Global Stratospheric Ozone Reduction - Air Pollution and Global Warming: History, Science, and Solutions - page 237

Chapter 11

Global Stratospheric Ozone Reduction

The stratospheric ozone layer began to form soon after
the Great Oxygenation Event, whereby oxygen started
to accumulate in the atmosphere due to photosynthe-
sis about 2.45 b.y.a. The ozone layer did not develop
fully until after 400 m.y.a., when green plants evolved
and molecular oxygen mixing ratios began to approach
their present levels. Absorption of UV radiation by
ozone is responsible for the temperature inversion that
defines the present-day stratosphere. This absorption is
critical for preventing UV radiation from reaching the
surface of the Earth, where it can harm life. The anthro-
pogenic emission of long-lived chlorine- and bromine-
containing compounds into the air since the 1930s, as
well as the slow transfer of these compounds to the
stratosphere, have caused a nontrivial reduction in the
global stratospheric ozone layer since the 1970s. In
addition, during September, October, and November
of each year since 1979, such emissions have caused
up to a 70 percent destruction of the ozone layer over
the Antarctic. Lesser reductions have occurred over the
Arctic in March, April, and May of each year. Recent
international cooperation has helped replace emissions
of most ozone-depleting gases with less destructive
ones, reducing ozone loss. However, global warming,
which warms the surface but cools the stratosphere, has
delayed the recovery of the ozone layer. In this chap-
ter, the natural stratospheric ozone layer, global ozone
reduction, and Antarctic/Arctic ozone destruction and
regeneration are discussed.

11.1. Structure of the Present-Day
Ozone Layer
Christian Schonbein discovered the ozone molecule in
1839 (Section 1.2.3.4). However, the ozone layer was
not identified until more than 70 years later. In 1879, the
French physicist Alfred Cornu (1841-1902) measured
asharp reduction in short wavelengths of UV light, rel-
ative to longer wavelengths, reaching the Earth's sur-
face, suggesting that an atmospheric constituent might
be absorbing the short wavelengths. In 1881, Walter
N. Hartley explained the absorption of short UV wave-
lengths as due to ozone high in the atmosphere. Finally,
in 1913, French physicists Charles Fabry (1867-1945)
and Henri Buisson (1873-1944) discovered and began
to quantify the thickness of the stratospheric ozone
layer.
About 90 percent of all ozone molecules in the atmo-
sphere reside in the stratosphere; most of the remaining
ozone molecules reside in the troposphere. Whereas
ozone molecules near the surface harm humans, ani-
mals, plants, trees, and structures, the same ozone
molecules, whether in the stratosphere or in polluted
air, shield the Earth from harmful UV radiation.
A common measure of the quantity of ozone in the air
is the ozone column abundance, which is the number
of ozone molecules above a square centimeter of the
ground, summed to the top of the atmosphere. When
this number is divided by 2.7 10 16 ,theresult is the ×