Experiments to determine the concentration of photochemically available Fe in ambient aerosol samples were carried out using a novel photochemical extraction procedure. Ambient aerosol samples, which were collected on Teflon filters, were suspended in an aqueous solution within a photochemical reactor and irradiated. Under these conditions, which were favorable to the photochemical weathering of aerosol particles, the relative amount of Fe(II)aq to Fetotal was shown to increase. The extent and rate of Fe(II)aq photoproduction was used to characterize the Fe in aerosol samples collected from Whiteface Mountain, New York, Pasadena, California, San Nicholas Island, California, and Yosemite National Park, California. Photochemically available Fe concentrations found ranged from <4 ng m-3 (0.07 nmole m-3) to 308 ng m-3 (5.52 nmole m-3), Fetotal concentrations ranged from 10 ng m-3 (0.18 nmole m-3) to 3400 ng m-3 (61 nmole m-3), and the percentage of photochemically available Fe to Fetotal ranged from 2.8% to 100%. Aerosol samples were also collected during biomass burning events in southern California; these samples showed insignificant changes in the photochemically available Fe (compared to nonbiomass burning samples) in conjunction with large increases of Fetotal. Calculations based on these experiments also provide further evidence that redox reactions of Fe in cloudwater could be an important in situ source of oxidants (⋅OH, HO⋅2/O-2). The estimated oxidant production rate in cloudwater based on these experiments is between 0 and 60 nM s-1, with an average value of 16 nM s-1. This estimated in situ oxidant production rate due to Fe chemistry is approximately equal to previous estimates of the oxidant flux to cloudwater from the gas phase. ¿ American Geophysical Union 1996