Evidence for bromine monoxide in the free troposphere during the Arctic polar sunrise

During the Arctic polar springtime, dramatic ozone losses occur not only in the stratosphere but also in the underlying troposphere(1), These troposph eric ozone loss events have been observed over large areas(2,3) in the plan etary boundary layer (PBL) throughout the Arctic(4,5), They are associated with enhanced concentrations of halogen species(1,6-9) and are probably cau sed by catalytic reactions involving bromine monoxide (BrO) and perhaps als o chlorine monooxide (ClO)(1,10-12). The origin of the BrO, the principle s pecies driving the ozone destruction, is thought to be the autocatalytic re lease of bromine from sea salt accumulated on the Arctic snow pack(1,6-9), followed by photolytic and heterogeneous reactions which produce and recycl e the oxide(10,11,14,15). Satellite observations have shown the horizontal and temporal extent of large BrO enhancements in the Arctic troposphere(16, 17), but the vertical distribution of the BrO has remained uncertain. Here we report BrO observations obtained from a high-altitude aircraft that sugg est the presence of significant amounts of BrO not only in the PBL but also in the free troposphere above it. We believe that the BrO is transported f rom the PBL into the free troposphere through convection over large Arctic ice leads (openings in the pack ice). The convective transport also lifts i ce crystals and water droplets well above the PBL18,19, thus providing surf aces for heterogeneous reactions that can recycle BrO from less-reactive fo rms and thereby maintain its ability to affect the chemistry of the free tr oposphere.