12 How does the ozone layer work?
Description
This article is from the Ozone Depletion FAQ, by Robert Parson rparson@spot.colorado.edu with numerous contributions by others.
12 How does the ozone layer work?
UV light with wavelengths between 240 and 320 nm is absorbed by
ozone, which then falls apart to give an O atom and an O2 molecule.
The O atom soon encounters another O2 molecule, however (at all times,
the concentration of O2 far exceeds that of O3), and recreates O3:
O3 + hv -> O2 + O
O + O2 -> O3
Thus _ozone absorbs UV radiation without itself being consumed_;
the net result is to convert UV light into heat. Indeed, this is
what causes the temperature of the stratosphere to increase with
altitude, giving rise to the inversion layer that traps molecules in
the troposphere. The ozone layer isn't just _in_ the stratosphere; the
ozone layer actually determines the form of the stratosphere.
Ozone _is_ destroyed if an O atom and an O3 molecule meet:
O + O3 -> 2 O2 ("recombination").
This reaction is slow, however, and if it were the only mechanism
for ozone loss, the ozone layer would be about twice as thick
as it is. Certain trace species, such as the oxides of Nitrogen (NO
and NO2), Hydrogen (H, OH, and HO2) and chlorine (Cl, ClO and ClO2)
can catalyze the recombination. The present ozone layer is a
result of a competition between photolysis and recombination;
increasing the recombination rate, by increasing the
concentration of catalysts, results in a thinner ozone layer.
Putting the pieces together, we have the set of reactions proposed
in the 1930's by Sidney Chapman:
O2 + hv -> O + O (wavelength < 240 nm) : creation of oxygen atoms O + O2 -> O3 : formation of ozone O3 + hv -> O2 + O (wavelength < 320 nm) : absorption of UV by ozone O + O3 -> 2 O2 : recombination .
Since the photolysis of O2 requires UV radiation while
recombination does not, one might guess that ozone should increase
during the day and decrease at night. This has led some people to
suggest that the "antarctic ozone hole" is merely a result of the
long antarctic winter nights. This inference is incorrect, because
the recombination reaction requires oxygen atoms which are also
produced by photolysis. Throughout the stratosphere the concentration
of O atoms is orders of magnitude smaller than the concentration of
O3 molecules, so both the production and the destruction of ozone by
the above mechanisms shut down at night. In fact, the thickness of the
ozone layer varies very little from day to night, and above 70 km
ozone concentrations actually _increase_ at night.
(The unusual catalytic cycles that operate in the antarctic ozone
hole do not require O atoms; however, they still require light to
operate because they also include photolytic steps. See Part III.)
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