Environmental context. Ozone depletion events (ODEs) in the Arctic lower atmosphere drive profound changes in the chemistry of nitrogen oxides (NO_x) because of the presence of bromine oxide (BrO). These are investigated using the isotopic composition of atmospheric nitrate (NO₃^–), which is a ubiquitous species formed through the oxidation of nitrogen oxides. Since BrO is speculated to play a key role in the atmospheric chemistry of marine regions and in the free troposphere, our studies contribute to the improvement of the scientific knowledge on this new topic in atmospheric chemistry.

Abstract. The triple oxygen isotopic composition of atmospheric inorganic nitrate was measured in samples collected in the Arctic in springtime at Alert, Nunavut and Barrow, Alaska. The isotope anomaly of nitrate (Δ¹⁷O = δ¹⁷O – 0.52δ¹⁸O) was used to probe the influence of ozone (O₃), bromine oxide (BrO), and peroxy radicals (RO₂) in the oxidation of NO to NO₂, and to identify the dominant pathway that leads to the production of atmospheric nitrate. Isotopic measurements confirm that the hydrolysis of bromine nitrate (BrONO₂) is a major source of nitrate in the context of ozone depletion events (ODEs), when brominated compounds primarily originating from sea salt catalytically destroy boundary layer ozone. They also show a case when BrO is the main oxidant of NO into NO₂.