Clouds mark the location of cracks in the
ice-covered Arctic ocean. Atmospheric mixing over the cracks can add mercury to
the Arctic food chain. Video copyright: Chris Linder Photography / University of
Washington
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Cloud plumes from cracks of open water in the
Arctic sea ice cover.
Image Credit: University of Hamburg, Germany
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Vigorous mixing in the air above large cracks in Arctic sea ice that expose seawater to cold polar air pumps atmospheric mercury down to the surface, finds a NASA field campaign. This process can lead to more of the toxic pollutant entering the food chain, where it can negatively affect the health of fish and animals who eat them, including humans.
Scientists measured increased concentrations of mercury near ground level
after sea ice off the coast of Barrow, Alaska, cracked, creating open seawater
channels called leads. The researchers were in the Arctic for the NASA-led
Bromine, Ozone, and Mercury Experiment (BROMEX) in 2012.
“None of us had suspected that we would find this kind of process associated
with leads,” said Son Nghiem, a scientist at NASA’s Jet Propulsion Laboratory,
Pasadena, Calif. Nghiem is the BROMEX principal investigator and a coauthor of a
paper reporting the discovery published in Nature on Jan. 15.
The mercury-pumping reaction takes place because open water in a lead is much
warmer than the air above it, according to study lead author Chris Moore of the
Desert Research Institute, Reno, Nev. Because of that temperature difference,
the air above the lead churns like the air above a boiling pot. “The mixing is
so strong, it actually pulls down mercury from a higher layer of the atmosphere
to near the surface,” Moore said. The mixing, marked by dense clouds spewing out
of the leads, extends up into the atmosphere about a quarter-mile (400 meters).
Moore estimates this may be the height where the mercury pumping occurs.
Almost all of the mercury in the Arctic atmosphere is transported there in
gaseous form from sources in areas farther south. Scientists have long known
that mercury in the air near ground level undergoes complex chemical reactions
that deposit the element on the surface. Once the mercury is completely removed
from the air, these reactions stop. However, this newly discovered mixing
triggered by leads in the sea ice forces down additional mercury to restart and
sustain the reactions.
Leads have become more widespread across the Arctic Ocean as climate change
has reduced Arctic sea ice cover. “Over the past decade, we’ve been seeing more
new sea ice rather than perennial ice that has survived for several years. New
ice is thinner and saltier and cracks more easily. More new ice means more leads
as well,” said Nghiem.
To understand the effects of the leads, the team took ground-based
measurements of mercury and other chemical species over the frozen Chukchi Sea
and over snow-covered land. They used images from the Moderate Resolution
Imaging Spectroradiometer (MODIS) instrument on NASA’s Terra satellite to
observe sea ice and a National Oceanic and Atmospheric Administration model of
air transport to gain insight into what was upwind of their mercury
measurements.
Co-author Daniel Obrist, also from the Desert Research Institute, said, “The
‘aha’ moment came when we combined the surface measurements with the satellite
data and model. We considered a bunch of chemical processes and sources to
explain the increased levels of mercury we observed, until we finally realized
it was this pumping process.”
Nghiem points out that this new finding has come at a turning point for
action on Arctic mercury pollution. The Minamata Convention, a global treaty to
curb mercury pollution in which Arctic vulnerability is particularly noted, has
been signed by 94 nations since it was opened for signatures in Oct. 2013.
Arctic mercury pollution originates almost entirely in nations as far south as
the tropics, from sources such as wildfires, coal burning and gold mining. “Once
the Minamata Convention has been ratified and becomes international law, we
expect this work to help assess its effectiveness,” Nghiem said.
The study also includes co-authors from Environment Canada, Toronto; the U.S.
Army Cold Regions Research and Engineering Laboratory, Fort Wainwright, Alaska;
and the University of Bremen, Germany, and was jointly funded by NASA,
Environment Canada and the Desert Research Institute.
For more information on BROMEX, visit:
The California Institute of Technology in Pasadena manages JPL for
NASA.
NASA
Guillermo Gonzalo Sánchez Achutegui
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