the ocean) or indirect production by condensation of organic...

Correlating the Viscosity and Rate of Water Diffusion in Semisolid Gel-Forming Aerosol Particles

“…Therefore, any method that disturbs the amount of water in aerosol must account for this. In our previous work, we demonstrated a method to measure the diffusion coefficient with minimal RH perturbation, yielding results that agreed well with other methods, such as isotope exchange, that do not impose a change in the total amount of water …”

“…Previous work has demonstrated the formation of viscous amorphous states at low RH values for pure organic particles containing oxygenated species. In particles containing sucrose, citric acid, and a range of other organic species, high viscosity is associated with reduced rates of water diffusion. ,, The viscosity data of Richards et al for pure gluconic acid, glucose, sorbitol, and glucuronic acid are reproduced in Figure S2, revealing that these samples also experience a clear increase in viscosity as the RH decreases. In this work, we characterized the diffusion coefficient of water in these samples, and the data are shown as a function of RH in Figure A–D (solid symbols).…”

“…The full procedure to characterize water diffusion was discussed in our earlier work . Briefly, a sample particle was brought to the measurement RH and allowed to equilibrate.…”

“…However, recent work has clearly demonstrated that this is not always the case, and it has been shown that when atmospherically relevant amounts of ammonium sulfate (AS), a common inorganic component of the atmosphere, is added to citric acid particles, the hygroscopicity decreases while the viscosity significantly increases. 14 Interestingly, the rate of water diffusion, inversely related to the viscosity according to the Stokes−Einstein relationship, was found to increasingly decorrelate from the viscosity as more AS was added. In mixtures of AS and sucrose, the viscosity has been shown to decrease when AS is added, as would be expected given the increased hygroscopic growth.…”

“…In our previous work, we demonstrated a method to measure the diffusion coefficient with minimal RH perturbation, yielding results that agreed well with other methods, such as isotope exchange, that do not impose a change in the total amount of water. 14 To connect viscosity and diffusion, a first approximation is the Stokes−Einstein (SE) relation, which assumes a large sphere diffusing through a viscous continuum. This approx-imation is known to break down when a small molecule, such as water, is diffusing through a viscous matrix consisting of finite-sized molecules.…”