Field and Laboratory Investigations of Organic Photochemistry on Urban Surfaces

Abstract:

In polluted urban environments, windows and building surfaces rapidly become coated with a complex film of chemicals, which enhances the dry deposition of particles and the partitioning of semi-volatile organic species to the surface. 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 photooxidative processing of semi-volatile organics contained within it. The present study represents a comprehensive field- and laboratory-based investigation of the film-phase photochemistry of polycyclic aromatic hydrocarbons (PAH), here used as proxies for light-absorbing semi-volatile organics present within the film.

Urban film sampling was conducted using a custom-built three-stage sampler housing, which was deployed in a central, high-traffic area in Leipzig, Germany. The sampler itself employs small glass beads as surrogate window surfaces and is designed such that only its uppermost stage is exposed to sunlight. Each stage is subdivided into 16 compartments, which allows for the study of film formation and evolution.

In the first phase of the study, the role of urban film as a photochemical sink for reactive organic species was determined by measuring total film PAH content and PAH abundance ratios as a function of atmospheric exposure time under both light and dark conditions.

In the second, more general, phase of the study, the organic and inorganic composition of collected film samples was compared to that of co-located PM₁₀ samples, and differences between the two sample types were used to gain insight into the relative importance of heterogeneous photochemical oxidation within the particle and film phases.

In the third phase of the study, film samples grown under dark conditions were exposed to gas-phase ozone in an atmospheric-pressure flat-bed reactor, and the kinetics of ozone-induced PAH loss were studied under both dark and illuminated conditions. Since previous work from our group has shown that the heterogeneous photooxidation of PAH occurs at different rates and via different mechanisms depending on its immediate environment, this in situ study of PAH reactivity provides substantial insight into the photochemical processing of this class of compounds in urban environments.