Atmospheric Chlorine Source Identified

Nitryl chloride detected in polluted marine areas

Elizabeth K. Wilson

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For the first time, scientists have detected in polluted marine atmospheres a molecule thought to be a potentially important source of ozone-damaging atmospheric chlorine. Nitryl chloride, or ClNO₂, has been difficult to detect in the atmosphere, though its presence has long been predicted from lab studies.

Now, sensitive instruments aboard the National Oceanic & Atmospheric Administration's research ship Ronald H. Brown have detected the molecule in surprisingly large quantities off the southern U.S. coast, according to James M. Roberts. An atmospheric chemist at NOAA, Roberts spoke on Dec. 11 at the American Geophysical Union meeting in San Francisco.

Nitryl chloride is formed at night from reactions of the air pollutant N₂O₅ with aqueous Cl⁻ from sea salt that has been redistributed onto smaller aerosols. "This reaction is happening at Cl⁻ concentrations much lower than people realized," Roberts said.

Sea salt can be aerosolized when waves break and when bubbles break on the surface. These aerosol particles, loaded with Cl⁻, then interact with species such as sulfates and nitrates found in polluted areas.

Nitryl chloride exists well into the morning, when it’s photolyzed to form NO₂ and Cl. There, in the lower portions of the atmosphere, the Cl atoms and the extra NO_x can drive photochemical ozone production, making air pollution worse.

"This may turn out to be one of the major sources of atmospheric chlorine from interactions of polluted atmospheres with sea salt," says University of California, Irvine, atmospheric chemistry professor Barbara J. Finlayson-Pitts, who has studied nitryl chloride.

Roberts' group is heading to the Arctic this spring to study the chemistry of ClNO₂ and other compounds that could be contributing to atmospheric chlorine.

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