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Bromine bomb drops toxic mercury fallout

Another reason to love old Arctic ice

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A “bromine explosion” in the Arctic back in 2008 has yielded a disturbing scientific analysis: the replacement of perennial sea-ice with younger seasonal ice could lead to mercury pollution in the Arctic.

In a new NASA-led study, American, Canadian, German and UK researchers believe they have identified the mechanism by which the melting ice cap alters the atmospheric concentration of bromine – and what happens to the bromine afterwards.

The study suggests that the reduction in perennial sea ice (reported by The Register on Wednesday) is intensifying the release of bromine into the atmosphere, with the twin results of depletion of ground-level ozone, and mercury being deposited into the Arctic.

The bromine processes – described as bromine explosions by team leader Son Nghiem from the Jet Propulsion Laboratory – take place because of the interaction between sea ice salt and sunlight in the Arctic’s low temperatures.

In the “bromine explosion”, the bromine released by the salty ice creates molecules of bromine monoxide in the atmosphere, which then reacts with gaseous mercury in the atmosphere. The resulting pollutant falls to Earth’s surface.

On the upside, bromine reacts with tropospheric ozone – which at ground level is a pollutant, in spite of its beneficial stratospheric role.

The study was launched with the aim of examining the nature of bromine explosions, which were first observed more than 20 years ago in Canada’s Arctic regions. The scientists “wanted to find if the explosions occur in the troposphere or higher in the stratosphere,” the NASA announcement says.

The satellite measurements used in this study came from six NASA, European Space Agency and Canadian Space Agency satellites, along with field observations, and a model of how air moves in the atmosphere.

The atmospheric model and the satellite observations agreed that Alaska’s Brooks Range, along with the Richardson and Mackenzie mountains in Canada, contain the atmospheric bromine, keeping it away from Alaska’s interior. This, the researchers say, means the bromine explosions take place below 2,000 meters.

The research was launched after observations in the spring of 2008 detected increased bromine concentrations in the atmosphere, observations which were verified by field observations.

2008, by no coincidence, was also the year in which perennial sea-ice reached its lowest recorded level (also discussed in this story). Its loss is partly replaced by seasonal ice, but the younger ice yields more bromine, for two reasons: it hasn’t undergone the leaching process that removes salt from sea ice; and it contains more of the high-salt crystals called “frost flowers”, which provide more salt to fuel bromine releases.

If sea-ice continues to be dominated by younger, saltier ice, Nghiem said, extreme Arctic cold spells will lead to more bromine explosions – and more mercury pollution. ®

Addendum: A commenter has asked a sensible question about the bromine: where does it come from? Forgive me for citing Wikipedia, but it's handy: it's present in typical seawater, "with a concentration of around 65 mg/L, which is around 0.2 percent of all dissolved salts." ®

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