Q.4
Section I: OZONE IN OUR ATMOSPHERE
Q1
Ozone is a gas that is naturally present in our atmosphere. Each ozone molecule contains three atoms of oxygen and is
denoted chemically as O3. Ozone is found primarily in two regions of the atmosphere. About 10% of atmospheric ozone is in
the troposphere, the region closest to Earth (from the surface to about 10–16 kilometers (6–10 miles)). The remaining ozone
(about 90%) resides in the stratosphere between the top of the troposphere and about 50 kilometers (31 miles) altitude. The
large amount of ozone in the stratosphere is often referred to as the “ozone layer.”
What is ozone and where is it in the atmosphere?
Ozone is a gas that is natu rally present in our atmos phere.
Ozone has the chemical formula O3 because an ozone
molecule contains three oxygen atoms (see Figure Q1-1).
Ozone was discovered in laboratory experiments in the mid-
1800s. Ozone’s presence in the atmosphere was later discov-
ered using chemical and optical measurement methods. The
word ozone is derived from the Greek word óζειν (oz ein),
meaning “to smell.” Ozone has a pungent odor that allows it
to be detected even at very low amounts . Ozone reacts rapidly
with many chemical compounds and is explosive in concen-
trated amounts. Electrical discharges are generally used to
produce ozone for industrial processes such as air and water
purification and bleaching of textiles and food products.
Ozone location. Most ozone (about 90%) is found in
the stratosphere, which begins about 10–16 kilometers (6–10
miles) above Earth ’s surface and extends up to about 50 kilo-
meters (31 miles) altitude. The stratospheric region with
the highest ozone concentration is commonly known as the
“ozone layer” (see Figure Q1-2). The ozone layer extends over
the entire globe w ith some variation in altitude and t hickness.
The remaining ozone, about 10%, is found in the troposphere,
which is the lowest region of the atmosphere, between Eart h’s
surface and the stratosphere.
Ozone abundance. Ozone molecules have a low relative
abundance in the atmosphere. In the stratosphere near the
peak concentration of the oz one layer, there are ty pically a few
thousand ozone molecules for every billion air molecules (1
billion = 1,000 million). Most air molecules are eit her oxygen
(O2) or nitrogen (N2) molecules. In the troposphere near
Earth’s surface, ozone is even less abundant, with a typical
range of 20 to 100 ozone molecules for each billion air mole-
cules. The highest surface values result when ozone is formed
in air polluted by human activities.
As an illustration of the low relative abundance of ozone
in our atmosphere, one can imagine bringing all the ozone
mole cules in the troposphere and stratosphere down to
Earth’s surface and uniformly distributing these molecules
into a layer of gas extending over the globe. The resulting
layer of pure ozone would have an average thickness of about
three millimeters (about one-tenth inch) (see Q4). Nonethe-
less, this extremely small fraction of the atmosphere plays a
vital role in protecting life on Earth (see Q3).
Figure Q1-1. Ozone and oxygen. A molecule of ozone
(O3) contains three oxygen (O) atoms bound together.
Oxygen molecules (O2), which constitute 21% of the
gases in Earth’s atmosphere, contain two oxygen atoms
bound together.
Oxygen
atom (O)
Oxygen
molecule (O2)
Ozone
molecule (O3)
Ozone and Oxygen
