Published: March 10, 2010

A new scientific study packs a double surprise about the chemistry happening in the air around us. Chlorine, a chemical usually kicked into the atmosphere by sea spray, is more abundant than expected in air far from any coastline, and looks to be interacting with manmade pollution at night in ways that might affect air quality and climate.

The high levels of inland, chlorine-containing gases confirm that human activities such as coal burning, biomass burning, and roadway de-icing are changing the chemistry of the air in the continental United States, according to the new study.

"Our findings show that the chemistry involving these chlorine-containing gases is not confined to the coastal and marine atmosphere," said study coauthor Nicholas Wagner, a researcher at the University of Colorado at Boulder-based Cooperative Institute for Research in Environmental Sciences, or CIRES. The paper is being published in Nature March 11.

Researchers at CIRES, the National Oceanic and Atmospheric Administration and the University of Washington looked at nitryl chloride, a gas that forms at night when airborne, chloride-containing particles interact with nitrogen oxide pollutants from combustion. The nitryl chloride breaks apart quickly as the sun rises to release chlorine atoms, which are highly reactive and can affect greenhouse gases and contribute to smog formation.

"Pollution strikes twice in this chemistry," said Steve Brown, a scientist at NOAA's Earth System Research Laboratory, or ESRL, in Boulder, Colo., and a co-author on the paper. "First it adds chloride-containing particles to the atmosphere, and then it transforms them at night to make nitryl chloride."

The ultimate release of the chlorine atoms from nitryl chloride after sunrise could be the missing piece in the longtime puzzle of the abundance and sources of highly reactive chlorine-containing gases in the atmosphere, according to the study. "Nighttime formation of nitryl chloride is a gateway to forming more highly reactive chlorine atoms," said Brown. "It changes the atmosphere's starting point for the next day."

When the research team was testing equipment in Boulder for a 2008 study of chlorine-related chemistry in the atmosphere above the North Atlantic, they found the nitryl chloride levels were comparable to those they expected to see at the ocean site, even though Boulder is 900 miles from the nearest coast. They followed up with an extensive study in Boulder in 2009 that confirmed their earlier observations.

"We expect this to be occurring other places as well," said Joel Thornton, University of Washington scientist and lead author of the study. Air quality measurements taken in a number of national parks across the United States indicate similar conditions in or near other non-coastal metropolitan areas, he said.

Nitryl chloride is a relative newcomer to the suite of gases studied by atmospheric scientists. A team of CIRES, NOAA, and University of New Hampshire researchers discovered the production of nitryl chloride in coastal regions in a 2006 field study.

Further work will be needed to sort out the implications of the latest findings for air quality and climate, according to the research team.

Other study co-authors on the Nature study included James Kercher, Theran Riedel, Glenn Wolfe and Becky Alexander from the University of Washington, Ann Middlebrook of NOAA's ESRL in Boulder, Julie Cozic, John Holloway, and William Dube of CIRES and NOAA's ESRL in Boulder and Patricia Quinn of NOAA's Pacific Marine Environmental Laboratory in Seattle.

A new study shows nighttime chemical interactions between airborne, chloride-containing particles and nitrogen oxide pollutants may be affecting air quality in the continental United States. Photo courtesy Josh Couts