The presence of chlorine radical species in the lower troposphere has been postulated, but only indirect evidence of its existence has been reported. We present the first direct spectroscopic observations of up to 15 ¿ 2 pmoles/mole-air chlorine oxide (ClO) in the mid-latitude boundary layer and confirm a previous discovery of the existence of bromine oxide (up to 6 ¿ 0.4 pmoles/mole-air BrO). Our observations from October 2000 suggest that reactive halogens are mobilized from salt on the flats around the Great Salt Lake, Utah. The results imply levels of 105 Cl atoms per cm3, which doubles the atmospheric oxidation capacity. Ozone concentrations are anti-correlated with the halogen oxide levels, particularly BrO, indicating that ozone depletion by catalytic reaction cycles may have occurred. If the mobilization of halogens from salt flats that was observed here is widespread, there are important implications for atmospheric chemistry and the biogeochemical cycling of mercury.