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 troposphere1. These tropospheric ozone loss events have been observed over large areas2,3 in the planetary boundary layer (PBL) throughout the Arctic4,5. They are associated with enhanced concentrations of halogen species1,6,7,8,9 and are probably caused by catalytic reactions involving bromine monoxide (BrO) and perhaps also chlorine monoxide (ClO)1,10,11,12. The origin of the BrO, the principle species driving the ozone destruction, is thought to be the autocatalytic release of bromine from sea salt accumulated on the Arctic snow pack10,11,13, followed by photolytic and heterogeneous reactions which produce and recycle the oxide10,11,14,15. Satellite observations have shown the horizontal and temporal extent of large BrO enhancements in the Arctic troposphere16,17, but the vertical distribution of the BrO has remained uncertain. Here we report BrO observations obtained from a high-altitude aircraft that suggest 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 from the PBL into the free troposphere through convection over large Arctic ice leads (openings in the pack ice). The convective transport also lifts ice crystals and water droplets well above the PBL18,19, thus providing surfaces for heterogeneous reactions that can recycle BrO from less-reactive forms and thereby maintain its ability to affect the chemistry of the free troposphere.