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08 How does the composition of the atmosphere change with altitude? (Or, how can CFC's get up to the stratosphere when they are heavier than air?)




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This article is from the Ozone Depletion FAQ, by Robert Parson rparson@spot.colorado.edu with numerous contributions by others.

08 How does the composition of the atmosphere change with altitude? (Or, how can CFC's get up to the stratosphere when they are heavier than air?)

In the earth's troposphere and stratosphere, most _stable_ chemical
species are "well-mixed" - their mixing ratios are independent of
altitude. If a species' mixing ratio changes with altitude, some
kind of physical or chemical transformation is taking place. That
last statement may seem surprising - one might expect the heavier
molecules to dominate at lower altitudes. The mixing ratio of
Krypton (mass 84), then, would decrease with altitude, while that
of Helium (mass 4) would increase. In reality, however, molecules
do not segregate by weight in the troposphere or stratosphere.
The relative proportions of Helium, Nitrogen, and Krypton are
unchanged up to about 100 km.

Why is this? Vertical transport in the troposphere takes place by
convection and turbulent mixing. In the stratosphere and in the
mesosphere, it takes place by "eddy diffusion" - the gradual mechanical
mixing of gas by motions on small scales. These mechanisms do not
distinguish molecular masses. Only at much higher altitudes do mean
free paths become so large that _molecular_ diffusion dominates and
gravity is able to separate the different species, bringing hydrogen
and helium atoms to the top. The lower and middle atmosphere are thus
said to be "well mixed."
[Chamberlain and Hunten] [Wayne] [Wallace and Hobbs]

Experimental measurements of the fluorocarbon CF4 demonstrate this
homogeneous mixing. CF4 has an extremely long lifetime in the
stratosphere - probably many thousands of years. The mixing ratio
of CF4 in the stratosphere was found to be 0.056-0.060 ppbv
from 10-50 km, with no overall trend. [Zander et al. 1992]

An important trace gas that is *not* well-mixed is water vapor. The
lower troposphere contains a great deal of water - as much as 30,000
ppmv in humid tropical latitudes. High in the troposphere, however,
the water condenses and falls to the earth as rain or snow, so that
the stratosphere is extremely dry, typical mixing ratios being about
5 ppmv. Indeed, the transport of water vapor from troposphere to
stratosphere is even less efficient than this would suggest, since
much of the small amount of water in the stratosphere is actually
produced _in situ_ by the oxidation of stratospheric methane. [SAGE II]

Sometimes that part of the atmosphere in which the chemical
composition of stable species does not change with altitude is
called the "homosphere". The homosphere includes the troposphere,
stratosphere, and mesosphere. The upper regions of the atmosphere
- the "thermosphere" and the "exosphere" - are then referred to as
the "heterosphere". [Wayne] [Wallace and Hobbs]

 

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