Finding Hydroxyl

ATMOSPHERIC CHEMISTRY

Expanding its role beyond dating ancient objects, carbon-14 is proving be a powerful tracer of the hydroxyl radical, a difficult-to-measure but all-important atmospheric cleanser that oxidizes greenhouse gases such as methane and pollutants such as carbon monoxide.

Until now, scientists have inferred atmospheric levels of the radical, which has a lifetime of one second, largely by measuring concentrations of methyl chloroform, a man-made molecule that reacts almost exclusively with OH. Nobel Laureate F. Sherwood Rowland, chemistry professor at the University of California, Irvine, introduced the method in the early 1980s. But methyl chloroform, an ozone-depleting compound, has been banned, and its concentrations in the atmosphere are dwindling.

The new study by Martin R. Manning, a director of the United Nations Intergovernmental Panel on Climate Change support unit at the National Oceanic & Atmospheric Administration in Boulder, Colo., and colleagues makes use of ¹⁴CO, a rare atmospheric gas also oxidized by OH. Like methyl chloroform, CO levels presumably decrease when there's more OH around.

CO lasts only a few months before it's oxidized, compared with the over-five-year lifetime of methyl chloroform. That means that with CO, researchers can observe short-term fluctuations in OH, such as those caused by major atmospheric events like the eruption of Mount Pinatubo in 1991. But unlike ¹²CO, which has innumerable sources and sinks, both natural and man-made, ¹⁴CO has the benefit of being largely produced during a cascade of reactions initiated by cosmic rays.

Manning's group collected an enormous trove of ¹⁴CO measurements, spanning 13 years, from the Southern Hemisphere, in Baring Head, New Zealand, and in Antarctica (Nature 2005, 436, 1001). Overall, they find, there's been no large rise or drop in atmospheric OH.

The researchers can infer fluctuations in OH levels lasting a period of a few months, however. The Pinatubo eruption, as well as a rash of forest fires in Indonesia in 1997, led to OH declines as large as 20%, they report. Their findings are similar to OH levels inferred from methyl chloroform measurements reported by atmospheric chemistry professor Ronald G. Prinn at Massachusetts Institute of Technology and colleagues (Geophys. Res. Lett. 2005, 32, 07809).

"This is a very interesting measurement" that Manning's group has produced, Rowland says.

"¹⁴CO should become the principal diagnostic tool for monitoring the oxidative capacity of the atmosphere," say Patrick Jöckel and Carl A. M. Brenninkmeijer of Max Planck Institute for Chemistry in Mainz, Germany, in a commentary accompanying the Nature report. "This tracer is a cosmic dowry for atmospheric chemists."

Manning says a confluence of technological and modeling advances made the work possible--most notably, the capability to accurately determine how much ¹⁴CO is produced in the atmosphere. Protons in cosmic rays hitting Earth's atmosphere produce neutrons, which react with ¹⁴N nuclei to produce excited ¹⁴C, which quickly reacts with O₂ to form ¹⁴CO. Complicating matters is the 11-year cycle of solar activity that affects cosmic radiation. The amounts of ¹⁴CO produced are also extremely small--only about 10 atoms per cm³--requiring sensitive detector technology.

Jos Lelieveld, director of the atmospheric chemistry department at Max Planck Institute for Chemistry, who also calls the work "very interesting," and Rowland both note that further measurements and modeling will be needed to nail down the inherent complexity and uncertainty of atmospheric OH.