An Introduction to Environmental Chemistry

eqn. 3.35

together with an additional reaction describing the destruction process for oxygen
atoms:

eqn. 3.36

Note the presence of the ‘third body’ M, which carries away excess energy during
the reaction. The third body would typically be O 2 or a nitrogen molecule (N 2 ).
Without this ‘third body’ the O 2 that formed might split apart again. Calcula-
tions that balance the production and destruction of O 3 considering only
reactions that involve the element oxygen (i.e. oxygen-only paths) give a fair
description of the O 3 observed in the stratosphere. The results of these calcula-
tions produce the correct shape for the vertical profile of O 3 in the atmosphere
and the peak O 3 concentration occurs at the correct altitude, but the predicted
concentrations are too high. This is because there are other pathways that destroy
O 3. Some involve hydrogen-containing species:

eqn. 3.37

eqn. 3.38

which sum:

eqn. 3.39

Similar reactions can be written for nitrogen-containing species, for example
nitric oxide (NO), which arises from supersonic aircraft, or nitrous oxide (N 2 O),
which crosses the tropopause into the stratosphere:

eqn. 3.40

eqn. 3.41

and N 2 O(g)can enter reaction 3.40 via the initial step:

eqn. 3.42

Reactions involving these species sum in such a way as to destroy O 3 and atomic
oxygen while restoring the OH or NO molecules. They can thus be regarded as
catalysts for O 3 destruction. In this case the catalysts are chemical species that
facilitate a reaction, but undergo no net consumption or production in the reac-
tion (see also Box 4.4). The important point of these catalytic reaction chains in
the chemistry of stratospheric O 3 is that a single pollutant molecule can be
responsible for the destruction of a large number of O 3 molecules.
It is now very well established that the most important of these catalytic
reaction chains affecting polar ozone loss are the ones based around chlorine-
containing species, as detailed below.

3.10.2 Ozone destruction by halogenated species

Natural chlorine in the stratosphere is mainly transferred there as methyl chlo-
ride (CH 3 Cl), which probably comes from marine and terrestrial biological

NO 2 ()gg+ÆO() 2 NO()g

NO 22 ()g+Æ +O()gggNO() O()

NO()gg g+Æ+O32 2() O() NO()g

OO O 32 ()gg+Æ() (^2) ()g
HO 22 ()g+Æ +O()gggOH()O()
OH()gg g+Æ+O32 2() O() HO()g
OOM O M()gg g++ Æ +() () 2 ()g ()g
OO O 32 ()gg+Æ() (^2) ()g
The Atmosphere 61