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An OASIS of science

Tracking the Arctic’s springtime chemistry

by Bob Henson

Holly Reay

The intense cold at OASIS froze all manner of objects—even eyelashes, as discovered by Holly Reay (Royal Holloway, University of London). (Photos by Carlye Calvin.)

The spring solstice had arrived, and flowers of frost were blooming off the Arctic coast. These delicate structures (see photo below) were part of the rich mix of phenomena studied in recent weeks near Barrow, Alaska, during the 2009 field phase of the OASIS project (Ocean–Atmosphere–Sea Ice–Snowpack).

OASIS drew several dozen researchers, about a third of them from NCAR, to a remote, frigid area where temperatures dipped below –35°C (–31°F) as late as mid-March. Bundled head to toe, the unfazed scientists and technicians ventured onto two field sites over two months, making some of the most extensive measurements ever on the chemical exchanges between polar air, snow, frost, brine, and sea ice. “It’s by far the most comprehensive campaign of this sort,” says NCAR’s John Orlando.

Sea of frost flowers

A sea of frost flowers

Part of International Polar Year, OASIS tackled a number of standing questions in polar chemistry, with the emphasis on the life cycle of pollutants that drift into the Arctic. Complex reactions, triggered by the Sun’s return in late winter and early spring, pull mercury into snow and ice, often in localized pulses known as mercury depletion events that can last for as long as a few days. Concurrent with this mercury depletion, ozone is also depleted to near-zero levels. How the mercury and ozone depletions are chemically linked, and why they tend to spike in such distinct times and places, are among the questions being probed by OASIS.

One common thread appears to be bromine oxide (BrO), a reactive halogen that’s been associated with ozone depletion events in many parts of the world. The planners of OASIS picked Barrow in part because satellite imagery showed that large amounts of BrO often prevail in the area. It didn’t take long for some of the scientists to confirm this. “For two weeks now we’ve been smelling halogens in the air around our clean snow area,” reported OASIS organizer Harry Beine (University of California, Davis) on
17 March.

Sandy Steffen prepares to analyse frost flowers

Atop a patch of ice cleared the day before, Sandy Steffen (Environment Canada) prepares to analyze a freshly formed batch of frost flowers.

Investigators deployed an array of other tools—including weather stations, lidars, and balloons as well as chemical and biological sensors—as they followed the trail of bromine and many other chemicals. To get a three-dimensional picture of the local processes, OASIS employed a tethered balloon and two instrumented towers, one 6 meters (20 feet) and the other 9 meters (30 feet) tall. Researchers also made day trips onto the sea ice, several kilometers offshore from the main field site. Armed sentries guarded the scientists to prevent any possible visits from polar bears.

Among the early findings: NCAR scientists Chris Cantrell, Becky Hornbrook, Lee Mauldin, and Ed Kosciuch, with CU graduate student Josh McGrath, discovered through the first such measurements in this type of environment that peroxy radicals (HO2 and other organic peroxy radicals, RO2) appear to be suppressed at the same time as ozone and mercury. The OASIS team will be poring over data on a wide range of other species that may help control or participate in the depletion events, including measurements collected by NCAR, Georgia Institute of Technology, Purdue University, the University of Wuppertal (Germany), the University of Toronto, and other collaborators. “These data will allow for a very detailed test of our understanding of the photochemistry occurring during these events as well as in unperturbed conditions,” says Orlando.

Sunset at the Oasis site.

Spring sunlight illuminates an instrumented tower used for OASIS.

With the springtime return of sunlight essential to the depletion episodes, several teams at OASIS focused on optical measurements. Stationed in a snow pit (see photo), Holly Reay and James France (Royal Holloway, University of London) inserted optical probes at various depths. Like photographers checking a camera’s exposure, Reay and France measured snow albedo by using spectrometers to compare the reflectance of the snow against a calibrated panel. “Our preliminary results look very promising,” says Reay. The results will help calculate photochemical reaction rates in snow and ice.

Even when the cold seemed endless, there were plenty of local weather variations to pique researchers’ interest. Graduate student Holger Sihler and colleague Patrick Boylan (University of Heidelberg), who conducted trace-gas measurements from atop the sea ice, were intrigued by patches of haze they observed. In one case, according to Sihler, the haze layer appeared to be only a few meters high and less than a kilometer wide. “Out on the ice, visibility was changing within minutes, which was astonishing for me,” he says. “Not only are the snow and ice very heterogeneous at Barrow, but the meteorology can also be very patchy.”

James France samples optical properties of snow.

James France (Royal Holloway, University of London) samples optical properties of the snow near Barrow.

Since mercury gets deposited onto snow and ice, OASIS carried out extensive observations of the character of the frozen surface, paying particular attention to frost flowers. Little known outside the world of polar research, these crystalline structures haven’t been studied much till recently. Forming atop fresh sea ice, they’re highly salty, which makes them potentially important in the chemistry of depletion events. Amanda Grannas (Villanova University) and Sandy Steffen (Environment Canada) were among those giving the frost flowers close scrutiny, with Grannas blogging about her field work for a Villanova site and for the Discovery Channel (see “On the Web”).

The Arctic could soon blossom with more frost flowers as the dramatic loss of year-round sea ice leads to more first-year ice in the winter and spring. While OASIS wasn’t focused on climate change per se, the project will shed light on chemistry shaping the future of this rapidly changing region, including concentrations of pollutants that could threaten marine life.

“We’re trying to find out how these chemicals get there, how the Arctic tolerates their intrusion, and what possible impacts there will be to the ecosystem should the Arctic Ocean melt,” says Jan Bottenheim (Environment Canada), who led the Canadian contingent in OASIS in his last project before retirement.

On the Web

OASIS Barrow 2009

30 Days of Light (Villanova OASIS blog)



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