AAAR 32nd Annual Conference
September 30 - October 4, 2013
Oregon Convention Center
Portland, Oregon, USA
Abstract View
Quantifying Water Diffusion in High-viscosity Atmospheric Aerosol Proxies
HANNAH PRICE, Benjamin Murray, Johan Mattsson, Daniel O'Sullivan, Theodore Wilson, Kelly Baustian, University of Leeds
Abstract Number: 589 Working Group: Aerosols, Clouds, and Climate
Abstract Some secondary organic aerosol (SOA) have been shown to be highly viscous under atmospheric conditions. These semi-solid/solid amorphous particles may equilibrate with the surrounding gas phase on very long timescales. This could have important consequences for understanding SOA growth, heterogeneous chemistry, water uptake and role as ice nuclei.
The timescales on which high-viscosity aerosol particles interact with gas species can be quantified if the diffusion coefficients of relevant gas species within aerosol particles are known. Previously, diffusion coefficients have been estimated from viscosity measurements using the Stokes-Einstein equation. However, this is not valid at high viscosity, with mobilities of different species diverging as diffusion slows. It is therefore more relevant to directly measure the diffusion coefficient.
In this work, diffusion coefficients of D$_2O in highly supersaturated aqueous solutions were measured using a Raman spectrometer. Disks of atmospheric SOA surrogates, e.g. sucrose and levoglucosan, were first allowed to equilibrate in gas with controlled H$_2O partial pressure. H$_2O vapor was replaced with D$_2O vapor of the same dewpoint and Raman spectroscopic information allowed us to fit an analytical solution of Fick’s second law to determine the diffusion coefficient. Results covering six orders of magnitude are reported, which compare well with literature data across the humidity range.
The results suggest that 150 nm semi-solid atmospheric aerosol particles equilibrate over milliseconds, whilst just below the glass transition the half lives for equilibration are on the order of seconds. Previously, a viscosity consistent with the glassy state has been assumed to indicate very long equilibration times. This work suggests this may not be the case: for the long timescales typically assumed to exist in amorphous solid aerosol, conditions must be deep into the glassy regime; simply identifying whether a particle is above or below its glass transition is insufficient if timescales are to be inferred.