Reactive bromine species contribute significantly to the destruction of ozone in the polar stratosphere(1). Reactive halogen compounds can have a strong effect not only on the chemistry of the stratosphere but also on that of the underlying troposphere. For example, severe ozone depletion events that are less persistent than those in the stratosphere occur in the Arctic(2) and Antarctic(3) boundary layer during springtime and are also associated with enhanced BrO abundances(2,4-10). Observations(5-8) of BrO (and ClO, which is less important) at ground level during these ozone depletion events have revealed halogen oxide mixing ratios of up to 30 parts per trillion-sufficient to destroy within one to two days the 30-40 parts per billion of ozone typically present in the boundary layer. The catalytic mechanism leading to so-called ＇tropospheric ozone holes＇ is well established(2,11), but the origin of the increased BrO concentrations and the spatial and temporal extent of these events remains poorly understood. Here we present satellite observations showing that tropospheric air masses enriched in BrO are always situated close to sea ice and typically extend over areas of about 300-2,000 km. The BrO abundances remain enhanced for periods of 1 to 3 days. These observations support the suggestion(7,9,10) that autocatalytic release of bromine from sea salt gives rise to significant BrO formation which, in turn, initiates ozone depletion in the polar troposphere.