Page 225 - Materials Chemistry, Second Edition

10 Life Cycle Impact Assessment 211

10.7.2 Environmental Mechanism

Stratospheric ozone concentrations result from a balance between O 3 formation and
destruction under the influence of solar (UV) radiation, temperature and the pres-
ence of other chemicals. The annual cycle of ozone destruction over the poles
develops under the presence of several influencing factors with its intensity directly
depending on their combined intensity: (1) meteorological factors (i.e. strong
stratospheric winds and low temperature) and (2) the presence of ozone depleting
chemicals.
Meteorological factors involve the formation of the “polar vortex”, a circum-
polar stratospheric wind phenomenon, in the polar night during the polar winter,
when almost no sunlight reaches the pole. This vortex isolates the air in polar
latitudes from the rest of Earth’s atmosphere, preventing ozone and other molecules
from entering. As the darkness continues, the air inside the polar vortex gets very
cold, with temperatures dropping below −80 °C. At such temperatures a special
type of clouds, called Polar Stratospheric Clouds (PSC), begins to form. Unlike
tropospheric clouds, these are not primarily constituted of water droplets, but of
tri-hydrated nitric acid particles, which can form larger ice particles containing
dissolved nitric acid in their core as temperature continues to drop. The presence of
PSC is crucial for the accelerated ozone depletion over the polar regions because
they provide a solid phase in the otherwise extremely clean stratospheric air on
which the ozone-degrading processes occur much more efficiently.
Chemical factors involve the presence of chlorine and bromine compounds in
the atmosphere as important contributors to the destruction of ozone. The majority
of the chlorine compounds and half of the bromine compounds that reach the
stratosphere stem from human activities.
Due to their extreme stability, chlorofluorocarbons (CFCs) are not degraded in
the troposphere but slowly (over years) transported into the stratosphere. Here, they
are broken down into reactive chlorine radicals under the influence of the very
energy-rich UV radiation at the upper layers of the ozone layer. One chlorine atom
can destroy very high numbers of ozone molecules, before it is eventually inacti-
vated through reaction with nitrogen oxides or methane present in the stratosphere.
The degradation and inactivation scheme is illustrated in a simplified form for a
CFC molecule in Fig. 10.11.
When they are isolated in the polar vortex and in the presence of PSC, these
stable chlorine and bromine forms come into contact with heterogeneous phases
(gas/liquid or gas/solid) on the surface of the particles forming the PSC, which
breaks them down and release the activated free chlorine and bromine, known as
“active” ozone depleting substances (ODS). These reactions are very fast and, as
explained, strongly enhanced by the presence of PSC, a phenomenon which was
neglected before the discovery of the ‘ozone hole’.
While this describes the fate mechanism leading to stratospheric ozone reduc-
tion, Fig. 10.12 shows the impact pathway leading to ozone depletion in the