Airborne Particulate-Bound Metals in Chicago: Seasonal and Spatial Variability

Airborne particulate matter (PM) samples were collected continuously at the University of Illinois Chicago (UIC) for 39.6 weeks throughout 2021, separated into three size fractions (0.3-1.2 µm, 1.2-2.5 µm, and > 2.5 µm), and analyzed via Inductively Coupled Mass Spectrometry for a suite of metals. Statistical methods including principal components analysis, Mahalanobis distances, correlation tests, Ward’s clustering, and box-and-whisker plots were used to evaluate the resulting concentrations for differences between size fractions and seasonal and spatial variability in comparison to contemporaneous data collected at U.S. Environmental Protection Agency (EPA) sites across the Chicago region. Overall, samples in the coarsest size fraction (PM>2.5) contain the most metal, contributed by the greatest number of elements; half the analyzed elements were rarely or never detected in the finer two size fractions (PM0.3-2.5) at UIC. The distribution of metals in PM0.3-2.5 exhibits seasonal variability, whereas the relative abundance of metals in PM>2.5 remains similar throughout the year. PM>2.5-bound metals are also more strongly correlated to one another than PM0.3-2.5-bound metals. In PM>2.5, concentrations of Ag, As, Ca, Cd, Fe, Mg, Mo, Pb, Sb, and Ti are positively correlated to southerly and/or southwesterly winds throughout the year; in PM0.3-2.5, only Mo concentrations are positively correlated to southwesterly winds. According to the EPA’s Toxics Emissions Inventory (2022), the industrial point sources responsible for the heaviest atmospheric metals emissions are located south of UIC. PM0.3-2.5 samples collected during 4/20-4/27, 6/2-6/9, and 9/8-9/15 contain unusually high concentrations of certain metals – not only at UIC but also at EPA sites. Despite contemporaneous spikes in certain elements at multiple sites, no metals are positively correlated across all sites except for Al, Cu, and Ti. Aluminum, Ca, Fe, Mg, and Ti are associated with one another at all sites. These five elements, along with Cu, likely share road dust as a major source. Because several of the most abundant metals (e.g., Al, Fe, Mn) throughout the study region are linked to health hazards, and because EPA data only represent atmospheric conditions during 1/3 of the year at most, further continuous monitoring of PM-bound metals is recommended.