Page 160 - Basic physical chemistry for the atmospheric sciences

146 Basic physical chemistry

it is a powerful air pollutant in the lower atmosphere. However, most
e
of the ozone in the atmosphere is located in the stratospher , which
extends from about 10 to 1 5 km (depending on latitude) up to about 45
km above the Earth' s surface. The presence of adequate concentra­
tions of ozone in the stratosphere is essential for plant and animal life
on Earth as we know it. This is because stratospheric ozone absorbs
the most biologically harmful of the sun' s UV radiation (called U V -B ,
which has a wavelength from 0 . 2 9 to 0 . 3 2 µ,m). Thus , decreases in
stratospheric ozone are accompanied by increases in the intensity of
UV-B radiation at the Earth' s surface and (disproportionate) increases
4
in biological cell damage, which can lead to skin cancer and damage
to plants. Since both the formation and depletion of ozone in the
stratosphere involve photochemical reactions, we will conclude this
chapter with a brief description of how ozone is formed in the strato­
sphere and some of the mechanisms by which it can be depleted both
naturally and by anthropogenic emissions.
A simple chemical scheme for maintaining steady-state concentra­
tions of ozone in an "oxygen-only" stratosphere was proposed by the
geophysicist Sidney Chapman in 1 9 30. The reaction scheme is5

j a
02 + h v � o + o (7.24)

kb
0 + 0 2 + M � 03 + M (7 .25)

jc
03 + h v� o + 2 0 (7.26)

kd
0 + 0 3 � 0 + 2 0 2 (7.27)
Until the early 1 9 60s it appeared that these reactions could explain
the main features of the steady-state distribution of ozone in the strato­
sphere. However, subsequent and more refined measurements of the
rate coefficients for Reactions (7. 2 4) to ( . 27) showed that the Chap­
7
man reactions generate ozone five times faster than they destroy it.
This is due primarily to the low value of kd in Reaction (7. 2 7). Since
the concentration of ozone in the stratosphere is not increasing at a
rapid rate, there must be a much faster route for destroying ozone
n
than indicated by the Chapman reactio s . The search for this fast
route, and the discovery of the sensitivity of stratospheric ozone con­
centrations to the presence of quite small amounts of certain trace