NO_x reactions on aqueous surfaces with gaseous HCl: Formation of a potential precursor to atmospheric Cl atoms

Chlorine atoms are highly reactive free radicals known to catalyze ozone depletion in the stratosphere and organic oxidation in the troposphere. They are readily produced photolytically upon irradiation of some stable Cl containing species, for instance, nitrosyl chloride, ClNO. We predict the formation of ClNO using ab initio molecular dynamics (AIMD) simulations of an NO₂ dimer on the surface of a thin film of water upon which gaseous HCl impinges. The reactant is chloride ion formed when HCl ionizes on the water film. The same mechanism for ClNO production may occur in humid environments when ONONO ₂ (the asymmetric NO₂ dimer examined here) comes in contact with either HCl or sea salt. The film of water serves to (1) stabilize ONONO₂ on the film surface so that it is localized and physically accessible for reaction, (2) provide the medium to ionize HCl, and (3) activate ONONO₂ making it more susceptible to nucleophilic attack by chloride. This substitution/elimination mechanism is new for NO_x chemistry on thin water films and could not be derived from studies on small clusters.