Unlike desert dust, which is primarily composed of crustal material, road dust is a complex mixture that also includes particles from road surface, brake, and tire wear; traction materials; semivolatile components of vehicle exhaust; and vegetative detritus, including soil and humic materials. As a result of its source profile, road dust also contains a number of toxic species, including heavy metals and polycyclic aromatic hydrocarbons (PAHs). Several road dust constituents, including soil, humic substances, and non-transition metal oxides, have been shown to produce the important environmental oxidant singlet oxygen (¹O₂) upon being illuminated. These observations suggest that road dust itself may be a photochemical source of ¹O₂ and thereby promote a variety of atmospherically important ¹O₂-mediated processes, including the oxidation of surface-sorbed PAHs.

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In the study, the team study, investigated the photochemical production of ¹O₂ by size-fractionated road dust collected in Edmonton, Alberta and compared the reactivity of this substrate to that of Arizona test dust, Niger sand, and Cape Verde dust.

Using a molecular probe technique (furfuryl alcohol, FFA), the research team showed that the illumination of aqueous road dust suspensions leads to the production of singlet oxygen—an important environmental oxidant. Although the physical properties of singlet oxygen only differ subtly from those of the more prevalent triplet ground state of O₂, singlet oxygen is far more reactive toward organic compounds.

In experiments conducted using size-fractionated road dust, the U Alberta team found that the surface area-normalized steady-state ¹O₂ concentration ([¹O₂]_ss) increased with decreasing particle size.

The team also observed correlations between [¹O₂]_ss and the dissolved organic carbon content and ultraviolet absorbance properties of dust extracts, which suggests the involvement of chromophoric water-soluble organic carbon in the observed photochemistry. [¹O₂]_ss in aqueous road dust extracts was lower than in the corresponding particle-containing samples, implying that the particle surface itself also participated in ¹O₂.

We found that when you shine light on road dust, it produces a reactive form of oxygen called singlet oxygen. It acts as an oxidant in the environment and can cause or influence other chemical reactions.

—Sarah Styler, corresponding author

Styler explained that if contaminants in road dust react with singlet oxygen, that means that sunlight could change the lifetime and potency of those contaminants in ways we don’t yet understand.

One group of chemicals that could react with singlet oxygen are a set of toxic components of combustion emissions, known as polycyclic aromatic hydrocarbons.

The team will next examine road dust from other places around the city, including residential, commercial, and park areas to understand better if and how the different composition of road dust will influence reactivity.

Resources

• Chelsea D. Cote, Stephanie R. Schneider, Ming Lyu, Sherry Gao, Lin Gan, Adam J. Holod, Thomas H. H. Chou, and Sarah A. Styler (2018) “Photochemical Production of Singlet Oxygen by Urban Road Dust” Environmental Science & Technology Letters doi: 10.1021/acs.estlett.7b00533