The stratospheric ozone layer is not a completely uniform stratum, nor does it
occur at the same altitude around the globe. It lies closest to the Earth over the
poles and rises to maximum altitude over the equator. In the stratosphere, ozone is
continuously being made and destroyed by natural processes. During the day the
Sun breaks down some of the oxygen molecules to single oxygen atoms, and these,
reacting with the oxygen molecules that have not been dissociated form ozone.
However, the sunlight also breaks down ozone by converting some of it back to
normal oxygen. In addition, naturally occurring nitrogen oxides enter into the cycle
and speed the breakdown reactions. The amount of ozone present at any one time is
the balance between the processes that create it and those that destroy it.
Since the splitting of the oxygen molecules depends directly upon the
intensity of solar radiation, the greatest rate of ozone production occurs over the
tropics. However, ozone is also destroyed most rapidly there, and wind circulation
patterns carry the ozone-enriched upper layers of the atmosphere away from the
equator. It turns out that the largest total ozone amounts are found at high
latitudes. On a typical day the amount of ozone over Minnesota, for example, is 30
percent greater than the amount over Texas, 900 miles farther south. The density
and altitude of the ozone layer also change with the seasons, the weather, and the
amount of solar activity. Nevertheless, at any one place above the Earth’s surface,
the long-term averages maintained by natural processes are believed to be
reasonably constant.
The amount of ozone near the Earth is only a small percent of the amount in
the stratosphere, and exchange of molecules between the ozone layer and the air at
ground level is thought to be relatively small. Furthermore, the ozone molecule is so
unstable that only a tiny fraction of ground-level ozone could survive the long trip
to the stratosphere, so the ozone layer will not be replenished to any significant
degree by the increasing concentrations of ozone that have been detected in recent
years near the earth’s surface. The long-term averages of ozone both near ground
level and in the stratosphere are regulated by continuous processes that are
constantly destroying and creating it in each of these places. This is why scientists
are so concerned about human beings’ injection into the stratosphere of chemicals
like nitrogen oxides, which are catalysts that facilitate the breakdown of ozone. If
the ozone layer is depleted significantly, more ultraviolet radiation would penetrate
to the Earth’s surface and damage many living organisms.

1. The passage suggests that factors contributing to the variation in the amount of ozone above different
areas of the Earth’s surface include which of the following?
I. Some of the ozone found at higher latitudes was produced elsewhere.
II. There is usually a smaller amount of naturally occurring nitrogen oxide over high latitudes.
III. The rate of ozone production over the poles is less than that over the tropics.
(A) II only
(B) III only
(C) I and II only
(D) I and III only
(E) I, II, and III

2. Which of the following best states the central idea of the passage?
(A) Naturally occurring nitrogen oxides as well as those introduced by humans threaten to deplete
the layer of ozone in the stratosphere.
(B) A delicate but reasonably constant balance exists between the natural processes that produce and
those that destroy ozone in the stratosphere.
(C) There is little hope that the increased concentrations of ground-level ozone observed in recent
years can offset any future depletion of stratospheric ozone.
(D) Meteorologically induced changes in the concentration of ozone in the stratosphere tend to cancel
themselves out over a period of time.
(E) Solar radiation not only produces and destroys ozone but also poses a hazard to human life.

3. The processes that determine the amount of ozone in a given portion of the stratosphere most
resemble which of the following?
(A) Automobile emissions and seasonal fog that create a layer of smog over a city
(B) Planting and harvesting activities that produce a crop whose size is always about the same
(C) Withdrawals and deposits made in a bank account whose average balance remains about the
same
(D) Assets and liabilities that determine the net worth of a corporation
(E) High grades and low grades made by a student whose average remains about the same from term
to term

4. According to the passage, which of the following has the least effect on the amount of ozone at a given
location in the upper atmosphere?
(A) Latitude
(B) Weather
(C) Season
(D) Ground-level ozone
(E) Solar activity