Why is there chlorine above Alaska?

While chlorine gas is extremely toxic, molecular chlorine is involved in reactions that purify the atmosphere, and could thus help prevent climate change.

An international research group discusses their unexpected discovery of chlorine in the air above the coastal town of Barrow, at the northernmost tip of Alaska in an article published on the website of Nature Geoscience.

Chlorine gas is toxic – so much so that it has been used in chemical warfare. The nefarious reputation of the gas is magnified by its ability to transform mercury in the atmosphere into a more toxic form.

On the other hand, molecular chlorine and radicals containing chlorine are in the same category as ozone and hydroxyl or nitrate radicals in that they can initiate an oxidising reaction in the atmosphere, a key way in which pollution is cleared.

The increase in oxygen can then change gases which are insoluble in water, like methane, into carbon dioxide and other more easily soluble substances. In addition to methane, the oxidation removes remnants of fossil fuels, solvents and other volatile organic compounds (VOC) from the atmosphere, which has a significant impact on air quality, acid rain and climate change.

The methane vacuum of the polar regions

Typically considered to be of little significance in terms of oxidation, chlorine becomes a key player in polar regions which have fewer hydroxyl radicals. The molecular chlorine involved in oxidation is generated in the atmosphere when light disintegrates chlorine gas or similar chlorine compounds.

In Barrow, the chlorine radicals often appear to function as primary oxidisers and to significantly increase VOC reactions. The spring 2009 study reported that 60% of oxidised methane was due to the influence of molecular chlorine, and only 40% to the influence of hydroxyl radicals.

“We must conduct more measurements on the concentration of different chlorine compounds in the areas above the polar regions to determine how widespread this phenomenon is,” states researcher Roy Mauldin from the University of Helsinki.

Brought with the wind

The chlorine concentrations above Barrow increased when the wind blew in from the Chuchki Sea in the Arctic Ocean. According to the researchers, their analysis of the incoming air masses indicates that the source of the chlorine is snow or the surface of the ice.

Particularly high concentrations of chlorine were measured above Barrow during the day. It seems that sunlight is needed to generate chlorine gas – not just disintegrate it.

The measurements made by the researchers in Alaska also reveal the connection between chlorine concentration and the amount of ozone. Apparently ozone is also needed for the creation of chlorine gas. The reactions of the chlorine radicals seem to play a significant role in the lack of ozone documented in Barrow in the spring.