Modeling urban films using a dynamic multimedia fugacity model

► Organic films accumulate on urban impervious surfaces. ► Films accumulate organic chemicals and act as a source or sink to the environment. ► The Multimedia Urban Model was extended to capture dynamic film behavior. ► Modeling results showed that films mainly act as a chemical sink from air. ► As film accumulates chemical it can become a source to the environment via wash-off. A thin film coats impervious urban surfaces that can act as a source or sink of organic pollutants to the greater environment. We review recent developments in the understanding of film and film-associated pollutant behavior and incorporate them into an unsteady-state version of the fugacity based Multimedia Urban Model (MUM), focusing on detailed considerations of surface film dynamics. The model is used to explore the conditions under which these atmospherically-derived films act as a temporary source of chemicals to the air and/or storm water. Assuming film growth of 2.1nmd−1 (Wu et al., 2008a), PCB congeners 28 and 180 reach air–film equilibrium within hours and days, respectively. The model results suggest that the film acts as a temporary sink of chemicals from air during dry and cool weather, as a source to air in warmer weather, and as a source to storm water and soil during rain events. Using the downtown area of the City of Toronto Canada, as a case study, the model estimates that nearly 1gd−1 of ∑5PCBs are transferred from air to film to storm water.