Letter Cite This: Environ. Sci. Technol. Lett. XXXX, XXX, XXX−XXX
pubs.acs.org/journal/estlcu
Photochemical Production of Singlet Oxygen by Urban Road Dust Chelsea D. Cote,§ Stephanie R. Schneider,†,§ Ming Lyu, Sherry Gao, Lin Gan,‡ Adam J. Holod, Thomas H. H. Chou, and Sarah A. Styler* Department of Chemistry, University of Alberta, Edmonton, Alberta, Canada T6G 2G2 S Supporting Information *
ABSTRACT: Road dust resuspension is a major source of particulate matter in many urban centers, especially those in which traction materials are applied to roadways in winter. Although many studies have investigated the composition and toxicity of road dust, nothing is currently known regarding its photochemical reactivity. Here, we show for the first time that road dust is photochemically active: in particular, we use a molecular probe technique to show that the illumination of aqueous road dust suspensions leads to the production of singlet oxygen (1O2), an important environmental oxidant. In experiments conducted using size-fractionated road dust, we found that the surface areanormalized steady-state 1O2 concentration ([1O2]ss) increased with decreasing particle size. We 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. Interestingly, [1O2]ss in aqueous road dust extracts was lower than in the corresponding particlecontaining samples, which implies that the particle surface itself also participated in 1O2 production. This work provides evidence that road dust photochemistry may influence the lifetime of urban pollutants that react via 1O2-mediated pathways.
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INTRODUCTION In both Canada and the United States, dust from paved and unpaved roads is the largest anthropogenic source of primary fine particulate matter (PM2.5).1,2 As a result of accumulation of sand and other winter traction materials applied to roadways, the level of dust emission from paved roads often peaks in the spring;3,4 however, road dust has also been shown to contribute to PM10 and PM2.5 loadings throughout the year in a wide variety of urban locations.5−8 Unlike desert dust, which is primarily composed of crustal material,9 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.10−14 As a result of its source profile, road dust also contains a number of toxic species, including heavy metals15 and polycyclic aromatic hydrocarbons (PAHs).16 Several road dust constituents, including soil,17,18 humic substances,19 and non-transition metal oxides,20 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.21 In this study, we investigated the photochemical production of 1O2 by size-fractionated road dust collected in Edmonton, Alberta, Canada, in September 2016. To determine whether road dust represents a unique reactive environment, we compared the reactivity of this substrate to that of Arizona © XXXX American Chemical Society
test dust, Niger sand, and Cape Verde dust. To determine the involvement of chromophoric water-soluble organic carbon (WSOC) in the observed photochemistry,22 we assessed correlations between the calculated steady-state 1O2 concentrations ([1O2]ss) in our experiments and the dissolved organic carbon (DOC) content and ultraviolet (UV) absorbance properties of dust extracts. To the best of our knowledge, this work provides the first experimental evidence that road dust is photochemically active.
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EXPERIMENTAL SECTION Experimental Approach. We used the well-known probe molecule furfuryl alcohol (FFA) to monitor 1O2 production.23 To ensure that [1O2]ss in our system was controlled by physical quenching by water and thus that FFA displayed first-order loss kinetics,24 we performed experiments using 25 μM FFA. Under these conditions, [1O2]ss was equal to the pseudo-first-order rate constant for FFA loss [kobs,dust, determined from the slope of a plot of ln(FFA/FFA0) vs time] divided by the secondorder rate constant for reaction of FFA with 1O2 [krxn,FFA; (9.6 ± 0.4) × 107 M−1 s−1 at 293 K25].24 Experimental Apparatus. Experiments were conducted in a custom-built photochemical reaction chamber consisting of a Received: November 23, 2017 Revised: December 29, 2017 Accepted: January 3, 2018
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DOI: 10.1021/acs.estlett.7b00533 Environ. Sci. Technol. Lett. XXXX, XXX, XXX−XXX
Letter
Environmental Science & Technology Letters temperature-controlled 12-cell sample compartment equipped with a magnetic stirring mechanism (Figure S1). Samples were illuminated using an Abet Technologies SunLite solar simulator (Figure S2), the spatial uniformity of which was verified using chemical actinometry of 2-nitrobenzaldehyde (Figure S3).26 Experimental Procedure. Experiments were conducted in 1 mL quartz vials containing 0.9 mL of aqueous 25 μM FFA and 0.15−10 mg of the sample of interest. Details regarding the choice of sample mass are presented in the text of the Supporting Information and in Figure S4. Some experiments were performed using solvent-rinsed samples (water, methanol, and hexane). In these experiments, 212−500 μm road dust samples were briefly shaken with the solvent at the same ratio of dust mass to water volume employed in the illumination experiments; after centrifugation, the supernatant was removed and the dust sample was dried overnight at 333 K. One set of experiments was performed using ashed road dust; in this case, an aliquot of 212−500 μm road dust was heated for 4 h at 648 K to remove the dust organic fraction.27 Some experiments were also performed using aqueous dust extracts; here, dust samples were extracted as described above, and the supernatant was filtered (0.2 μm nylon filter, VWR) prior to use. Samples were gently stirred for the duration of illumination (0−4 h), which ensured equilibration with ambient O2 levels (i.e., ∼3 x 10−4 M dissolved O2 at 293 K28). After illumination, samples were filtered and analyzed via high-performance liquid chromatography with UV−visible detection (HPLC−UV) as described in the Supporting Information. Control experiments in the absence of dust showed minimal FFA loss (see Figure 1); no dust-mediated FFA loss was observed in the dark. Chemicals and Environmental Samples. FFA (98% reagent grade), 2-nitrobenzaldehyde (≥99.9%), deuterated water (D2O, 99.9 atom % D), and HPLC-grade acetonitrile (≥99.9%) were obtained from Sigma-Aldrich and used as received. Deionized water was obtained from a Millipore Synergy UV ultrapure water system. Arizona test dust (0−3 μm) was obtained from Powder Technology Inc. Niger sand (
