Halogens Hover In Antarctic

THE FIRST DIRECT atmospheric measurements of iodine oxide and bromine oxide above the Antarctic snowpack unexpectedly indicate that the chemicals participate in destroying ozone and change the oxidizing potential of the region's atmosphere, according to a new study (Science 2007, 317, 348).

John M. C. Plane and Alfonso Saiz-Lopez at the University of Leeds, in the U. K., and colleagues present results from a yearlong field study at Halley Station near the Antarctic coast. They directly measured concentrations of IO and BrO with a long-path differential-optical-absorption spectrometer suspended approximately 5 meters above the snowpack. Previous measurements of IO and BrO concentrations were obtained indirectly from tropospheric data or satellites.

"This work provides new and rather surprising results indicating that there are very high concentrations of photoactive iodine species in the Antarctic-an order of magnitude higher than any reported in the Arctic and higher than those reported anywhere in the atmosphere," comments Robert A. Duce, professor emeritus of oceanography and atmospheric sciences at Texas A&M University. He and his students made the first atmospheric measurements of elemental iodine and bromine in the Antarctic during the 1970s.

In the past two decades, researchers have closely examined the role that bromine chemistry has played in depleting ozone in the Arctic during sunlit periods. Iodine has also been shown to efficiently instigate the formation of particles that contribute to cloud formation and other processes that affect global climate. Reactive halogen compounds that affect atmospheric chemistry are thought to originate in the ocean or are anthropogenic.

Plane's group reports peak concentrations of 20 parts per trillion by volume for both IO and BrO in Antarctic spring. BrO concentrations were similar to springtime Arctic levels, but IO has not been detected there. The researchers note that seasonal changes in halogen oxide concentrations essentially follow the diurnal cycle of solar radiation, which indicates that both compounds are produced photochemically. They also found that the chemicals have a lifetime of less than two hours.

These high levels of halogen oxides will change the Antarctic atmosphere's capacity to oxidize various compounds by increasing the NO₂/NO ratio and decreasing the HO₂/OH ratio, the researchers suggest. They used a photochemical model to estimate the impact that halogen chemistry has on the atmosphere during the spring. Their model indicates synergistic effects: Iodine seems to remove more ozone than bromine, but when iodine and bromine act together, ozone losses quadruple.

"Most atmospheric chemistry models do not include halogen chemistry," notes Jochen Stutz, associate professor of atmospheric chemistry at the University of California, Los Angeles. He adds that these latest results punctuate the importance of including halogens in global modeling schemes.