Physical and Chemical Characterization of Urban Grime Sampled from Two Cities

Abstract

Urban grime can be an important substrate for heterogeneous reactions in cities. Studies performed using laboratory-prepared urban grime proxies and urban grime collected from the field indicate that the physicochemical properties of urban grime can greatly affect heterogeneous reaction rate constants. We investigated several properties of urban grime collected from two cities in the north-east region of the United States of America. Optical and Raman microscopy indicated that urban grime collected from these regions consists primarily of particles as opposed to a uniform film. Total carbon analysis and ion chromatography were used to determine the bulk composition of the urban grime from both cities. Comparing these results to reported compositions of urban grime from cities in Canada and Europe showed strong similarities between different locations, with some variations for specific ions, such as higher chloride levels in North American cities and higher sulfate levels in some European cities. Absorbance spectra demonstrated that urban grime can absorb sunlight across the ultraviolet region. It may therefore be able to participate in photochemical reactions (either hindering them via processes such as the inner filter effect or enhancing them via processes such as photosensitization).

Factors affecting wavelength‐resolved ultraviolet irradiance indoors and their impacts on indoor photochemistry

“…46,47 The effect in UV wavelengths relevant to indoor photochemistry has not been quantified. Recently, our research group reported that urban grime absorbs solar radiation throughout the actinic region, 48 which suggests that UV irradiance indoors could be reduced at high urban grime surface coverages. However, the results also imply that urban grime surface coverage on windows will often be very low.…”

Evaluating Environmental Film Maturation through a Deliquescence–Efflorescence Model

“…Because film compositions are often dominated by particles, measuring changes to particulate morphology (e.g., sizes and shapes) can help us estimate a film's adsorptive capacity. 2,12 Outdoor film formation models have been reported, but pathways and the effects of film maturation are unknown or remain poorly defined. 11,14 Formation models generally suggest that (semi-)volatile organic molecules and particles first encounter a viscous sublayer surrounding solid objects, then cross this layer, and finally interact physically with the underlying surface material.…”

“…16,17 In fact, the precursor film may only be a few nanometers thick, and recent studies have suggested that the majority of a film's ultimate mass comprised of super-micron-sized particles. 1,2,14 these studies, the organic portion of the film has been quantified at 10−40% of the film's total mass with 10% of that being monocarboxylic acids and 35% being aliphatic species. 17 The inorganic portion has been identified to be 60−90% of the total mass with ∼7% of this material being nitrates.…”

“…18 The values listed are generic findings explaining the chemical composition, but it is accepted that composition will vary significantly by location and season. 2,18 Indoor film chemistry has evolved parallel to outdoor film studies. Concepts from this science can also be applied with limitations understanding the assumptions made.…”

“…These trends are also important with respect to recent findings that films on vertical surfaces may have more particlelike composition. 2 Mixed Films. The mixed films show a small increase in the average number of particles per mm 2 (Figure 3a, middle column).…”