CFC's Reaching Stratosphere
Why do CFC's reach the stratosphere if they are heavier
than the air?
CFCs in the atmosphere exist as gases. Gases have rather weak intermolecular attractions
and they exhibit the property of diffusion -- that is, their molecules are in incessant
vibrational, rotational, and translational motion. Even though their densities are
greater than that of air, CFCs will ultimately mix with and diffuse about in air.
In time, some will reach the uppermost parts of earth's atmosphere.
Gases, heavier than air, reach high altitudes by convection -- the physical
mixing of masses of the atmosphere. Once there CFC's undergo photochemical
chain reactions so that a small quantity of CFC's propagate
atmosphere-damaging chemical reactions.
Almost any molecule or particle that is not too much heavier
than air can be transported long distances horizontally or
to great heights vertically by atmospheric motions. For instance,
consider the great heights to which thunderstorms can reach,
well into the stratosphere. As the air rises in the thunderstorm,
it carries with it whatever pollution and particles that were
near the surface. Thus they can end up in the stratosphere. Not
being too much heavier than air and with little for mechanisms to
remove them (little vertical motion, little water vapor, no
precipitation, etc.), CFCs can reside in the stratosphere
for a long time before they literally fall out down towards the
David R. Cook
Atmospheric Research Section
Environmental Research Division
Argonne National Laboratory
Click here to return to the Weather Archives
Update: June 2012