An investigation was carried out to identify some of the oxidants formed during the sunlight photolysis of authentic cloud and fog water samples from North Carolina, Virginia, New York, Washington, and Oregon. Constituents (chromophores) present in the atmospheric water samples absorb solar ultraviolet radiation, and thereby initiate formation of peroxyl radicals (RO2 and HO2), and singlet molecular oxygen, O2(1Δg). In the sunlit cloud and fog waters studied, (1) photostationary state total concentrations of unspeciated peroxyl radicals ranged from 1 to 30 nanomolar (nM), and (2) photostationary state concentrations and aqueous-phase photochemical formation rates of O2(1Δg) ranged from 27 to 1100 femtomolar (fM) and 5 to 200 nanomolar/second (nM/s), respectively. The chromophores responsible for initiating these photochemical reaction have not yet been identified. Although these studies contain uncertainties, we conclude that aqueous-phase photochemical reactions are a significant, and in some cases dominant, source of peroxyl radicals and O2(1Δg) in tropospheric cloud and fog drops. This represents a heretofore unrecognized source of these oxidants in atmospheric water drops and the first discussion and report of their detection in authentic condensed phases of the atmosphere. ¿ American Geophysical Union 1992