Polar stratospheric descent of NOy and CO and Arctic denitrification during winter 1992-1993

Observations inside the November 1994 Antarctic stratospheric vortex and in side the April 1993 remnant Arctic stratospheric vortex by the Atmospheric Trace Molecule Spectroscopy (ATMOS) Fourier transform spectrometer are repo rted. In both instances, elevated volume mixing ratios (VMRs) of carbon mon oxide (CO) were measured. A peak Antarctic CO VMR of 60 ppbv (where 1 ppbv = 10(-9) per unit volume) was measured at a potential temperature (Theta) o f 710 K (similar to 27 km), about 1 km below the altitude of a pocket of el evated NOy (total reactive nitrogen) at a deep minimum in N2O (<5 ppbv). Th e Arctic observations also show a region of elevated vortex CO with a peak VMR of 90 ppbv at 630-670 K (similar to 25 km) but no corresponding enhance ment in NOy, perhaps because of stronger dynamical activity in the northern hemisphere polar winter and/or interannual variability in the production o f mesospheric NO. By comparing vortex and extravortex observations of NOy o btained at the same N2O VMR, Arctic vortex denitrification of 5 +/- 2 ppbv at 470 K (similar to 18 km) is inferred. We show that our conclusion of sub stantial Arctic winter 1992-1993 denitrification is robust by comparing our extravortex observations with previous polar measurements obtained over a wide range of winter conditions. Correlations of NOy with N2O measured at t he same Theta by ATMOS in the Arctic vortex and at midlatitudes on board th e ER-2 aircraft several weeks later lie along the same mixing line. The res ult demonstrates the consistency of the two data sets and confirms that the ER-2 sampled fragments of the denitrified Arctic vortex following its brea kup, An analysis of the ATMOS Arctic measurements of total hydrogen shows n o evidence for significant dehydration inside the vortex.