Urban grime, the collective term for pollutants blanketing most city surfaces, are actually substances that affect chemical reactions on the surfaces they are on, according to a new study.

A study led by researchers from the Department of Chemistry at Syracuse University has collected urban grime gathered from two cities in the northeastern United States. A closer look at these materials revealed that they absorb sunlight, potentially serving as important substrates for chemical reactions in cities. Researchers published their findings at the American Chemical Society's Earth and Space Chemistry journal.

Studying Lab-Prepared and Field-Collected Urban Grime

The proponents of the study have previously analyzed a proxy material for urban grime prepared in the laboratory. Later on, they collected urban grime samples from their native Syracuse, New York, and the city of Scranton, Pennsylvania - taking samples from more than 40 locations in both cities. In their field collection activity, they placed vertical quartz plates on the areas and left for 30 days before recovery and analysis.

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Researchers then conducted both optical and Raman microscopy in an attempt to characterize the composition of the samples. They observed that the urban grime they have collected is mostly made up of particles and not a uniform film.

They also conducted carbon analysis and ion chromatography to understand the bulk chemical composition of the urban grime samples, hoping to look for consistency or similarity across all collected specimens. Researchers used reported urban grime compositions from Canada and Europe as reference for those gathered from the two US cities - finding strong similarities across separate locations, noting that there were some variations in terms of specific ions. One particular substance, chloride, was found to be in higher concentrations for North American cities (Canada and US), while sulfate levels were higher among European cities.

There were also specific findings to be points of interest for the researchers. Among the locations, in Syracuse, for example, there is an outlier in the case of Jefferson I, which had the largest ionic concentration of nitrate among all samples in the city. On the other hand, their Lafayette 3 location was the outlier for levels of potassium ions, contributing 4.5 percent compared to the mean of 1.4 percent from all different samples.

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A Potential Factor for Heterogeneous Chemical Reactions

Based on the studies conducted, urban grime from the field strongly indicates that the physicochemical properties of these materials affect heterogeneous reaction rate constants.

To illustrate this property, researchers took the wavelength-resolved light extinction, or scattering of aerosol particles, for the 19 Syracuse samples and 22 Scranton samples. It showed that, in general, urban grime could absorb light, including those wavelengths that are included in natural sunlight.

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This suggests that as urban grime partially absorbs sunlight, the Sun itself can affect - either by speeding it up or slowing it down - on chemical reactions, which affect air and water quality in cities.