In the past decade, extensive work by Diamond and co-workers (3-7) has shown that exposed outdoor surfaces in urban areas become rapidly coated with a complex mixture of chemical compounds ("urban surface film"), most readily encountered as "window grime." This film grows via accretion from the atmosphere and is removed by rain washoff, or revolatilization processes, yielding an (estimated) steady-state thickness of several tens to hundreds of nanometers. Chemical analysis of these films has been carried out, both in a "broad brush" approach, (6, 7) which identified the compound classes present, and more detailed studies., (3-5) that determined the specific compounds within these classes.
Interestingly, organics make up only 5-10% (by mass) of the films; most of the identified mass is nitrate (-7%), sulphate (-8%) and various metals (18%). Among the organics, fatty acids, alkanes, carbohydrates and aromatics are all observed, as well as trace contaminants such as PAHs, PCBs and PBDEs. Field measurements have determined that, for the most part, the partitioning of such trace gases between the atmosphere and the film is related to the octanol-air partitioning coefficient, Koa, (4) suggesting that it is the organic fraction which controls the deposition of airborne compounds to the film. This property has been exploited by using the urban films as passive samples of ambient air pollutants. (3, 8, 9) From a laboratory perspective, it suggests that proxy films composed of octanol, mixed with other components, might mimic the chemical environment of "real" urban films.
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