Tiny tempests above cracks in Arctic sea ice help pull down toxic mercury and ozone from the sky — an unexpected new source of mercury pollution in the polar environment, according to research published today (Jan. 15) in the journal Nature.
Low concentrations of mercury vapor, from sources such as coal-fired power plants and gold mining, pollute the atmosphere everywhere on Earth. The gas can travel thousands of miles from its source, even reaching the North and South poles.
Mercury leaves the atmosphere above the Arctic every spring. About 20 years ago, scientists discovered how it escapes: a strange chemistry triggered by the sun that takes place mainly along coastal areas. When the sun peeks above the horizon after a long, dark winter, the solar rays jump-start chemical reactions that quickly remove mercury and ozone from the lowest layers of the atmosphere. (The ozone destroyed during this process is a pollutant, not the protective ozone in Earth's stratosphere, a layer of the atmosphere above the one humans live in, called the troposphere.)
One player in this chemical chain, molecular chlorine, recently was measured for the first time in the Arctic at surprisingly high levels of up to 400 parts per million, according to a separate study published Sunday (Jan. 12) in the journal Nature Geoscience. The high chlorine levels were tracked above Barrow, Alaska, in spring 2009. (Parts per million is a unit of volume that denotes, in this case, that for every million molecules of air in the region, 400 of them are chlorine.)
The mercury, a neurotoxin to humans and wildlife, ends up on snow and ice, and not all of it goes back into the atmosphere after the summer melt. "This adds hundreds of tons of mercury to the Arctic every year," said Daniel Obrist, an atmospheric scientist at the Desert Research Institute in Nevada and a co-author of today's Nature study.
The chemical reactions stop once they "eat" all the mercury and ozone in the air just above Earth's surface. But recently, a campaign to better understand this unusual Arctic chemistry discovered that roiling air currents above cracks in Arctic sea ice — similar to the swirling turbulence above a pot of boiling water — can suck down more mercury from higher in the sky, about a quarter-mile (400 meters) up, restarting the chemistry.
"This came as a surprise," Obrist told LiveScience. "We would not have thought that this physical mixing would lead to a resupply of mercury."
While studying mercury chemistry during the Bromine, Ozone and Mercury Experiment (BROMEX) field project near Barrow in 2009 and 2012, researchers discovered higher-than-expected concentrations of mercury above these sea-ice "leads," or cracks.
"When the leads open, we see a very quick increase in mercury concentrations," said Chris Moore, a co-author of the Nature study and an atmospheric scientist at the Desert Research Institute. "They jump from essentially zero to global background levels within a couple of hours." (The global background level is the atmospheric concentration of mercury; in the Arctic, it is 1.3 to 1.5 nanograms per cubic meter.)
Here's what happens: When Arctic sea ice cracks apart, relatively warm ocean water meets frigid polar air, causing atmospheric turbulence, Moore said. This mixes up the layered Arctic atmosphere, which would otherwise prevent the sunlight-triggered chemistry from reaching mercury higher in the sky.
The Arctic sea ice undergoes its biggest cracking and fracturing in the spring, at the same time as the sun reappears after winter. This raises the question of what will happen as the extent of Arctic sea ice changes in response to global warming.
"We really need to understand how these environmental processes may change in the future," Moore said.
"This is a very dynamic process, and it will change from year to year, depending on how much seasonal sea ice we have," he added. (Seasonal sea ice is year-old ice, unlike perennial ice that lasts longer than one freeze-thaw season.) "This transition to an Arctic that has more seasonal sea ice means there is potential for this mechanism to happen over a larger and larger area," Moore said.