Reproducing springtime Arctic tropospheric ozone depletion events in an outdoor mesocosm sea-ice facility

Abstract. The episodic build-up of gas-phase reactive bromine species over sea ice and snowpack in the springtime Arctic plays an important role in the boundary layer, causing annual concurrent depletion of ozone and gaseous elemental mercury during polar sunrise. Extensive studies have shown that these phenomena, known as bromine explosion events (BEEs), ozone depletion events (ODEs) and mercury depletion events (MDEs), respectively, are all triggered by gas-phase reactive bromine species that are photochemically activated from bromide via multi-phase reactions under freezing air temperatures. However, major knowledge gaps exist in both fundamental cryo-photochemical processes causing these events and meteorological conditions that may affect their timing and magnitude. Here, we report an outdoor mesocosm-scale study in which we successfully reproduced ODEs at the Sea-ice Environmental Research Facility (SERF) in Winnipeg, Canada. By monitoring ozone concentrations inside large, acrylic tubes over bromide-enriched artificial seawater during entire sea ice freeze-and-melt cycles, we observed mid-day photochemical ozone loss in winter in the boundary layer air immediately above the sea ice surface in a pattern that is characteristic of BEE-induced ODEs in the Arctic. The importance of UV radiation and the presence of a condensed phase (experimental sea ice or snow) in causing such surface ozone loss was demonstrated by comparing ozone concentrations between UV-transmitting and UV-blocking acrylic tubes under different air temperatures. The ability of reproducing BEE-induced ODEs at a mesocosm scale in a non-polar region provides a new approach to systematically studying the cryo-photochemical and meteorological processes leading to BEEs, ODEs, and MDEs in the Arctic, their role in biogeochemical cycles across the ocean-sea ice-atmosphere interfaces, and their sensitivities to climate change.