Mass Accommodation ofWater: Bridging the Gap BetweenMolecular Dynamics Simulations and Kinetic Condensation Models

摘要

The condensational growth of submicrometer aerosol particles to climate relevant sizes is sensitive to their ability to accommodate vapor molecules, which is described by the mass accommodation coefficient. However, the underlying processes are not yet fully understood. We have simulated the mass accommodation and evaporation processes of water using molecular dynamics, and the results are compared to the condensation equations derived from the kinetic gas theory to shed light on the compatibility of the two. Molecular dynamics simulations were performed for a planar TIP4P-Ew water surface at four temperatures in the range 268–300 K as well as two droplets, with radii of 1.92 and 4.14 nm at T = 273.15 K. The evaporation flux from molecular dynamics was found to be in good qualitative agreement with that predicted by the simple kinetic condensation equations. Water droplet growth was also modeled with the kinetic multilayer model KM-GAP of Shiraiwa et al. [Atmos. Chem. Phys. 2012, 117, 2777 []]. It was found that, due to the fast transport across the interface, the growth of a pure water dropletis controlled by gas phase diffusion. These facts indicate that thesimple kinetic treatment is sufficient in describing pure water condensationand evaporation. The droplet size was found to have minimal effecton the value of the mass accommodation coefficient. The mass accommodationcoefficient was found to be unity (within 0.004) for all studied surfaces,which is in agreement with previous simulation work. Additionally,the simulated evaporation fluxes imply that the evaporation coefficientis also unity. Comparing the evaporation rates of the mass accommodationand evaporation simulations indicated that the high collision flux,corresponding to high supersaturation, present in typical moleculardynamics mass accommodation simulations can under certain conditionslead to an increase in the evaporation rate. Consequently, in suchsituations the mass accommodation coefficient can be overestimated,but in the present cases the corrected values were still close tounity with the lowest value at ≈0.99.