The topic of stratospheric ozone depletion has received major attention over the past few years, both in terms of specific phenomena like the polar ozone holes, and the threat of a gradual thinning of Earth's protective ozone shield at all latitudes. The problem arises because odd-nitrogen- and halogen-containing molecules are able to reach the stratosphere, where they participate in photochemical reactions which accelerate the rates at which ozone is destroyed, thereby altering the natural balance between production and removal. TES, in common with a number of other instruments, has the ability to measure many of the species which participate in these reactions. However, TES can also observe the region of the polar vortices (to "82E latitude") during the polar night when they are inaccessible to instruments relying on backscattered sunlight (e.g., TOMS) or solar occultation (e.g., SAGE).

Troposphere-Stratosphere Exchange Mechanisms

In attempting to understand and limit stratospheric ozone depletion, we need to address the processes whereby the depletor molecules (mainly N₂O and halocarbons) reach the stratosphere and the reservoir species for reactive nitrogen and halogen (such as HNO₃ and HCl) make the return journey into the troposphere where they can be removed by rainout. To first order, the tropopause is quite an efficient barrier against vertical motion, which means that sporadic events (in cumulus towers, for instance) and diffusive processes combine in unknown proportions to inject tropospheric gases into the stratosphere. Equally obscure processes, probably involving tropopause-folding events, bring stratospheric gases (including ozone) down to the surface. The global budgets of these transfers, which are crucial to any long-term understanding of the evolution of the ozone layer, are very difficult to quantify, because so little is known about the processes involved.

The key to understanding the vertical-transfer processes in the region of the tropopause is obtaining, on a global and seasonal basis, vertical profiles of species which originate in either the troposphere or the stratosphere and therefore normally have large gradients across the tropopause. H₂O, N₂O and CH₄ are the best candidates in the former category and O₃ itself in the latter. Changes in these gradients will allow the transfer process to be modeled and their statistics of occurrence established.

The problem with obtaining such measurements is that instruments flown before TES will either have lower spectral resolution (and therefore very limited penetration into the troposphere), or are solar occultation instruments with very limited coverage. Preliminary calculations suggest that TES resolution is sufficient to resolve "microwindows" at wavelengths where the opacity is controlled by absorptions of all the important species named above, thus permitting limb observations well down into the troposphere (to 4 km when the absence of high clouds permits).