MadSci Network: Chemistry

Re: Is it a good thing to have more oxygen than nitrogen in air?

Date: Mon Sep 22 09:48:15 2003
Posted By: In Koo Kim, Grad student, Physical Chemistry, Harvard
Area of science: Chemistry
ID: 1064155165.Ch

Dear Eager Student,

  The atmosphere is composed, by volume, of 21% molecular oxygen, 78%,
molecular nitrogen, and 1% other compounds most of which is carbon dioxide
and water vapor.   The last 1% varies a lot depending on the regional
climate and altitude, but the other 99% is pretty much constant throughout
the atmosphere until you reach ultra high altitudes.  These molar
percentages are a result of an equilibrium established on Earth through
four and a half billion years of volcanism, comet impacts,  and
chemical-biological processes. If the Earth were at a different distance
from the sun, if the life had never formed on Earth, or if any number of
other factors were different, the equilibrium would be different.
  The only thing nature knows as “good” are those that are in a state of
equilibrium.  Anything out of equilibrium is quickly changed until an
equilibrium is set.  The process of photosynthesis and respiration is a
cycle which results in an equilibrium between atmospheric carbon dioxide
and oxygen.  If you suddenly had, say 23% instead of 21% oxygen in the
atmosphere, the Earth's atmosphere would be out of its equilibrium and
things would proceed very rapidly to re-establish an equilibrium.  If you
took chemistry, you may be familiar with Le Chatlier's principle.  It
basically states that a chemical reaction will proceed so that an excess of
a compound in a reversible reaction will drive a reaction in the direction
to decrease the excess compound. The reaction here is :

O2 + carbon compound <-> CO2 + H2O

A reaction to the right is respiration.  The opposite direction is
photosynthesis (assuming you have light).

  So according to LeChatlier's principle, an increase in oxygen will result
in a shift in the equilibrium so that respiration will proceed more
readily.  Fire, is an instance of chemical respiration and that is the
respiration pathway most likely affected by the increased oxygen.  We would
see a marked increase in the number and spread of wildfires. You may even
get a fire that would be impossible to put out until there was nothing left
to burn.  And eventually, a new equilibrium would be established.  This new
equilibrium, would necessarily include much more carbon dioxide in the
atmosphere.  This, of course, would lead to increased global warming, which
would lead to increased desertification, which would lead to less food...
and so on and so forth.
  There are of course, many other equilibriums that would be affected.  A
change in any one equilibrium has a way of affecting numerous other
equilibriums since the various chemical and biological components of the
Earth's surface are interdependent.  As per your question regarding,
nitrogen, there are a slew of reactions involving nitrogen, including ones
that produce tropospheric smog and destroy stratospheric ozone.
So you see, I'm not sure if it would be a “good” thing to have more oxygen
in the atmosphere. But my instincts as a scientist and human being is that
it's best to let a good thing be.

In Koo Kim

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