Ask A Scientist Weather Archive

  
name         Marķa
status       student
age          17

Question -   Why do CFC's reach the stratosphere if they are heavier
than the air?
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Maria,

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.

Regards,
ProfHoff 866
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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.

Vince Calder
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Maria,

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
Earth.

David R. Cook
Atmospheric Research Section
Environmental Research Division
Argonne National Laboratory
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