| Abstract: A number of key problems in atmospheric chemistry
are shaped by the strength and character of the various mechanisms acting
to move and mix air in the upper troposphere. These transport processes
are examined from a mechanistic perspective, with primary emphasis on the
tropopause and middle-troposphere regions in the extratropics. The roles
of vertical and horizontal transport "barriers" are explored,
including the processes by which such barriers are created and are overcome.
These transport considerations lead to a hypothesis concerning the processes
that shape the tropopause itself. Some perspectives are offered on the
still immature subject of transport in the upper troposphere of the tropics.
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