The fate of NO(x) (=NO + NO(2)) is important to understand because NO(x) is a significant player in air quality determination through its role in O(3) formation. Here we show that renoxification of the urban atmosphere may occur through the photolysis of HNO(3) deposited onto urban grime. The photolysis occurs 4 orders of magnitude faster than in water with J values at noon on July 1 in Toronto of 1.2 × 10(-3) s(-1) for nitrate on urban grime and 1.0 × 10(-7) s(-1) for aqueous nitrate. Photolysis of nitrate present on urban grime probably follows the same mechanism as aqueous nitrate photolysis, involving the formation of NO(2), OH, and possibly HONO. Thus NO(x) may be rapidly returned to the atmosphere rather than being ultimately removed from the atmosphere through film wash off.

Always cite the published version, so the author(s) will receive recognition through services that track citation counts, e.g. Scopus. If you need to cite the page number of the author manuscript from TSpace because you cannot access the published version, then cite the TSpace version in addition to the published version using the permanent URI (handle) found on the record page. Deposition of atmospheric constituents -either gas phase or particulate -onto urban 16 impervious surfaces gives rise to a thin "urban grime" film. The area exposed by these 17 impervious surfaces in a typical urban environment is comparable to, or greater than, that 18 of particles present in the urban boundary layer, however it is largely overlooked as a site 19 for heterogeneous reactions. Here we present the results of a field campaign to determine 20 and compare the chemical composition of urban grime and of particles collected 21 simultaneously during the autumn of 2014 at an urban site in central Leipzig, Germany. 22We see dramatically reduced ammonium and nitrate levels in the film as compared to 23 particles suggesting a significant loss of ammonium nitrate, thus enhancing the mobility 24 of these species in the environment. Nitrate levels are 10% lower for films exposed to 25 sunlight compared to those that were shielded from direct sun, indicating a possible 26 mechanism for recycling nitrate anion to reactive nitrogen species. Finally, chloride 27 levels in the film suggest that it could represent an unrecognized source of continental 28 chloride available for ClNO 2 production even in times of low particulate chloride. Such 29 source and recycling processes could prove to be important to local and regional air 30 quality. 31

Abstract. Impervious surfaces are ubiquitous in urban environments and constitute a substrate onto which atmospheric constituents can deposit and undergo photochemical and oxidative processing, giving rise to “urban grime” films. HNO3 and N2O5 are important sinks for NOx in the lower atmosphere and may be deposited onto these films, forming nitrate through surface hydrolysis. Although such deposition has been considered as a net loss of NOx from the atmosphere, there is increasing evidence that surface-associated nitrate undergoes further reaction. Here, we examine the gas phase products of the photochemistry of real, field-collected urban grime using incoherent broadband cavity-enhanced absorption spectroscopy (IBBCEAS). Gas phase nitrogen oxides are emitted upon illumination of grime samples and their production increases with ambient relative humidity (RH) up to 35 % after which the production becomes independent of RH. These results are discussed in the context of water uptake onto and evaporation from grime films.

Impervious surfaces, especially in urban environments, are coated with a film composed of a complex mixture of substances, referred to as urban grime. Despite its ubiquity, the factors that dictate urban grime composition are still not well understood. Here, we present the first study of the seasonal variation in composition of water-soluble inorganic ions present in urban grime, performed by analyzing samples collected in Toronto for 4-week intervals over the course of a year. A clear seasonality in the composition is evident, with NaCl dominating in the winter months and Ca2+ and NO3 – dominant in the summer. We compare the grime composition to the water-soluble ion composition of PM2.5 and PM10 in order to infer chemistry occurring within the grime and find evidence that chemistry occurring within the urban grime matrix could provide a source of ClNO2 and NH3 to the urban atmosphere. The uptake of water by urban grime also shows a clear seasonality, which may be driven by the changing proportions of nitrate salts and/or oxidized organic compounds over the year.

In polluted urban environments, windows and building surfaces are coated with a complex film of chemicals. Despite its high surface-to-volume ratio and direct exposure to sunlight, few studies have directly investigated the role that this "urban film" may play in promoting the chemistry and photochemistry of semivolatile organic species contained within it. Here, we report results from a field investigation of the organic composition of urban film and particulate matter (PM 10 ) samples collected at an urban site in Leipzig, Germany, in which we provide clear evidence for the influence of anthropogenic processes on film composition. In this study, we find that the ratio of water-soluble organic carbon (WSOC) to the total ionic content of film samples decreases with atmospheric exposure time, which suggests that urban film growth proceeds first via the condensation of semivolatile species, and that the coating thus formed enhances the dry deposition of particles. Further, we find that the polycyclic aromatic hydrocarbon (PAH) abundance profiles in light-exposed films are different from those in films collected under light-shielded conditions, which represents the first direct evidence that urban films serve as a photochemical sink for semivolatile organic pollutants. Finally, we find that the PAH and n-alkane profiles of urban film samples differ substantially from colocated PM 10 samples, which we suggest reflects both the contribution of settled coarse particulate matter to the overall film composition and the influence of in-film oxidative processes. Together, these results highlight the unique reactive environment afforded by urban film and underscore the need for further studies of urban surface chemistry.