Reproducing polar springtime bromine explosion events, ozone depletion events and atmospheric mercury depletion events in an outdoor mesocosm sea-ice facility

Every year during polar sunrise, a series of photochemical events are observed episodically in the troposphere over the Arctic and Antarctic, including bromine explosion events (BEEs), ozone depletion events (ODEs), and mercury depletion events (MDEs). Extensive studies show that all these events are triggered by gas-phase reactive bromine species that are photochemically activated from sea-salt bromide via multi-phase reactions under freezing air temperatures. However, major knowledge gaps exist in both fundamental cryo-photochemical processes and local meteorological conditions that may affect the timing and magnitude of those events. Here, we present an outdoor mesocosm-scale experiment in which we studied the depletion of surface ozone and gaseous elemental mercury at the Sea-ice Environmental Research Facility (SERF) in Winnipeg, Canada, in an urban and non-polar region. Temporal changes in ozone and gaseous elemental mercury concentrations inside acrylic tubes were monitored over bromide-enriched artificial seawater during entire sea ice freeze-and-melt cycles and open water periods. Mid-day photochemical loss of both gas species was observed in the boundary layer air immediately above the sea ice surface, in a pattern that is characteristic of BEE-induced ODEs and MDEs in the Arctic. The importance of UV radiation and sea ice presence in causing such observations was demonstrated by sampling from UV-transmitting and UV-blocking acrylic tubes under different air temperatures. The ability of reproducing mesocosm-scale BEE-induced ODEs and MDEs in a non-polar region provides a new platform with opportunities to systematically study the cryo-photochemical mechanisms 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 a changing climate.