Composition and sources of atmospheric trace elements at three urban sites in Helsinki, Finland

Abstract

This study applies a positive matrix factorization (PMF) model to identify the sources of ambient fine (PM2.5) and coarse (PM2.5-10) particulate matter at two traffic sites and one urban background site in Helsinki, Finland. Over 700 24-h PM2.5 and PM10 filter samples were analysed using energy dispersive X-ray fluorescence, which quantified 12–19% of PM2.5 and 22–29% of PM10 mass. In the coarse particle fraction, Si had the highest concentrations, followed by Fe, Al, and Ca, while S, Fe, and either Na or Ca dominated the fine particle fraction. A 5-factor solution best represented the elemental sources, identifying Dust, Non-Exhaust Emissions (NEE), Aged Salt, Fresh Salt, and a mixed factor called here as Combustion/Secondary inorganic aerosol (SIA). Dust had the highest contribution at all sites, except for PM2.5 at the urban background site, where Combustion/SIA was dominant. Dust was characterized by Al, Si, and Ti, while Combustion/SIA by S, V, Ni, Pb, and Se. Fresh salt was dominated by Cl, and Aged salt by Na and Mg. NEE was enriched in Cr, Fe, Cu, Zn, Ba, and Zr, suggesting primarily brake wear with possible tyre-wear contributions. Size distributions, measured with a 13-stage impactor at one of the sites, showed that elements related to Dust peaked at 5.6–10 μm and Combustion/SIA at 0.2–0.6 μm. Elements associated with NEE exhibited bimodal behaviour (<1 μm and 3–10 μm). In addition to ambient measurements, brake wear particles were studied in the laboratory to support the interpretation of source apportionment analysis.