AAAR 31st Annual Conference
October 8-12, 2012
Hyatt Regency Minneapolis
Minneapolis, Minnesota, USA
Abstract View
Molecular Dynamics of Evaporation and Mass Accommodation of Water for Various Droplet Sizes
JAN JULIN, Manabu Shiraiwa, Ulrich Pöschl, Ilona Riipinen, Stockholm University
Abstract Number: 630 Working Group: Aerosol Physics
Abstract We have studied the mass accommodation coefficient of water molecules on water surfaces using molecular dynamics (MD) simulations. The mass accommodation coefficient describes the ability of a surface to uptake gas phase molecules, and is an important quantity in understanding the condensational growth of atmospheric aerosol particles. Unfortunately, even for water-on-water a discrepancy exists between reported coefficients, with values ranging from 0.1 to 1. In MD simulations for water the coefficient has been consistently found to be one. However, to our knowledge, the studies have so far considered only planar surfaces. In order to study possible aerosol size-dependent factors affecting mass accommodation we have performed mass accommodation simulations for different droplet surfaces at T=273.15 K, as well as the planar surface. Additionally, we have also performed simulations for the planar surface in various temperatures to elucidate any temperature dependence. The simulations were performed using the TIP4P-ew water potential.
The simulation setup is the familiar one: incident molecules are generated at set time intervals at a distance from the surface, and shot towards the surface. A total of 1000 impact events are simulated per target surface. We also simulated evaporation from a surface without any incident molecules present, and found the evaporative flux to be consistent with the theoretical prediction calculated using equilibrium vapor pressure. Comparing to the evaporation events observed in the mass accommodation simulations, we can identify the evaporation induced by the incoming molecules. Thus we can identify the magnitudes of the two options for non-accommodation: the direct reflection of incoming molecules and the exchange of an incident molecule with a surface molecule. In any case, the mass accommodation coefficient is close to unity, even for droplets with 2 nm radius. The results from MD simulations are further used to assess and improve assumptions used in kinetic mass-transport models.