A sea change in the Arctic atmosphere
Thinning sea ice in spring affects ozone chemistry, with implications for mercury contamination
Arctic warming has thinned the blanket of sea ice that stretches across the Arctic Ocean in springtime and a study led by a scientist from the Cooperative Institute for Research in Environmental Sciences (CIRES) shows that this change is altering the chemistry of the atmosphere at ground level in the region. Those atmospheric changes may, in turn, be increasing the amount of toxic mercury contaminating the region.
“We have observed a substantial increase in springtime surface ozone-depletion events in the Arctic during the last 40 years,” said Samuel Oltmans, lead author of a new study published in the Journal of Geophysical Research. “We haven’t been measuring mercury long enough to know yet if mercury deposition, itself, is getting worse, but that is likely.”
The sea ice and mercury changes are linked chemically through bromine, which is released from seawater. The sea ice surface controls the way bromine escapes the ocean and reaches the atmosphere. When they do reach the air, bromine molecules can do two things: chemically scrub out ozone, and transform mercury from a fairly unreactive form to a reactive form. Reactive mercury can fall out of the atmosphere into snow, ice and the ocean.
“It is this converted form of mercury that can be incorporated into Arctic food chains in the ocean and on land,” said Oltmans, an atmospheric scientist with CIRES and NOAA’s Earth System Research Laboratory. At high enough levels, mercury can harm organisms’ nerves, brains, reproductive systems and more, depending on the form of the metal and type of exposure.
Oltmans detailed the relationship between sea ice, ozone and mercury this week during the Global Monitoring Annual Conference at NOAA’s Earth System Research Laboratory in Boulder, Colo.
Scientists have long known that the chemical bromine can escape seawater and get into the atmosphere, where it can trigger reactions that destroy ground-level ozone, which is both a naturally coccurring component of the atmosphere and a pollutant. High levels of ozone can damage people’s lungs and plants, so in general, less ozone is good for air quality.
But in the Arctic, there’s a catch: when surface ozone-scrubbing events occur, changes in the atmosphere also transform airborne mercury into its more reactive form. It is well-known that when atmospheric mercury is transformed into reactive form, much of it disappears from the atmosphere and accumulates in the snow and ice below.
Recently, other scientists have shown that thin ”annual” sea ice (ice that formed during the previous winter and is riddled with cracks) is better at promoting bromine-related ozone destruction than thick, sturdy multi-year ice. Springtime sea ice in the Arctic is much thinner than it was in the past, with far less multi-year ice.
In the new study, Oltmans and his colleagues pull the story together, showing that ground-level ozone-depletion events are significantly more common during the month of March in recent years than they were just a few decades years ago.
“In the first half of the record, ozone depletion events were rare, occurring less than 15 percent of the time in March,” Oltmans said. But since 1993, ozone depletion events have occurred more than 25 percent of the time.
He and his colleagues showed that the increase in ozone-scrubbing events is not related to changes in wind patterns in the Arctic; rather, it appears that the thinner ice is promoting the changes in chemistry.
And that, Oltmans suspects, is bad news for Arctic ecosystems already well-known to suffer from mercury contamination. If warming continues, leading to more ozone-depletion events, that may well also mean that mercury is slipping more easily into Arctic food webs.
“In addition to measuring ozone, we will also want to routinely track mercury levels,” Oltmans said, “since it is the mercury compounds that have the most direct environmental implications for the Arctic.”