To text describing this figure   To MLS home page    To MLS scientific results page lower stratospheric HNO3 in the 1992-1993 polar vortices
This figure illustrates the evolution of nitric acid (HNO3) in the 1992 southern hemisphere winter, and the 1992-93 northern winter, as observed by MLS. Maps of HNO3 are shown for selected days: (1) before temperatures reached the PSC threshold, (2) during the period when temperatures were low enough for PSCs, and (3) after temperatures had warmed above the PSC threshold. The data have been interpolated to the 465 K isentropic surface (approximately 20 km). The color bar gives HNO3 abundances in parts per billion. White contours are potential vorticity values (2.5 and 3.0 x 10-5 K m2 kg-1 s-1 in the north, -2.5 and -3.0 x 10-5 K m2 kg-1 s-1 in the south) which identify the approximate edge of the polar vortex. Green contours on the ClO maps, and black contours on the HNO3 maps, show the 195 K (outer contour) and 190 K (inner contour) temperature contours, when these temperatures are present.

On 3 December in the north, and 28 April in the south, there is more HNO3 inside the vortex, as could be due both to increased heterogeneous chemical production of HNO3 at colder temperatures and descent of HNO3-rich air from above. By 2 June a deficit of HNO3 is seen in the southern hemisphere region where temperatures are sufficiently cold for PSC formation (which removes HNO3 from the gas-phase), and by 17 August the inner vortex of the southern hemisphere appears essentially devoid of HNO3. The HNO3 deficit remains on 1 November, after the lower stratosphere has warmed above the threshold for PSCs (this happened in early September), indicating irreversible removal of HNO3 from the lower stratosphere.

In the northern hemisphere a relative deficit in HNO3 is seen on 22 February, as expected, where temperatures are low enough for PSCs. On 14 March, when temperatures had warmed above the PSC threshold, no deficit of HNO3 in the vortex is seen - indicating, in contrast to the southern hemisphere, no irreversible removal of HNO3 from the stratosphere. See Santee, et al. [1995] for further discussion of these data and results.

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