Automobile catalyst emissions have resulted in the occurrence of elevated Pt, Pd, and Rh concentration in the urban and roadside environment. We investigate the chronology of platinum group elements (PGE) accumulation in dated sediments from an urban lake near Boston, MA. Chronological profiles demonstrate that Pt, Pd, and Rh concentrations increased following the introduction in catalysts with accumulation rates 6-16 times larger in 1992-2002 than prior to the introduction of catalysts. Ratios of these elements closely match their ratios in catalysts, providing further evidence of an automobile source. Iridium and Ru accumulation in sediments also increased following the introduction of catalysts, and while past Os contamination is associated with leather tanning, recent changes in the isotopic composition of Os indicate another anthropogenic source for this element. The PGE have similar geochemical properties and are difficult to separate from one another, and therefore, we suggest that automobile catalyst emissions also result in increasing environmental concentrations of Ir, Ru, and Os, which occur as impurities in catalysts. An automobile catalyst source of Ir and Os is supported by elevated concentrations of these elements in a tunnel dust sample.