AAAR 33rd Annual Conference
October 20 - October 24, 2014
Rosen Shingle Creek
Orlando, Florida, USA
Abstract View
Resolving the Surface and Bulk Accommodation of Atmospherically Relevant Compounds with Molecular Dynamics Simulations
JAN JULIN, Paul M. Winkler, Neil Donahue, Paul E. Wagner, Ilona Riipinen, Stockholm University
Abstract Number: 265 Working Group: Aerosol Physics
Abstract The condensational growth of aerosol particles needs to be accurately understood in order to correctly predict cloud condensation nuclei (CCN) concentrations. A large fraction of the condensable vapors partaking in the growth process are different organic compounds with a variety of structural features. The CCN in turn grow to cloud droplets by condensation of water vapor. The quantity describing the vapor uptake ability of the aerosol particles is the mass accommodation coefficient, defined as the fraction of gas phase molecules that will accommodate to the condensed phase. The coefficient has proven to be challenging to determine experimentally. A further complication arises from the fact that the experiments are interpreted using aerosol dynamic models which do not distinguish between aerosol bulk and surface, yet the coefficient can be defined either as surface or bulk accommodation coefficient. The difference between the two is whether surface adsorption or bulk absorption is required for accommodation.
We present molecular dynamics (MD) simulation results on the condensation of various organic molecules on organic surfaces, as well as water on ice. MD simulations provide information on mass accommodation coefficient directly from the fates of individual molecules as they arrive on a surface, and MD can distinguish between surface and bulk accommodation. We find that for nonane MD simulations and expansion chamber measurements agree well. We have examined the effect of both the molecular structure of the organic compounds and the phase-state of the condensed phase on the mass accommodation process. Our results highlight both the sensitivity of the bulk accommodation coefficient to the division between surface and bulk, and the necessity to include a time-dependence in the definition of the bulk accommodation coefficient. Consequently, our results emphasize the need for experimental methods that can probe aerosol surface and bulk separately to reliably constrain the mass accommodation coefficient.