Atmospheric bromine deposition as a source of seasonal snowpack bromide reservoirs for Arctic bromine explosion events: evidence from linking chemistry-climate simulations with snow observations using a layered snow model

Abstract. Bromine explosion events (BEs) are an important source of reactive bromine in the Arctic boundary layer and contribute to episodic ozone depletion in spring. While bromide-rich sea ice and snow are generally regarded as the primary source, the extent to which atmospheric deposition of bromine can establish and replenish a snowpack bromide reservoir remains poorly constrained. We combine simulations with the ECHAM/MESSy Atmospheric Chemistry model (EMAC), field observations, and laboratory analyses of snow samples collected around Ny-Ålesund (Svalbard) in April 2024 to investigate the role of atmospheric bromine deposition for BEs. To connect atmospheric deposition fluxes with observed snow chemistry, we developed the Snow Model with Evolving Layers (SMELs), a simple offline snow model that tracks bromide accumulation within a dynamically evolving snowpack.

EMAC simulations indicate that bromine dry deposition from very short-lived substances (VSLS) and sea-salt aerosol (SSA) dehalogenation can establish snow bromide reservoirs sufficient to sustain typical Arctic springtime BrO abundances. Bromine species partitioning upon deposition suggests that a lack of HBr deposition may suppress heterogeneous Br₂ production during fall. SMELs reproduces the observed vertical bromide enhancement and depletion patterns and demonstrates that finite snowpit sampling can introduce substantial biases. The strong agreement between modeled and observed normalized bromide profiles suggests that bromine deposition and post-depositional processing operate on regional scales around Ny-Ålesund. Our results identify atmospheric bromine deposition as an important seasonal bromide reservoir and highlight the role of bromine speciation and snowpack processes in controlling Arctic bromine activation.