A study by scientists is offering new insight into what happens to mercury deposited onto Arctic snow from the atmosphere using a 'fingerprinting' method.
In the Arctic, mercury remains in its benign gaseous form through the dark winter, because there's no sunlight to drive oxidation and little bromine to catalyze the process.
But in polar springtime, that all changes.
As sea ice breaks up, water vapor rises in great clouds through the openings in the ice, bringing with it bromine from the sea water.
The bromine enters the atmosphere, where it conspires with sunlight to convert mercury gas into the reactive form.
The activated mercury sticks to snowflakes and ice crystals in the air and travels with them onto the surface of the snow.
This leads to what's known as a mercury depletion event.
The normally steady levels of mercury in the atmosphere quickly drop to near zero, as concentrations of mercury on the surface of the snow rise to extremely high levels.
"The more we learned, the more we realized that the sunlight shining on the snow typically will cause much of the oxidized mercury to become reduced and return to the atmosphere as a gas. And it turns out that its re-release to the atmosphere has a striking "fingerprint" that we can use to study the progress of this reaction through time," said Joel Blum, the John D. MacArthur Professor of Geological Sciences.
The fingerprint is the result of a natural phenomenon called isotopic fractionation, in which different isotopes of mercury react to form new compounds at slightly different rates.
In the work, the researchers confirmed, through sample collection and experiments, that mass-independent fractionation (MIF) occurs during the sunlight-driven reactions in snow, resulting in a characteristic MIF fingerprint that is absent in atmospheric mercury.
In MIF, the behavior of the isotopes depends not on their absolute masses but on whether their masses are odd or even.
"This finding allowed us to use the MIF fingerprint to estimate how much mercury was lost from the snowpack and how much remained behind, with the potential to enter Arctic ecosystems," said U-M graduate student Laura Sherman, the research paper's first author.
"Our experiments showed that a significant portion of mercury deposited to snow was re-emitted. Any mercury that is not re-emitted is likely to retain the unique fingerprint, so we hope future researchers will be able to use our discovery to track mercury through Arctic ecosystems," she added. (ANI)