From: parson_r@cubldr.colorado.edu (Robert Parson)
Subject: Chemical Composition of the Atmosphere - Why Mass Doesn't Matter
Organization: University of Colorado, Boulder
Date: Sun, 24 Jan 1993 05:07:11 GMT

 In my various posts last week, I included  data on the  "mixing ratios"
(relative concentrations measured in parts-per-  billion by volume, i.e. the
relative number of molecules of a  given type) of various chlorine compounds as
a function of altitude.  I argued that the decrease in organic chlorine "mixing
ratios"  with altitude, and the  corresponding increase in inorganic chlorine
mixing ratios, is strong  evidence that the inorganic chlorine is being
generated from the organic  chlorine. 

 It has since occured to me that readers might find this confusing, as I
 rely on a fact that is not well known outside the atmospheric science
 community - molecular mass is _not_ relevant in the troposphere and
 stratosphere.  I found this very surprising when I first learned about
 it, and I have seen at least one Freshman Chemistry textbook get it wrong.

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. This may seem surprising - a direct application
of the Maxwell-Boltzmann distribution ("barometric distribution"  for this
case) seems to imply that the heavier molecules dominate at lower altitudes. 
The mixing ratio of Krypton (mass 84), then, would decrease with altitude,
since Kr is much heavier than Nitrogen (mass 28) or Oxygen (mass 32),
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 80 km,
 although of course the overall density drops off. 

Why is this? Vertical transport in the troposphere takes place by convection
and turbulent mixing.  In the stratosphere and in the next layer up, the
"mesosphere", it takes place by "eddy diffusion" - the gradual mechanical
mixing of gas by small scale motions.  These mechanisms do not distinguish
molecular masses.  Only at much higher altitudes do mean free paths become
large enough that _molecular_ diffusion dominates and gravity is able to
separate the different species.

Experimental measurements of the fluorocarbon CF4 verify this homogeneous
mixing.  CF4 has an extremely long lifetime in the stratosphere - many
thousands of years.  The mixing ratio of CF4 in the strat.  was found to be
0.056-0.060 ppb from 10-50 km, with no overall trend.  

[R. Zander et al, J. At. Chem.  _15_, 171, 1992.]

 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 the next layer
 up, the "mesosphere". The upper regions of the atmosphere are then 
 referred to as the "heterosphere".  (The transition between the two
 regimes is gradual, and in principal depends upon the particle mass -
 one does not find many sand grains or cannonballs in the mesosphere. In
 practice, large molecules are destroyed chemically long before they rise
 high enough for molecular diffusion to take over, and for all species of
 interest the transition takes place far above the stratosphere.)

 Robert
Xref: sparky sci.environment:14851 soc.culture.usa:10138