SPREAD OF DENITRIFICATION FROM 1987 ANTARCTIC AND 1988-1989 ARCTIC STRATOSPHERIC VORTICES

Vertical profiles of N2O and NOy taken by the ER-2 outside the vortex are used to construct average vertical profiles of F(NOy)= NOy/(A-N2O), where A is the tropospheric content of N2O three years prior to the measurements. The southern hemisphere had less nitrous oxide in the range 400 < theta < 470 K, by up to 25% relative to the northern hemisphere. F(NOy) is the ratio of NOy produced to N2O lost in a stratospheric air mass since entry from the troposphere. The profiles of F(NOy) have the following characteristics: (1) Relative to 1991-1992, a year without denitrification Inside or outside the vortex, the northern hemisphere in 1988-1989 showed denitrification outside the vortex ranging up to 25% and averaging 17% above theta = 425 K. (2) Relative to the northern hemisphere in 1991-1992, the southern hemisphere in 1987 showed denitrification outside the vortex ranging up to 32% and averaging 20% above theta = 400 K. (3) Below theta = 400 K the southern hemisphere showed enhancements of F(NOy) relative to the northern hemisphere in 1991-1992 ranging up to 200% at theta = 375 K, outside the vortex. Corresponding profiles of residual water, R(H2O) = H2O - 2 [1.6 - CH4], are considered and shown to be consistent with those of F(NOy) in the sense that they show deficits outside the Antarctic vortex, which was both dehydrated and denitrified, but not outside the 1988-1989 Arctic vortex, which was denitrified but not dehydrated. R(H2O) is the water content of stratospheric air with the contribution from methane oxidation subtracted. Comparison of F(NOy) and R(H2O) below 400 K outside the Antarctic vortex leads to the suggestion that dehydration in the Antarctic vortex occurs by the sedimentation of ice crystals large enough to fall out of the stratosphere, whereas denitrification occurs mainly on mixed nitric acid-water crystals which evaporate below the base of the vortex at theta = 400 K but above the tropopause.