Abstract. Previous studies have suggested that the adverse health effects from ambient particulate matter (PM) are linked to the formation of reactive oxygen species (ROS) by PM in cardiopulmonary tissues. While hydroxyl radical (^•OH) is the most reactive of the ROS species, there are few quantitative studies of ^•OH generation from PM. Here we report on ^•OH formation from PM collected at an urban (Fresno) and rural (Westside) site in the San Joaquin Valley (SJV) of California. We quantified ^•OH in PM extracts using a cell-free, phosphate-buffered saline (PBS) solution with or without 50 μM ascorbate (Asc). The results show that generally the urban Fresno PM generates much more ^•OH than the rural Westside PM. The presence of Asc at a physiologically relevant concentration in the extraction solution greatly enhances ^•OH formation from all the samples. Fine PM (PM_2.5) generally makes more ^•OH than the corresponding coarse PM (PM_cf, i.e. with diameters of 2.5 to 10 μm) normalized by air volume collected, while the coarse PM typically generates more ^•OH normalized by PM mass. ^•OH production by SJV PM is reduced on average by (97 ± 6) % when the transition metal chelator desferoxamine (DSF) is added to the extraction solution, indicating a dominant role of transition metals. By measuring calibration curves of ^•OH generation from copper and iron, and quantifying copper and iron concentrations in our particle extracts, we find that PBS-soluble copper is primarily responsible for ^•OH production by the SJV PM, while iron often makes a significant contribution. Extrapolating our results to expected burdens of PM-derived ^•OH in human lung lining fluid suggests that typical daily PM exposures in the San Joaquin Valley are unlikely to result in a high amount of pulmonary ^•OH, although high PM events could produce much higher levels of ^•OH, which might lead to cytotoxicity.