Hydroxyl radicals remove hundreds, perhaps thousands, of organic gases from the atmosphere and are often regarded as am index of the oxidizing capacity of the Earth\u27s atmosphere (Thompson 1992). In recent years, there have been growing concerns that, over the past century and now, human activities may be depleting hydroxyl concentrations by adding huge amounts of carbon monoxide and methane to the atmosphere. Reduction in the hydroxyl concentrations can then indirectly lead to more global warming, stratospheric ozone depletion, and other disturbances in atmospheric chemistry. Carbon monoxide (CO) is a key component in the determination of hydroxyl radical (OH) concentrations: increases of CO would lead to a decline of OH. Global increases of CO had been observed in the 1980s (Khalil and Rasmussen 1985, 1988, 1990), but now it appears that the atmospheric concentrations of CO are falling. Here we will report data from Antarctica that suggest recent decreases in the concentration of CO
