C. M. Volk, ^* J. W. Elkins, D. W. Fahey, R. J. Salawitch, G. S. Dutton, J. M. Gilligan, M. H. Proffitt, M. Loewenstein, J. R. Podolske, K. Minschwaner, J. J. Margitan, K. R. Chan

Airborne in situ observations of molecules with a wide range of lifetimes (methane, nitrous oxide, reactive nitrogen, ozone, chlorinated halocarbons, and halon-1211), used in a tropical tracer model, show that mid-latitude air is entrained into the tropical lower stratosphere within about 13.5 months; transport is faster in the reverse direction. Because exchange with the tropics is slower than global photochemical models generally assume, ozone at mid-latitudes appears to be more sensitive to elevated levels of industrial chlorine than is currently predicted. Nevertheless, about 45 percent of air in the tropical ascent region at 21 kilometers is of mid-latitude origin, implying that emissions from supersonic aircraft could reach the middle stratosphere.

C. M. Volk, G. S. Dutton, and J. M. Gilligan are with the Climate Monitoring and Diagnostics Laboratory (CMDL), National Oceanic and Atmospheric Administration (NOAA), Boulder, CO 80303, and the Cooperative Institute for Research in Environmental Sciences (CIRES), University of Colorado, Boulder, CO 80309, USA. J. W. Elkins is with NOAA/CMDL, Boulder, CO 80303, USA. D. W. Fahey is with the NOAA Aeronomy Laboratory, Boulder, CO 80303, USA. R. J. Salawitch and J. J. Margitan are with the Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA. M. H. Proffitt is with the NOAA Aeronomy Laboratory, Boulder, CO 80303, and CIRES, University of Colorado, Boulder, CO 80309, USA. M. Loewenstein, J. R. Podolske, and K. R. Chan are with the NASA Ames Research Center, Moffett Field, CA 94035, USA. K. Minschwaner is with the Department of Physics, New Mexico Institute of Mining and Technology, Socorro, NM 87801, USA.
^* To whom correspondence should be addressed.
Present address: Department of Physics, Vanderbilt University, Nashville, TN 37235, USA.