Saturday, March 3, 2012

Climate Change: NASA finds sea ice decline driving rise in Arctic air pollutants

 Drastic reductions in Arctic sea ice in the last decade
may be intensifying the chemical release of bromine into the
atmosphere, resulting in ground-level ozone depletion and the deposit
of toxic mercury in the Arctic, according to a new NASA-led study.

The connection between changes in the Arctic Ocean's ice cover and
bromine chemical processes is determined by the interaction between
the salt in sea ice, frigid temperatures and sunlight. When these
mix, the salty ice releases bromine into the air and starts a cascade
of chemical reactions called a "bromine explosion." These reactions
rapidly create more molecules of bromine monoxide in the atmosphere.
Bromine then reacts with a gaseous form of mercury, turning it into a
pollutant that falls to Earth's surface.

Bromine also can remove ozone from the lowest layer of the atmosphere,
the troposphere. Despite ozone's beneficial role blocking harmful
radiation in the stratosphere, ozone is a pollutant in the
ground-level troposphere.

A team from the United States, Canada, Germany, and the United
Kingdom, led by Son Nghiem of NASA's Jet Propulsion Laboratory in
Pasadena, Calif., produced the study, which has been accepted for
publication in the Journal of Geophysical Research- Atmospheres. The
team combined data from six NASA, European Space Agency and Canadian
Space Agency satellites, field observations and a model of how air
moves in the atmosphere to link Arctic sea ice changes to bromine
explosions over the Beaufort Sea, extending to the Amundsen Gulf in
the Canadian Arctic.

"Shrinking summer sea ice has drawn much attention to exploiting
Arctic resources and improving maritime trading routes," Nghiem said.
"But the change in sea ice composition also has impacts on the
environment. Changing conditions in the Arctic might increase bromine
explosions in the future."

The study was undertaken to better understand the fundamental nature
of bromine explosions, which first were observed in the Canadian
Arctic more than two decades ago. The team of scientists wanted to
find if the explosions occur in the troposphere or higher in the

Nghiem's team used the topography of mountain ranges in Alaska and
Canada as a "ruler" to measure the altitude at which the explosions
took place. In the spring of 2008, satellites detected increased
concentrations of bromine, which were associated with a decrease of
gaseous mercury and ozone. After the researchers verified the
satellite observations with field measurements, they used an
atmospheric model to study how the wind transported the bromine
plumes across the Arctic.

The model, together with satellite observations, showed the Alaskan
Brooks Range and the Canadian Richardson and Mackenzie mountains
stopped bromine from moving into Alaska's interior. Since most of
these mountains are lower than 6,560 feet (2,000 meters), the
researchers determined the bromine explosion was confined to the
lower troposphere.

"If the bromine explosion had been in the stratosphere, 5 miles [8
kilometers] or higher above the ground, the mountains would not have
been able to stop it and the bromine would have been transported
inland," Nghiem said.

After the researchers found that bromine explosions occur in the
lowest level of the atmosphere, they could relate their origin to
sources on the surface. Their model, tracing air rising from the
salty ice, tied the bromine releases to recent changes in Arctic sea
ice that have led to a much saltier sea ice surface.

In March 2008, the extent of year-round perennial sea ice eclipsed the
50-year record low set in March 2007, shrinking by 386,100 square
miles (one million square kilometers) -- an area the size of Texas
and Arizona combined. Seasonal ice, which forms over the winter when
seawater freezes, now occupies the space of the lost perennial ice.
This younger ice is much saltier than its older counterpart because
it has not had time to undergo processes that drain its sea salts. It
also contains more frost flowers -- clumps of ice crystals up to four
times saltier than ocean waters -- providing more salt sources to
fuel bromine releases.

Nghiem said if sea ice continues to be dominated by younger saltier
ice, and Arctic extreme cold spells occur more often, bromine
explosions are likely to increase in the future.

Nghiem is leading an Arctic field campaign this month that will
provide new insights into bromine explosions and their impacts.
NASA's Bromine, Ozone, and Mercury Experiment (BROMEX) involves
international contributions by more than 20 organizations.