 |
| Titel |
Quantifying water diffusion in secondary organic material |
| VerfasserIn |
Hannah Price, Benjamin Murray, Johan Mattsson, Daniel O'Sullivan, Theodore Wilson, Yue Zhang, Scot Martin |
| Konferenz |
EGU General Assembly 2014
|
| Medientyp |
Artikel
|
| Sprache |
Englisch
|
| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250099952
|
| Publikation (Nr.) |
 EGU/EGU2014-15807.pdf |
|
|
|
|
|
| Zusammenfassung |
| Recent research suggests that some secondary organic aerosol (SOA) is highly viscous under
certain atmospheric conditions. This may have important consequences for equilibration
timescales, SOA growth, heterogeneous chemistry and ice nucleation. In order to quantify
these effects, knowledge of the diffusion coefficients of relevant gas species within aerosol
particles is vital.
In this work, a Raman isotope tracer method is used to quantify water diffusion
coefficients over a range of atmospherically relevant humidity and temperature
conditions. D2O is observed as it diffuses from the gas phase into a disk of aqueous
solution, without the disk changing in size or viscosity. An analytical solution of
Fick’s second law is then used with a fitting procedure to determine water diffusion
coefficients in reference materials for method validation. The technique is then
extended to compounds of atmospheric relevance and α-pinene secondary organic
material.
We produce water diffusion coefficients from 20 to 80 % RH at 23.5°C for sucrose,
levoglucosan, M5AS and MgSO4. For levoglucosan we show that under conditions where a
particle bounces, water diffusion in aqueous solutions can be fast (a fraction of a second for a
100 nm radius). For sucrose solutions, we also show that the Stokes-Einstein relation breaks
down at high viscosity and cannot be used to predict water diffusion timescales with
accuracy.
In addition, we also quantify water diffusion coefficients in α-pinene SOM from 20-80%
RH and over temperatures from 6 to -30°C. Our results suggest that, at 6°C, water diffusion
in α-pinene SOA is not kinetically limited on the second timescale, even at 20% RH. As
temperatures decrease, however, diffusion slows and may become an increasingly limiting
factor for atmospheric processes. A parameterization for the diffusion coefficient of water in
α-pinene secondary organic material, as a function of relative humidity and temperature,
is presented. The implications for atmospheric processes such as ice nucleation
and heterogeneous chemistry in the mid- and upper-troposphere will be discussed. |
|
|
|
|
|
|