This week a new study published in Nature and co-authored by Drs. Chris Moore and Daniel Obrist of Nevada's Desert Research Institute establishes, for the first time, a link between Arctic sea ice dynamics and the region's changing atmospheric chemistry potentially leading to increased amounts of mercury deposited to the Earth's northernmost and most fragile ecosystems.
The opening and closing of sea ice leads (large cracks in the ice that expose warmer seawater to the cold polar atmosphere) create a pumping effect, explained Moore, an assistant research professor in DRI's atmospheric science division, that in turn causes atmospheric depletion events to cease. The depletion events are coupled with the destruction of ozone and ultimately the deposition of atmospheric mercury onto snow and ice, a portion of which can enter Arctic ecosystems during snowmelt.
"The atmospheric mixing created when thinner, seasonal sea ice opens to form leads is so strong," Moore said, "that it actually pulls down mercury from a higher layer of the atmosphere to near the surface."
Scientists have long known that complex chemical reactions involving sunlight deposit mercury out of the air to the surface, but these processes normally stop once the mercury near ground level is completely removed. The newly discovered mixing induced by sea ice leads forces down additional mercury to restart and sustain these reactions.
Moore and his colleagues, including researchers from NASA's Jet Propulsion Laboratory in Pasadena, Calif., measured increased concentrations of mercury near ground level after leads opened near Barrow, Alaska, in 2012 during the NASA-led Bromine, Ozone, and Mercury Experiment (BROMEX) field project. They also used images from the Moderate Resolution Imaging Spectroradiometer 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.
Obrist, also a research professor at DRI and co-leader of the Institute's Environmental Mercury Laboratory, said, "the 'aha' moment came when we combined satellite data with the air transport model and surface measurements. We considered a variety of chemical processes and sources to explain the increased levels of mercury we observed, until we finally realized it was this pumping process."
The authors estimate the mercury pumping occurs about a quarter-mile (400 meters) above the Arctic surface, the height where visible roiling clouds spewing out of sea ice leads extend.
Moore said while the initial findings support needed actions to curb mercury pollution across the globe, future research will be needed to establish the degree to which changes in sea ice dynamics across the Arctic alter ozone chemistry and impact mercury deposition throughout the sensitive region.
The funding for this research was supported jointly by NASA, Environment Canada, and the Desert Research Institute.
- Christopher W. Moore, Daniel Obrist, Alexandra Steffen, Ralf M. Staebler, Thomas A. Douglas, Andreas Richter, Son V. Nghiem. Convective forcing of mercury and ozone in the Arctic boundary layer induced by leads in sea ice. Nature, 2014; DOI: 10.1038/nature12924
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