Heterogeneous atmospheric chemistry has predominantly been concerned with processes involving airborne aerosol particles. This is mainly for historical reasons: acid rain and ozone hole (and depletion, generally) chemistry has demanded an understanding of chemical processes taking place on and at the surface of such particles. (I) More recently, the interest has also been motiviated by trying to understand and quantify the cloud-condensing properties of aerosols, and how these properties may change as the particles are aged in the atmosphere. However, it is certainly true that similar chemical interactions may take place among gas-phase species and other heterogeneous media, such as bodies of water, vegetation and soils. In urban environments there also exists the possibility of reactions taking place on the large and stationary surfaces of the films coating buildings, roadways, etc. Organic films (self-assembled monolayers or neat liquids) have been used as laboratory proxies for aerosol surfaces (2) because of experimental challenges often associated with exploring reactions using "real" particles. However, reactions on urban films could display rather different features than the corresponding processes in aerosol, gas or
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aqueous phases, due to the different chemical environment of the exposed urban surfaces. Yet, there are few explorations of the chemistry taking place on such surfaces. The following is a brief review of our recent work in this direction.
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