Global Stratospheric Ozone Reduction - Air Pollution and Global Warming: History, Science, and Solutions - page 241

stratospheric ozone is produced after photolysis of
molecular oxygen, whereas tropospheric ozone is pro-
duced after photolysis of nitrogen dioxide. Next, reac-
tions that naturally produce and destroy stratospheric
ozone are described.
In the stratosphere, far-UV wavelengths shorter than
245 nm break down molecular oxygen by

O( 1 D )(g) O(g)

O 2 (g) + h →· +· < 175 nm

Molecular Excited Atomic

oxygen atomic oxygen

(11.1)

oxygen

O(g) O(g)

O 2 (g) + h →· +· 175 < < 245 nm

Molecular Atomic

(11.2)

oxygen oxygen

The first reaction is important only at the top of the
stratosphere because wavelengths shorter than 175 nm
do not penetrate lower. Neither reaction is important in
the troposphere. Excited atomic oxygen from Reaction
11.1 rapidly converts to the ground state by

M
→·

(11.3)

O( 1 D )(g)

O(g)

·

Excited Atomic

Figure 11.6. Sydney Chapman (1888-1970). American
Institute of Physics, Emilio Segr `eVisualArchives,
Physics Today collection.

atomic oxygen

oxygen

Ozone then forms by

stratosphere. He further postulated that Reactions 11.2,
11.4, 11.6, 11.7, and

M
→

(11.4)

O(g)

· + O 2 (g) O 3 (g)

Atomic Molecular Ozone

oxygen oxygen

(11.8)

M
→

O(g) O(g)

· +· O 2 (g)

This reaction also occurs in the troposphere, where
the O(g) originates from NO 2 (g) photolysis rather than
from O 2 (g) photolysis. Ozone is destroyed naturally in
the stratosphere and troposphere by

Atomic Molecular

oxygen oxygen

controlled the natural formation and destruction of
ozone in the stratosphere (Chapman, 1930). These reac-
tions comprise what is known today as the Chap-
man cycle and describe the process fairly well. Some
Chapman reactions are more important than are oth-
ers. Reactions 11.2, 11.4, and 11.6 affect ozone the
most. The non-Chapman reaction, Reaction 11.5, is also
important.
Some of the Chapman cycle reactions can be used
to explain why the altitudes of peak ozone concentra-
tion and mixing ratio occur where they do, as illus-
trated in Figure 11.7. Oxygen density, like air density,
decreases exponentially with increasing altitude. UV
radiation intensity, conversely, decreases with decreas-
ing altitude. Peak ozone densities occur where suffi-
cient radiation encounters sufficient oxygen density,
which is near 25 to 32 km. At higher altitudes, oxygen
density is too low for its photolysis by Reactions 11.1

O( 1 D )(g)

O 3 (g) + h → O 2 (g) +· < 310 nm

Ozone Molecular Excited

(11.5)

oxygen atomic

oxygen

O(g)

O 3 (g) + h → O 2 (g) +· > 310 nm

Ozone Molecular Atomic

(11.6)

oxygen oxygen

Stratospheric ozone is also destroyed by

O(g) (11.7)

· + O 3 (g) → 2O 2 (g)

Atomic Ozone Molecular

oxygen oxygen

In 1930, the English physicist Sydney Chapman
(1888-1970; Figure 11.6) suggested that UV pho-
tolysis of molecular oxygen produced ozone in the