Basic
chemistry of ozone depletion
Please
understand that this is a greatly simplified "readers digest" version
of the central reactions of interest in the story of Ozone depletion.
For further information I recommend
- T.E.Graedel
and P.J.Crutzen
- _Atmospheric
Change: an Earth System Perspective_2nd ed._
- Freeman,
New York (1993) Starting on page 141.
for
a more in-depth treatment of ozone chemistry that is still very readable
by the science-layman.
Formation
and destruction of ozone
Sunlight
is the major energy source for both making and destroying stratospheric
ozone: 
When
an Oxygen molecule absorbs a photon of light with a wavelength shorter
than 200 nanometers (1 billionth of a meter) the energy splits the
molecule into two Oxygen atoms. One of these atoms can react with
another Oxygen molecule to form an Ozone molecule.
Up
to 98% of the sun's high-energy ultraviolet light(UV-B and UV-C)
are absorbed by the destruction and formation of atmospheric ozone.
The global exchange between ozone and oxygen is on the order of
300 million tons per day.
Halogens
are a chemical family containing fluorine, chlorine, bromine and iodine;
any carbon compound containing them is known as a halocarbon. While
all halogens have the ability to catalyze ozone breakdown, they have
an unequal impact on the ozone layer.
A catalyst is a compound which can alter the rate of a reaction
without permently being altered by that reaction, and so can react
over and over again.
In
this fashion it is estimated that one molecule of chlorine can degrade
over 100,000 molecules of ozone before it is removed from the stratosphere
or becomes part of an inactive compound. These inactive compounds,
for example ClONO2, are collectively called 'resevoirs'. They hold
chlorine in an inactive form but can release an active chlorine
when stuck by sunlight.
The relative potency of the different halogens depends a great deal
on the stability of the resevior compounds. Hydrogen fluoride, HF,
is so very stable that fluorocarbons have relatively no known impact
on ozone. Bromine resevoirs, such as HBr and BrONO2, are much more
easily broken up by sunlight ; causing bromine to be from 10 to
100 times more effective than chlorine at destroying ozone. From
30-60% of bromocarbons released to the atmosphere are man-made (methyl
bromide fumigants and halon fire extinguishers) and both compounds
will soon be restricted by international agreement.
Chlorine
Removal
In
the stratosphere the major mechanisms for chlorine removal involve
the formation of HCl:
- OH
+ ClO ---->HCl + O2
- O2H
+ Cl ------> HCl + O2
HCl
is water soluble and is eventually precipitated out of the stratosphere
by water droplets or crystals. The estimated lifetime of HCl in the
stratosphere is about 2 years. CH4 and other Hyrogen-containing organics
compounds, including HCFC's, can also convert active chlorine to HCl.
In the troposphere and lower stratosphere NO, NO2 and OH radicles
are catalysts in ozone degredation. But they are equally able to
tie up tropospheric chlorine into resevoir compounds, which adhere
to water particles and get rained out of the troposphere.
Author: Brien Sparling
Return
to the ozone homepage
| 
