Australia: The Land Where Time Began

A biography of the Australian continent 

Arctic Surface Snowpacks - Molecular Bromine, Photochemical Production

Ozone levels in the lower troposphere decline episodically, almost to zero, after sunrise in the spring (1). Increased atmospheric levels of reactive bromine initiate ozone depletion events (2-5). The oxidative capacity of the troposphere in the Arctic is changed which leads to the removal of many transported gas pollutants, that include mercury, under these conditions (6), though it has been uncertain what the sources and mechanisms that lead to the increase of reactive bromine in the Atmosphere are, which has been a limiting factor in simulations of the Arctic atmospheric chemistry as the sea ice landscape rapidly changes (7,8). In this study the authors¹ examined the potential for molecular bromine to be produced in various samples of saline snow and ice, with and without sunlight and ozone, the experiments being carried out in an outdoor snow chamber in Alaska. When surface snow, that had been collected from above tundra and first-year sea ice, was exposed to sunlight was the only time they were able to detect molecular bromine. The authors¹ suggest that a photochemical mechanism facilitates the oxidation of bromine, and they found that this was most efficient with more acidic samples that were characterised by enhanced ratios of bromide to chloride ratios. When the snow was exposed to ozone the concentrations of molecular bromine increased significantly, which the authors¹ say is consistent with the mechanism of interstitial air amplification. Enhanced bromine oxide levels near the surface of the snow was confirmed by aircraft-based observations. The authors¹ suggest the photochemical production of molecular bromine in surface snow is a major source of reactive bromine, and in the Arctic spring the episodic depletion of ozone in the troposphere is a result of this.

Sources & Further reading

Pratt, Kerri A., Kyle D. Custard, Paul B. Shepson, Thomas A. Douglas, Denis Pohler, Stephan General, Johannes Zielcke, et al. "Photochemical Production of Molecular Bromine in Arctic Surface Snowpacks." Nature Geosci 6, no. 5 (05//print 2013): 351-56.