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Chemistry Over Colorado

Chlorine Surprise: Reactions may process up to one-fifth of inland nitrogen oxide emissions

by Jyllian N. Kemsley
March 15, 2010 | A version of this story appeared in Volume 88, Issue 11

Credit: William Dubé/NOAA
Theran P. Riedel (left) and Glenn M. Wolfe of the University of Washington set up a mass spectrometer to test for atmospheric ClNO2 in Colorado.
Credit: William Dubé/NOAA
Theran P. Riedel (left) and Glenn M. Wolfe of the University of Washington set up a mass spectrometer to test for atmospheric ClNO2 in Colorado.

Atmospheric chlorine chemistry thought to happen only in the air above seawater also occurs far inland, report researchers from the University of Washington and the National Oceanic & Atmospheric Administration.

This chemistry has been well established over coastal regions, where Cl enters the atmosphere through sea spray. Chloride ions react with N2O5 formed from the emission of NOx compounds to generate ClNO2. ClNO2 then leads to Cl and NO2. Cl in turn reacts with methane and other volatile hydrocarbons, removing them from the atmosphere but also producing oxidants that can yield ozone.

Atmospheric chemists, however, hadn’t thought such chemistry was important above inland areas such as Colorado. The discovery was serendipitous, says Joel A. Thornton, a professor of atmospheric sciences at the University of Washington who led the new work with NOAA research chemist Steven S. Brown.

Thornton and colleagues were loading instruments into containers in Boulder, Colo., in preparation for an Atlantic Ocean cruise when they decided to turn on a mass spectrometer to get a background reading before they hit the water. To their surprise, they saw almost as much ClNO2 in the Colorado air as in coastal air. After returning from the cruise, they took their instruments back to Colorado to investigate further.

Combining their measurements with those obtained by national atmospheric monitoring networks, the researchers found that ClNO2 is produced inland at significant quantities—one-third to one-half of those seen in coastal areas (Nature 2010, 464, 271). They further estimate that 8–22% of NOx emissions in the U.S. cycle through ClNO2.

In hindsight, the fact that ClNO2 chemistry occurs in mid-continental areas is perhaps not surprising, says Barbara J. Finlayson-Pitts, a chemistry professor at the University of California, Irvine. Inland sources of chloride, principally from HCl emissions, include burning coal, biomass, and waste, as well as industrial processes such as semiconductor and petroleum manufacturing.

“I think perhaps we’re just lacking measurements of HCl in continental regions and recognition of how ubiquitous it is,” Finlayson-Pitts says.



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