There is increasing evidence that at polar sunrise sunlight-induced changes in the composition of the lower Arctic atmosphere (0–2 km) are taking place that are important regarding the tropospheric cycles of ozone, bromine, sulphur oxides1, nitrogen oxides2 and possibly iodine3. Here we focus on recent ground-level observations from the Canadian baseline station at Alert (82.5° N, 62.3° W) and from aircraft that show that ozone destruction is occurring under the Arctic surface radiation inversion during March and April as the Sun rises. The destruction might be linked to catalytic reactions of BrOx radicals and the photochemistry of bromoform, which appears to have a biological origin in the Arctic Ocean. This may clarify previously unexplained regular springtime occurrences of ozone depletion at ground level in a 10-year data record at Barrow, Alaska4, as well as peaks in aerosol bromine observed throughout the Arctic in March and April3. Current information does not allow us to offer more than a speculative explanation for the chemical mechanisms leading to these phenomena.