Page 193 - Fundamentals of Air Pollution

II. Ozone Holes 159

positive (to increase temperature). For the foregoing five models the range
of ratios was from unity (no feedback) to 1.55 (a fairly strong positive
feedback). All of these models consider two cloud types, stratiform and
convective, but there are differences in the way these are calculated. It can
be concluded that considerable work will be needed before the treatment
of clouds can be considered satisfactory (3). Some arguments have been
made that cloudiness may provide a negative feedback to temperature
increases (2), that is, cause a decrease in temperature. These results indicate
that this is not likely.
Penner (11) has pointed out that short-lifetime constituents of the atmo-
sphere such as nitrogen oxides, carbon monoxide, and nonmethane hydro-
carbons may also play roles related to global warming because of their
chemical relations to the longer-lived greenhouse gases. Also, SO 2 with a
very short life interacts with ozone and other constituents to be converted
to particulate sulfate, which has effects on cloud droplet formation.

II. OZONE HOLES

During the mid-1980s, each September scientists began to observe a
decrease in ozone in the stratosphere over Antarctica. These observations
are referred to as "ozone holes." In order to understand ozone holes, one
needs to know how and why ozone is present in the earth's stratosphere.
Stratospheric ozone is in a dynamic equilibrium with a balance between
the chemical processes of formation and destruction. The primary compo-
nents in this balance are ultraviolet (UV) solar radiation, oxygen molecules
(O 2), and oxygen atoms (O) and may be represented by the following
reactions:

where hv represents a photon with energy dependent on the frequency of
light, v, and M is a molecule of oxygen or nitrogen. The cycle starts with
the photodissociation of O2 to form atomic oxygen O (Eq. 11-1). O atoms
react with O 2 in the presence of a third molecule (O 2 or N 2) to form O 3 (Eq.
11-2). Ozone absorbs UV radiation and can undergo photodissociation to
complete the cycle of formation and destruction (Eq. 11-3). At a given
altitude and latitude a dynamic equilibrium exists with a corresponding
steady-state ozone concentration. This interaction of UV radiation with
oxygen and ozone prevents the penetration of shortwave UV to the earth's
surface. Stratospheric ozone thus provides a UV shield for human life and
biological processes at the earth's surface.