Arctic Atmosphere Contains Surprisingly High Levels of Molecular Chlorine

The atmosphere above the northern Alaskan town of Barrow has unprecedentedly high levels of molecular chlorine, according to researchers who say the trend could continue as seasonal variation in ice increases each year. 

Molecular chlorine, derived from sea salt released by melting sea ice, reacts with sunlight to produce highly reactive chlorine atoms. These atoms are responsible for speeding up the degradation of methane and the oxidation of mercury to more toxic forms.  

"Molecular chlorine is so reactive that it's going to have a very strong influence on atmospheric chemistry," said Greg Huey, a professor in the School of Earth and Atmospheric Sciences at the Georgia Institute of Technology. 

Published in the journal Nature Geoscience, the study was carried out over a six-week period in the spring of 2009, during which the scientists detected levels as high as 400 parts per trillion.

"No one expected there to be this level of chlorine in Barrow or in polar regions," Huey said. 

According to Huey, the biggest source of the molecular chlorine is the sodium chloride in sea salt. Exactly how sea salt becomes molecular chlorine is unclear, however.

"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."

What is clear is a change in the region's sea ice: Ice that remains intact from one winter to another is decreasing, leading to a larger area of melted ice and a rise in the amount of ice that comes and goes throughout the seasons. Such seasonal variation could be behind the astonishing amount of molecular chlorine in the atmosphere, the researchers hypothesize.

"There is definite climate change happening in the Arctic," Huey said. "That's changing the nature of the ice, changing the volume of the ice, changing the surface area and changing the chemistry of the ice."