Competitive Adsorption of Trace Gases on Ice at Tropospheric Temperatures: A Grand Canonical Monte Carlo Simulation Study

The coadsorption of two atmospheric
trace gases on ice is characterized
by using, for the first time, grand canonical Monte Carlo (GCMC) simulations
performed in conditions similar to those of the corresponding experiments.
Adsorption isotherms are simulated at tropospheric temperatures by
considering two different gas mixtures of 1-butanol and acetic acid
molecules, and selectivity of the ice surface with respect to these
species is interpreted at the molecular scale as resulting from a
competition process between these molecules for being adsorbed at
the ice surface. It is thus shown that the trapping of acetic acid
molecules on ice is always favored with respect to that of 1-butanol
at low pressures, corresponding to low coverage of the surface, whereas
the adsorption of the acid species is significantly modified by the
presence of the alcohol molecules in the saturated portion of the
adsorption isotherm, in accordance with the experimental observations.
The present GCMC simulations thus confirm that competitive adsorption
effects have to be taken into consideration in real situations when
gas mixtures present in the troposphere interact with the surface
of ice particles.