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Atmospheric Chemistry and Physics An interactive open-access journal of the European Geosciences Union
https://doi.org/10.5194/acp-2016-1052
© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.
Research article
12 Jan 2017
Review status
A revision of this discussion paper was accepted for the journal Atmospheric Chemistry and Physics (ACP) and is expected to appear here in due course.
An efficient approach for treating composition-dependent diffusion within organic particles
Simon O'Meara1, David O. Topping1,2, Rahul A. Zaveri3, and Gordon McFiggans1 1Centre for Atmospheric Science, School of Earth & Environmental Sciences, University of Manchester, Manchester, M13 9PL, UK
2National Centre for Atmospheric Science (NCAS), University of Manchester, Manchester, M13 9PL, UK
3Atmospheric Sciences and Global Change Division, Pacific Northwest National Laboratory, Richland, Washington 99352, USA
Abstract. Mounting evidence demonstrates that under certain conditions the rate of component partitioning between the gas- and particle-phase in atmospheric organic aerosol is limited by particle-phase diffusion. To date, however, particle-phase diffusion has not been incorporated to regional atmospheric models. An analytical rather than numerical solution to diffusion through organic particulate matter is desirable because of its comparatively small computational expense in regional models. Current analytical models assume diffusion to be independent of composition, and therefore use a constant diffusion coefficient. To realistically model diffusion, however, it should be composition-dependent (e.g. due to the partitioning of components that plasticise, vitrify or solidify). This study assesses the modelling capability of an analytical solution to diffusion corrected to account for composition dependence against a numerical solution. Results show reasonable agreement when the gas-phase saturation ratio of a partitioning component is constant and particle-phase diffusion limits partitioning rate (< 10 % discrepancy in estimated radius change). However, when the saturation ratio of the partitioning component varies a generally applicable correction could not be found, indicating that existing methodologies are incapable of deriving a general solution. Until such time as a general solution is found, caution should be given to sensitivity studies that assume constant diffusivity. The correction was implemented in the polydisperse multi-process Model for Simulating Aerosol Interactions and Chemistry (MOSAIC), and is used to illustrate how the evolution of number size distribution may be accelerated by condensation of a plasticising component onto viscous organic particles.

Citation: O'Meara, S., Topping, D. O., Zaveri, R. A., and McFiggans, G.: An efficient approach for treating composition-dependent diffusion within organic particles, Atmos. Chem. Phys. Discuss., https://doi.org/10.5194/acp-2016-1052, in review, 2017.
Simon O'Meara et al.
Simon O'Meara et al.
Simon O'Meara et al.

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Short summary
To simulate diffusion an analytical expression is desired because it takes less calculation time than a differential equation. Here a correction is found for the analytical solution for when diffusivity is dependent on composition, thereby making it more widely applicable than before. As a consequence, we are able to more realistically evaluate the rate limitation (if any) imposed by particle phase diffusion on component partitioning between the gas and particle phase.
To simulate diffusion an analytical expression is desired because it takes less calculation time...
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