Timescales of water transport in viscous aerosol: measurements on sub-micron particles and dependence on conditioning history

Abstract

Evaporation studies of single aqueous sucrose aerosol particles as a function of relative humidity (RH) are presented for coarse and fine mode particles down into the submicron size range (600 nm < r < 3.0 μm). These sucrose particles serve as a proxy for biogenic secondary organic aerosols that have been shown to exist, under ambient conditions, in an ultraviscous glassy state, which can affect the kinetics of water mass transport within the bulk phase and hinder particle response to changes in the gas phase water content. A counter-propagating Bessel beams (CPBBs) optical trapping setup is employed to monitor the real-time change in the particle radius with RH decreasing from 75% to 5%. The slow-down of the size change upon each RH step and the deviation from the theoretical equilibrium hygroscopic growth curve indicate the onset of glassy behavior in the RH range of 10-40%. Size-dependent effects were not observed within the uncertainty of the measurements. The influence of the drying time below the glass transition RH on the timescale of subsequent water condensation and re-equilibration for sucrose particles is explored by optical tweezers measurements of micron-sized particles (3 μm < r < 6 μm). The timescale for water condensation and re-equilibration is shown to increase with increasing drying time, i.e. the time over which a viscous particle is dried below 5% RH. These studies demonstrate the importance of the history of the particle conditioning on subsequent water condensation and re-equilibration dynamics of ultraviscous and glassy aerosol particles.

The effect of viscosity and diffusion on the HO&lt;sub&gt;2&lt;/sub&gt; uptake by sucrose and secondary organic aerosol particles

“…The reason for this strong dependence upon pH has been discussed previously and is due to the partitioning of HO 2 with its conjugate base O − 2 , as shown by Reaction (R4), affecting the effective Henry's law coefficient and the effective rate coefficients (Thornton et al, 2008). Although it was not possible to measure the actual pH of the aerosol particles, it was possible to estimate the concentration of copper(II) sulfate (which is a weak acid) within the aerosol particles using the known growth factors of sucrose aerosol particles (Lu et al, 2014). The pH of 0.05 and 0.1 M copper(II) sulfate solutions (which were calculated to be the extremes of the possible copper concentrations over the RH range) were then measured using a pH meter (Jenway, 3310) as being in the range of 4.10 ± 0.05.…”

Angular scattering of light by a homogeneous spherical particle in a zeroth-order Bessel beam and its relationship to plane wave scattering

“…[18][19][20][21][22] In the field of aerosol science, Bessel beams are very appealing for the study of single aerosol particles and instrumentation 2 incorporating such beams has been recently demonstrated. [23][24][25][26][27][28][29] However, there remains the need for an approach to accurately characterize a particle once it has been trapped in such a beam. For a homogeneous spherical particle the radius and refractive index must be accurately determined.…”

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

“…The occurrence of partial-shell morphologies and their importance for the timescales of physical and chemical evolution of aerosols has recently come to the fore requiring novel analytical techniques to fully assess their occurrence and impacts (Buajarern et al 2007a(Buajarern et al , 2007bKwamena et al 2010;Reid et al 2011;Dennis-Smither et al 2012;Shiraiwa et al 2012;Stewart et al 2015;Metcalf et al 2016). For a core-shell morphology, diffusion through the shell dictates the rate at which gas-phase components partition into similar aerosol components as the system re-equilibrates (Vaden et al 2011;Davies et al 2012;Abramson et al 2013;Lu et al 2014). The composition at the air-particle interface also dictates the particle's ability to undergo chemical reactions with gas-phase reactants, and the rates and mechanisms of those reactions (Evans 2005;Alexander et al 2009;Shindell et al 2009;Abbatt et al 2012;Gaston and Thornton 2016).…”