Uptake and reaction of atmospheric organic vapours on organic films

Abstract

Films composed in whole or in part of organic compounds represent an important atmospheric interface. Urban surfaces are now known to be coated with a film ("grime") whose chemical composition somewhat resembles that of urban atmospheric aerosols. Such films may act as media in which atmospheric trace gases may be sequestered (leading to their removal from the gas phase); they may also act as reactive media, either as a "solvent" or as a source of reagents. Organic coatings on aqueous surfaces are also important, not just on ocean and lake surfaces ("biofilms") but also on the surfaces of fogwaters and atmospheric aerosol particles. We have initiated experimental uptake studies of trace gases into simple proxies for urban organic films using two techniques: a Knudsen cell effusion reactor and a laser-induced fluorescence method. We will discuss our first results on non-reactive uptake of organic compounds by organic films we use as proxies for urban grime coatings. In general, the measured uptake coefficients appear to track the octanol-air partition coefficients, at least qualitiatively. We have also measured the kinetics of reactions between gas-phase ozone and small polycyclic aromatic hydrocarbons (PAHs), when these are adsorbed at the air-aqueous interface or incorporated into an organic film. Reactions at the "clean" air-water interface and at a coated interface consisting of a monolayer of various amphiphilic organic compounds all follow a Langmuir-Hinshelwood mechanism, in which ozone first adsorbs to the air-aqueous interface, then reacts with already adsorbed PAH. By contrast, the reaction in the pure organic film occurs in the bulk phase. Under some circumstances, heterogeneous oxidation of PAHs by ozone may be as important in the atmosphere as their gas phase oxidation by OH.