More Ar near Earth's surface

Testing a 180-year-old theory of chemist John Dalton, researchers have measured a diffusive separation between heavy and light molecules in the near-surface layer of Earth's atmosphere. Ralph F. Keeling and coworkers at Scripps Institution of Oceanography, La Jolla, Calif., found a detectable separation effect through precise measurements of the argon/nitrogen ratio (Science 2006, 311, 1429). On nights in February and April 2005, they took air samples in Borrego Sink, a 2-km depression in a California desert plain 140 meters above sea level that is rimmed by mountains. Ar/N₂ ratios were found to be higher near the surface relative to the air aloft and the air back in La Jolla. Dalton proposed that gravity would produce changes in atmospheric composition at different elevations, but subsequent measurements could find no separation below 100 km, where presumably the tendency to separate is overwhelmed by turbulent mixing. The Scripps group believes its observations were attributable to temperature gradients and aided by low winds.