Halogens such as bromine and chlorine in the Arctic's lower atmosphere can deplete ozone and oxidize mercury and dimethyl sulphide. But a lack of data means that little is known about the exact reactions that take place. Now, a team from Germany has measured concentrations of the gases above the Arctic sea ice for the first time.
“The bromine oxide (BrO) concentrations that drive the ozone and mercury depletion by a so called ‘bromine explosion’ during spring in polar regions are higher over the ocean than so far observed at the coast,” Denis Pöhler of the University of Heidelberg told environmentalresearchweb. “That implies that the driving mechanism is likely much stronger than so far assumed.”
Operating as part of the Ocean Atmosphere Sea Ice Snowpack (OASIS) investigation, Pöhler and colleagues used the long-path differential optical absorption spectroscopy (LP-DOAS) remote sensing technique to measure levels of halogen oxides above sea ice in the Amundsen Gulf, east of Beaufort Sea and south of Banks Island, in March and April 2008. The team used Canada’s CCGS Amundsen icebreaker, which was involved in the Circumpolar Flaw Lead project for International Polar Year, to access the sea ice.
“The consequences of the presence of reactive halogens in the troposphere range from the destruction of ozone via changes in the balance of important chemical families to the increased deposition of toxic compounds and a potential impact on global climate,” said Pöhler. “However, observations of reactive halogen species in the troposphere are still very sparse and more measurements are necessary to better quantify the sources and global distribution of these species.”
The LP-DOAS technique employs a light-emitting/receiving telescope and a retroreflector set up several kilometres apart. As a result, the kit is difficult to deploy and measurements are scarce; until this study they had only been taken inland or at the coast. (Although it’s also possible to use passive techniques such as multi-axis (MAX)-DOAS or satellite measurements to detect halogen oxides, these rely on scattered sunlight and are only feasible in daylight.)
Pöhler and colleagues positioned the telescope for the LP-DOAS on board the Amundsen icebreaker and put the retroreflector out on sea ice between 1.1 and 3.7 km away. Typically, measurements were taken at a distance of around 25 km from the nearest coast.
The team found that levels of bromine oxide had an average mixing ratio in the polar boundary layer of 41 pmol/mol, the highest level observed to date. Scientists believe that acidified sea-salt surfaces, such as brine on fresh sea ice, sea salt on snow, aerosols, or possibly frost flowers, are able to release bromine.
Brief contact of an air mass with sea ice appeared sufficient to release high levels of bromine and start the destruction of ozone. “Thus the halogen release can occur on a very small scale, without any special geographic requirements,” said Pöhler. “This implies that the involved processes can likely occur everywhere in polar areas after short contact of the air masses with the sea ice. We conclude that the area where such ‘bromine explosions’ and ozone and mercury depletions arise are much larger than so far expected.”
Below about –15°C, maximum BrO levels increased almost linearly as temperatures decreased. “This implies that a warming can cause a reduction of the halogen release and thus a reduction of ozone and mercury depletion,” said Pöhler. “For a global estimation more measurements are required.”
In another first, the team measured full daily cycles of halogen and ozone concentrations directly over sea ice. They hope to employ these daily cycles in models to help understand the chemical processes involved.
The type of sea ice – for example first-year or multiyear – did not have a clear influence on halogen concentration. Pöhler says the halogen release seems to occur generally, with higher emissions at lower temperatures and high solar radiation.
Other halogens can speed up the rate of bromine atom recycling. The researchers believe they are the first to detect OClO in the Arctic, indicating the presence of chlorine as well as bromine. So it seems that chlorine is likely to be involved in the depletion processes.
Although iodine oxide (IO) has been seen in Antarctica, the team did not find any measurable traces of the gas in the Arctic; their kit had a detection limit of 0.3 pmol/mol.
The researchers also carried out fieldwork for the OASIS project in Barrow in spring 2009, and plan to take further measurements. “The data will be compared with satellite observations and passive ground-based instruments,” said Pöhler. “We plan to use these data to improve chemical models.”
The researchers reported their work in PNAS.