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downward to the region of 40 km, NO may have an important influence on the ozone density of the stratosphere (Huang and Brasseur, 1993). While the catalytic destruction of ozone in this region is relatively well understood (see Chapter 3), the transport of thermospheric-generated nitric oxide into the region is not. Because the photochemical destruction lifetime is on the order of one day throughout the sunlit mesosphere, NO must move downward during the polar night to reach the upper stratosphere. Two-dimensional modeling studies, as well as spacecraft observations of middle atmospheric ozone abundances, suggest that this may indeed be a viable coupling mechanism (Garcia et al., 1984; Solomon and Garcia, 1984). Changes in nitrate content of antarctic snow associated with solar activity may reflect these processes (Dreschhoff and Zeller, 1990).

In addition to solar-related processes, the lowest layers of the upper atmosphere are influenced by turbulent breaking of atmospheric gravity waves and tides and by sporadic, intermittent compositional exchanges of atomic oxygen and nitric oxide. These complex turbulent exchange processes influence long-lived species such as carbon dioxide, carbon monoxide, water vapor, and atomic and molecular hydrogen. Although theoretical studies have indicated that chemical, dynamical, and radiative interactions are a viable coupling mechanism, the whole question of thermospheric/lower atmospheric exchange is not well understood, primarily because our atmosphere between about 50 and 200 km is virtually unexplored on a global basis.