Molecular Chlorine Found at Elevated Levels in Arctic Atmosphere

Molecular chlorine found at elevated levels in Arctic atmosphere

Scientists have recently performed a study investigating the presence of molecular chlorine above Barrow, Alaska. Situated 300 miles north of the Arctic Circle, Barrow is home to snow-covered ice pack that spans out towards the coast’s horizon.

Photolysis Generates Highly Reactive Chloride Radicals

For the first time, the levels of molecular chlorine have been monitored in the Arctic. The research team was surprised to find unprecedentedly elevated levels of the molecule within the atmosphere. Greg Huey, a professor from the School of Earth and Atmospheric Science at the Georgia Institute of Technology, in Atlanta, reiterated this point:

“No one expected there to be this level of chlorine in Barrow or in polar regions.”

Molecular chlorine, derived from the sea salt of melting sea ice, can react with sunlight to generate chlorine radicals. In turn, these chlorine radicals may then go on to oxidize a number of chemical components in the atmosphere, including elemental mercury and methane. In addition, radicals also trigger activation of bromine chemistry, leading to an accelerated rate of oxidization of elemental mercury.

When molecular chlorine undergoes photolysis, the resulting radicals can expedite oxidation of mercury to toxic forms, as well as degradation of the tropospheric zone – the lowest part of the Earth’s atmosphere, comprising around 80 percent of the atmosphere’s total mass. The oxidized mercury is more reactive and, therefore, enters the Arctic ecosystem.

First author of the study Jin Liao checks the instrumentation in Barrow
First author of the latest study, Jin Liao, checks the research team’s scientific instruments in Barrow, Alaska.

The researchers employed chemical ionization mass spectrometry to determine the molecular chlorine levels in the Arctic marine boundary layer in Barrow, during the spring of 2009. After performing analysis over a timeframe of six weeks, the scientists were astonished to discover levels that climbed to 400 parts per trillion. The researchers indicate that this figure is remarkably high, particularly when considering chlorine radicals have a limited lifespan within the atmosphere, constituting highly reactive oxidants.

The molecular chlorine levels rose to their maximum level in the early morning and during late afternoon. However, during the night-time, the chlorine levels sharply dropped to barely perceptible levels.

Halogen Release Caused by Climate Change?

Prior research has reported on the high concentrations of oxidized mercury in a number of polar regions, including Barrow. Human activity is conjectured to be responsible for mercury becoming so widespread and persistent within the environment. Specifically, coal burning and metal mining have enhanced mobilization of mercury, increasing its atmospheric ubiquity, as well as its presence in various waters and soils.

In Barrow, snow-covered ice pack extends in every directly except inland
In Barrow, the ice pack extends in almost every direction, except inland. Image credit: Greg Huey.

However, the levels of elemental mercury and ozone are typically depleted from the atmosphere during the springtime in Barrow. This phenomenon is believed to have been triggered by the release of halogens, such as bromine and chlorine, from the melting sea ice.

Although the exact mechanism remains elusive, Huey maintains that most of the detected molecular chlorine originates from sodium chloride in sea salt. Since the sea ice between each successive winter appears to be diminishing, the group posit this marked elevation in molecular chlorine to be attributable to climate change. Huey argues that climate changes is “… changing the nature of sea ice…” along with the surface area, volume and chemistry of ice throughout the region.

“We don’t really know the mechanism. It’s a mystery to us right now… 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.”

The study was published in the Jan. 12 issue of the journal Nature Geoscience, and was supported by the National Science Foundation (NSF).

By James Fenner


Nature Geoscience
Press Release
Science 2.0

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