Scientists studying the atmosphere above Barrow, Alaska, have discovered unprecedented¹ levels of molecular² chlorine（氯） 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 constituents³ of the atmosphere including methane⁴ and elemental mercury, as well activate⁵ bromine（溴） chemistry, which is an even stronger oxidant of elemental mercury. Oxidized mercury is more reactive and can be deposited to the Arctic ecosystem⁶.

 

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.

 

"No one expected there to be this level of chlorine in Barrow or in polar regions," said Greg Huey, a professor in the School of Earth and Atmospheric⁷ Sciences at the Georgia Institute of Technology in Atlanta.

 

The study was published January 12 in the journal Nature Geoscience and was supported by the National Science Foundation (NSF), part of the international multidisciplinary OASIS⁸ program.

 

The researchers directly measured molecular chlorine levels in the Arctic in the spring of 2009 over a six-week period using chemical ionization mass spectrometry. At first the scientists were skeptical⁹ of their data, so they spent several years running other experiments to ensure their findings were accurate.

 

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 ozone¹⁰ 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 depleted¹¹ from the atmosphere when halogens -- chlorine and bromine -- are released into the air from melting sea ice.

 

"Molecular chlorine is so reactive that it's going to have a very strong influence on atmospheric chemistry," Huey said.

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