After this 2012 blog post on Arctic pollution, it's time to return to the subject, if only because Arctic sea ice is proving to be a significant factor in changes in atmospheric chemistry, leading to increases in mercury concentrations in the Arctic. This happens through sea ice melting during summer, and an increase in leads and cracks during winter.
In the past couple of days two papers were published that show the relationships. The first, Convective forcing of mercury and ozone in the Arctic boundary layer induced by leads in sea ice by Moore et al., was published in Nature and deals with bromine. This Science Daily article explains it well:
Cracked Sea Ice Stirs Up Arctic Mercury Concern
Jan. 15, 2014 — 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.
The second study, High levels of molecular chlorine in the Arctic atmosphere by Liao et al., was published in Nature Geoscience and deals with chlorine.
For a background story we turn to the Georgia Tech News Center:
High Levels of Molecular Chlorine Found
in Arctic Atmosphere
Scientists studying the atmosphere above Barrow, Alaska, have discovered unprecedented levels of molecular chlorine in the air, a new study reports.
Molecular chlorine, from sea salt released by melting sea ice, reacts with sunlight to produce chlorine atoms. These chlorine atoms are highly reactive and can oxidize many constituents of the atmosphere including methane and elemental mercury, as well activate bromine chemistry, which is an even stronger oxidant of elemental mercury. Oxidized mercury is more reactive and can be deposited to the Arctic ecosystem.
The study is the first time that molecular chlorine has been measured in the Arctic, and the first time that scientists have documented such high levels of molecular chlorine in the atmosphere.
The level of molecular chlorine above Barrow was measured as high as 400 parts per trillion, which is a high concentration considering that chlorine atoms are short –lived in the atmosphere because they are strong oxidants and are highly reactive with other atmospheric chemicals.
Molecular chlorine concentrations peaked in the early morning and late afternoon, and fell to near-zero levels at night. Average daytime molecular chlorine levels were correlated with ozone concentrations, suggesting that sunlight and ozone may be required for molecular chlorine formation.
Previous Arctic studies have documented high levels of oxidized mercury in Barrow and other polar regions. The major source of elemental mercury in the Arctic regions is coal-burning plants around the world. In the spring in Barrow, ozone and elemental mercury are often depleted from the atmosphere when halogens — chlorine and bromine — are released into the air from melting sea ice.
In Barrow, snow-covered ice pack extends in every directly except inland. The ultimate source of the molecular chlorine is the sodium chloride in sea salt, Huey said, most likely from the snow-covered ice pack. How the sea salt is transformed into molecular chlorine is unknown.
“We don’t really know the mechanism. It’s a mystery to us right now,” Huey said. “But the sea ice is changing dramatically, so we’re in a time where we have absolutely no predictive power over what’s going to happen to this chemistry. We’re really in the dark about the chlorine.”
Scientists do know that sea ice is rapidly changing, Huey said. The sea ice that lasts from one winter to the next winter is decreasing. This has created a larger area of melted ice, and more ice that comes and goes with the seasons. This seasonal variation in ice could release more molecular chlorine into the atmosphere.
Read the rest here.