Origin Of Arctic Bromine Identified

As sea ice covers less and less of the Arctic Ocean—coverage in August 2012 was the lowest ever recorded—achieving a better understanding of the chemistry of the Arctic environment is becoming increasingly important. Scientists say the clock is ticking in their quest to comprehend the interactions of seawater, snow, and air in the region.

Researchers have known for some time that reactions in ice and snow affect the Arctic atmosphere. Fieldwork in Alaska led by Purdue University scientists provides new details on a critical portion of Arctic chemistry. The scientists found that bromine (Br₂), a precursor of bromine radicals (Br^•) that cleanse pollutants from the atmosphere, forms in surface snow exposed to sunlight (Nat. Geosci., DOI: 10.1038/ngeo1779).

In the Arctic, bromide (Br^–) from seawater somehow gets turned into Br₂, which winds up in the air. In the atmosphere, sunlight then converts Br₂ to Br^•, which reacts with pollutants such as sulfur dioxide and mercury and helps remove them.

Scientists previously did not understand exactly where or how Br^– forms Br₂ and Br^• because they have had trouble connecting atmospheric observations with laboratory experiments, says University of Toronto chemistry professor Jonathan Abbatt in a commentary accompanying the report.

To pin down the source of these bromine species, a group led by Purdue postdoctoral researcher Kerri A. Pratt and chemistry professor Paul B. Shepson looked for evidence of Br₂ formation in Barrow, Alaska, as part of a multi-institutional project called the Bromine, Ozone & Mercury Experiment.

Pratt and colleagues collected samples of sea ice, brine icicles formed beneath uplifted sea ice, and different layers of snow from on top of both ice and land. The researchers then used mass spectrometry to look for Br₂ in the samples. They found that Br₂ was produced in surface snow, that sunlight was required, and that ozone (O₃) increased Br₂ production.

Combining their new results with previous studies, Pratt and her colleagues suggest a mechanism for Br₂ production that starts with photolysis of other compounds in snow to produce hydroxyl radicals (^•OH). These radicals then react with seawater-derived Br^- in the snow to form Br₂. The Br₂ can either get swept into the atmosphere by wind or react further in snow in a cascade of events that incorporate more Br^– into Br₂.

Pratt’s team notes that the ice sampled in the study had formed over the previous winter. The team did not study older sea ice. Further work will be necessary to understand to what degree diminishing sea ice will affect atmospheric bromine chemistry, Pratt says.