12 March 2010
A new scientific study packs a double surprise about the chemistry happening in the atmosphere around us: chlorine is more abundant than expected in the air at sites far from the ocean's sea spray sources. And, it interacts with manmade pollution at night – a time period that's not so sleepy for the atmosphere – in ways that might ultimately affect air quality and climate.
Chloride gets kicked up into the air naturally by sea spray, but the high levels of chlorine-containing gases discovered by the researchers in the middle of the continental U.S. confirm that human activities such as coal burning, biomass burning, and roadway deicing are now kicking in too.
"Our findings show that the chemistry involving these chlorine-containing gases is not confined to the coastal and marine atmosphere," said study coauthor Nick Wagner, a researcher at CSD with the Cooperative Institute for Research in Environmental Sciences (CIRES) at CU. The paper was published online in Nature on March 11th.
Researchers at NOAA, CIRES, 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," says Steve Brown, a CSD scientist and a coauthor 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. "Nighttime formation of nitryl chloride is a gateway to forming more highly reactive chlorine atoms," says Brown. "It changes the atmosphere's starting point for the next day."
Researchers at NOAA, CIRES, and the University of Washington were getting ready for a 2008 study of the chloride-related chemistry in the atmosphere above the North Atlantic, and were testing their equipment in Boulder. They found that 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," says Joel Thornton, University of Washington scientist and lead author of the study. Air quality measurements taken in a number of national parks across the U.S. imply similar conditions in or near other non-coastal metropolitan areas.
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.
Joel A. Thornton 1, James P. Kercher 1, Theran P. Riedel 1, Nicholas L. Wagner 2,3, Julie Cozic 2,3, John S. Holloway 2,3, William P. Dubé 2,3, Glenn M. Wolfe 1, Patricia K. Quinn 4, Ann M. Middlebrook 2, Becky Alexander1, Steven S. Brown 2, A large atomic chlorine source inferred from mid-continental reactive nitrogen chemistry Nature, doi:10.1038/nature08905, 11 March 2010.