Inhaled ambient particulate matter (PM) causes adverse health effects, possibly by generating reactive oxygen species (ROS), including hydrogen peroxide (HOOH), in the lung lining fluid. There are conflicting reports in the literature as to which chemical components of PM can chemically generate HOOH in lung fluid mimics. It is also unclear which redox-active species are most important for HOOH formation at concentrations relevant to ambient PM. To address this, we use a cell-free, surrogate lung fluid (SLF) to quantify the initial rate of HOOH formation from 10 transition metals and 4 quinones commonly identified in PM. Copper, 1,2-naphthoquinone, 1,4-naphthoquinone, and phenanthrenequinone all form HOOH in a SLF, but only copper and 1,2-naphthoquinone are likely important at ambient concentrations. Iron suppresses HOOH formation in laboratory solutions, but has a smaller effect in ambient PM extracts, possibly because organic ligands in the particles reduce the reactivity of iron. Overall, copper produces the majority of HOOH chemically generated from typical ambient PM while 1,2-naphthoquinone generally makes a small contribution. However, measured rates of HOOH formation in ambient particle extracts are lower than rates calculated from soluble copper by an average (±1σ) of 44 ± 22%; this underestimate is likely due to either HOOH destruction by Fe or a reduction in Cu reactivity due to organic ligands from the PM.