Web Release Date: August 4,
Molecular Dynamics Simulations of Atmospheric Oxidants at the Air-Water Interface: Solvation and Accommodation of OH and O3
and
Environmental Molecular Science Institute and Department of Chemistry, University of California, Irvine, California 92697-2025, Center for Biomolecules and Complex Molecular Systems, Institute of Organic Chemistry and Biochemistry, Academy of Sciences of the Czech Republic, Flemingovo nam. 2, 16610 Prague 6, Czech Republic, and Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, Washington 99352
Received: March 15, 2005
In Final Form: June 15, 2005
Abstract:
A comparative study of OH, O3, and H2O equilibrium aqueous solvation and gas-phase accommodation on
liquid water at 300 K is performed using a combination of ab initio calculations and molecular dynamics
simulations. Polarizable force fields are developed for the interaction potential of OH and O3 with water. The
free energy profiles for transfer of OH and O3 from the gas phase to the bulk liquid exhibit a pronounced
minimum at the surface, but no barrier to solvation in the bulk liquid. The calculated surface excess of each
oxidant is comparable to calculated and experimental values for short chain, aliphatic alcohols. Driving forces
for the surface activity are discussed in terms of the radial distribution functions and dipole orientation
distributions for each molecule in the bulk liquid and at the surface. Simulations of OH, O3, and H2O impinging
on liquid water with a thermal impact velocity are used to calculate thermal accommodation (S) and mass
accommodation (
) coefficients. The values of S for OH, O3, and H2O are 0.95, 0.90, and 0.99, respectively.
The approaching molecules are accelerated toward the liquid surface when they are approximately 5 Å above
it. The molecules that reach thermal equilibrium with the surface do so within 2 ps of striking the surface,
while those that do not scatter into the gas phase with excess translational kinetic energy in the direction
perpendicular to the surface. The time constants for absorption and desorption range from approximately 35
to 140 ps, and the values of for OH, O3, and H2O are 0.83, 0.047, and 0.99, respectively. The results are
consistent with previous formulations of gas-phase accommodation from simulations, in which the process
occurs by rapid thermal and structural equilibration followed by diffusion on the free energy profile. The
implications of these results with respect to atmospheric chemistry are discussed.
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