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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