Stratospheric transport and tracer lifetimes from airborne in situ observations

Global-scale transport and chemistry of long-lived trace species in the stratosphere are studied using airborne in situ observations. A new instrument, the Airborne Chromatograph for Atmospheric Trace Species IV (ACATS-IV) was developed and operated on board the NASA ER-2 high-altitude aircraft from February to November 1994 during the aircraft campaign "Airborne Southern Hemisphere Ozone Experiment/Measurements for Assessing the Effects of Stratospheric Aircraft" (ASHOE/MAESA). ACATS-IV provided simultaneous measurements of mixing ratios of the long-lived source gases N;Tropical and mid-latitude observations are used in conjunction with a tropical tracer model to quantify for the first time the rate of mass transport between the tropical and mid-latitude lower stratosphere. It is shown 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 thus found to be 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% of air in the tropical ascent region at 21 km is of mid-latitude origin, implying that emissions from supersonic aircraft could reach the middle stratosphere, where enhancements of nitrogen oxides are expected to lead to reductions in ozone.;The environmental impact of the measured source gases depends, among other factors, on the rate at which they release ozone-depleting chemicals in the stratosphere, i.e. on their stratospheric lifetimes. Based on theoretical concepts, methods are developed and applied to semi-empirically derive stratospheric lifetimes of the measured species from observed extratropical tracer-tracer correlations. For most of the species, our best lifetime estimates are significantly shorter than currently recommended lifetimes, which are based largely on photochemical model calculations. Because the derived stratospheric lifetimes are identical to atmospheric lifetimes for most of the species considered, the shorter lifetimes would imply a faster recovery of the ozone layer following the phase-out of industrial halocarbons than currently predicted.