Tropospheric bromine monoxide in Ny-Ålesund: source analysis and impacts on atmospheric chemistry

Abstract. Arctic tropospheric bromine monoxide (BrO) plays a critical role in atmospheric chemistry, particularly during springtime ozone depletion events. While sources such as sea ice, open ocean, aerosols, and snowpack have been proposed, their relative contributions remain uncertain. In this study, we addressed this uncertainty using long-term Multi-Axis Differential Optical Absorption Spectroscopy observations of BrO and aerosol profiles in Ny-Ålesund, Svalbard (78.92° N, 11.93° E), collected during March–May 2017–2023. Supporting datasets included BrO satellite retrievals, backward trajectories, and sea salt aerosol (SSA) simulations. We found a strong correlation between BrO and aerosol extinction (r = 0.51–0.76), suggesting a close association between BrO enhancements and airborne particles. Five-day backward trajectories (0–3 km) showed significant BrO correlation with sea ice contact time, particularly under strong winds. Observed BrO also correlated with modelled blowing-snow-sourced SSA concentrations and bromine emission fluxes from blowing snow. During bromine explosion events (BEEs), air mass contact with sea ice (52.0 %, 0–3 km) far exceeded that with open ocean (6.8 %), highlighting sea ice as the dominant bromine source. Within the boundary layer (<500 m), multi-year ice contributed more than first-year ice (56.1 % vs. 23.8 %) during BEEs, underscoring its importance. Snowpack-sourced bromine fluxes also correlated with BrO, although disentangling release processes remains challenging. These results provide evidence linking BrO to sea-ice and SSA processes, advancing understanding of Arctic bromine activation and its implications for ozone depletion.