Abstract The trace metal contents and iron isotope composition of size-resolved aerosols were determined in a parking structure in Tempe, AZ, USA. Particulate matter (PM) < 2.5 µm in diameter (the fine fraction) and PM > 2.5 µm were collected. Several air toxics (e.g., arsenic, cadmium, and antimony) were enriched above the crustal average, implicating automobiles as an important source. Extremely high levels of fine copper (up to 1000 ng m − 3 ) were also observed in the parking garage, likely from brake wear. The iron isotope composition of the aerosols were found to be + 0.15 ± 0.03‰ and + 0.18 ± 0.03‰ for the PM < 2.5 µm and PM > 2.5 µm fractions, respectively. The similarity of isotope composition indicates a common source for each size fraction. To better understand the source of iron in the parking garage, the elemental composition in four brake pads (two semi-metallic and two ceramic), two tire tread samples, and two waste oil samples were determined. Striking differences in the metallic and ceramic brake pads were observed. The ceramic brake pads contained 10–20% copper by mass, while the metallic brake pads contained about 70% iron, with very little copper. Both waste oil samples contained significant amounts of calcium, phosphorous, and zinc, consistent with the composition of some engine oil additives. Differences in iron isotope composition were observed between the source materials; most notably between the tire tread (average = + 0.02‰) and the ceramic brake linings (average = + 0.65‰). Differences in isotopic composition were also observed between the metallic (average = + 0.18‰) and ceramic brake pads, implying that iron isotope composition may be used to resolve these sources. The iron isotope composition of the metallic brake pads was found to be identical to the aerosols, implying that brake dust is the dominant source of iron in a parking garage.