Coarse mode aerosols were collected at three sites in the Los Angeles area, two in Riverside, CA, one upwind and the other downwind of a major freeway, and also on the campus of the University of California, Los Angeles (UCLA). Coarse mode aerosol mass, H(2)O(2), and H(2)O(2) normalized to aerosol mass averaged 46 +/- 22 microg/m(3), 17 +/- 8 ng/m(3), and 0.48 +/- 0.32 ng/microg at the upwind Riverside site and 97 +/- 27 microg/m(3), 34 +/- 14 ng/m(3), and 0.37 +/- 0.18 ng/microg at the downwind Riverside site, respectively. H(2)O(2), which appears to be generated by the particles (Arellanes, C.; Paulson, S. E.; Fine, P. M.; Sioutas, C. Environ. Sci. Technol. 2006, 40, 4859-4866), was uncorrelated with particle mass, but was strongly correlated with soluble iron, zinc, and copper (r = 0.47-0.67, p = 0.00-0.01). H(2)O(2) levels were not affected by the addition of dithiothreitol, a marker for quinone redox activity. H(2)O(2) levels were sensitive to the pH of the particle extraction solutions, increasing as the pH was decreased. The initial rate of H(2)O(2) generation by coarse mode aerosols was 7.8 (+/-5.7) x 10(-8) M min(-1), similar to initial rates of hydroxyl radical generation from dissolved Fe(2+), Cu(2+), and Zn(2+) solutions. The results support the notion that the majority of coarse mode H(2)O(2) generation is mediated by a small set of transition metals.