How the Ozone Layer Forms and Protects
Distance Makes the Heart Grow Fonder
one day and the need to
It,s pretty confusing when we hear about the dangers of ozone depletion
when it's miles away from us
reduce ozone the next. ozone gas is extremely helpful -- but only
of ozone gas near the
production
in the ozone layer. When car exhaust and other things cause the
Read Hor'v Ozone Pollutiorl
surface of the earth, it pollutes the air and causes health problems.
Wor:lt.s to learn more.
(uV) Iight. If you
Most ecosystems rely on the ozone to protect them from harmful ultraviolet
wavelengths of light
know much about thl lisht spectrum, you'll remember that the varying
range of light that's
determine the color orilind or rgnt. Llltraviolet light falls outside of the
visible to the human eye, much like microwaves, X-rav and radio waves.
us. UV light from the
When it comes to UV light, what we don't know (or don't see) can hurt
a sunny day' But skin
suu's rays burns our skiriand freckles our noses when we're outside on
skin cancer and
to
lead
can
ut.-irt are the least of our worries. Exposure to uv light
",
cataracts, and can damage the body's itnmune s)'stetn [source: EBA].
UV rays' Ninety percent
Thankfully, the ozone layer protects us from most of the sun's harmful
starting at six to 11 miles
of the atmospheri" ororr" is in the earth's stratosphere -- the altitude
(48.3 kilometers) above
miles
30
(9.6 to 12.7 kilometers) above the earth and extending to about
conducive to the formation
the earth [source: Fahey]. The stratosphere provides a natural setting
the earth'
envelops
of the ozone, *her" gas lorrns a protective layer that completely
gas in what is known as the
Ozone gas forms in the stratosphere when UV sunlight hits oxygen
ozone-oxygen cYcle:
sun hits a
The first stage of this cycle occurs when short-wavelength UV light from_the
bond
oxygen
the
moiecule of o*ygen gas. The light has so much energy that it breaks
process, the
holding the atom"s to[ether, thus creating two oxygen atoms. Through this
a
leaves
still
this
oxygen essentiaily absorbs the short-wavelength UV light, but
ozone comes in'
significant amount of UV light with longer wavelenglhs, which is where
Then, when an
When UV light hits oxygen gas, it breaks it down to two oxygen atoms'
gas'
ozone
oxygen atom meets oxygen gas, it forms
In the second stage, each of the two remaining oxygen atoms will then latch onto two oxygen
gas molecules, creating two separate ozone molecules fsource: Ei1!gy].
Short-wavelength UV light has enough energy to break apart ozone molecules (which are
more volatile and easier to separate than oxygen molecules). Thus, in the third stage of
the cycle, the ozone gas then breaks into one oxygen gas molecule and an oxygen atom,
hence absorbing much of the remaining UV light.
If you're wondering why
these processes "absorb" UV light, it is because they create exothermic
meaning
they
release
heat. Essentially, oxygen and ozone convert UV light to heat.
reactions,
Together, ozone and oxygen gas are effective at absorbing about 98 percent of the harmful UV
light
How could this happen? And how could CFCs be partly responsible? Chemists Mario Molina
and Sherwood Rowland were awarded the Nobel Prize in 1995 for their theories and research
that explained how this might work. Scientists knew that chlorine and bromine are both
substances that can destroy ozone. It turns out that some natural and man-made chemical
compounds containing chlorine and bromine are able to rise up to the stratosphere where the
conditions allow them to react with and destroy ozone. The earth's natural production of these
substances accounts for 17 percent of the chlorine and 30 percent of the bromine in the
stratosphere [source: Fahey].
NASA
When chlorine encounters ozone, chlorine monoxide and an oxygen molecule form (destroying
the ozone). When chlorine monoxide encounters an oxygen atom, the chlorine is released to
wreak havoc on more ozone.
Molina and Sherwood explained that CFCs, which are man-made, gradually rise up into the
ozone layer, where ultraviolet light breaks the compounds apart, which releases chlorine fsource:
Nobel Foundation]. A chlorine atom can steal an oxygen atom from an ozone molecule, creating
oxygen gas and chlorine monoxide (ClO), which effectively destroys the ozone molecule
[source: Chemical Heritage]. But the chlorine atom isn't done yet; a chlorine atom can break
from its oxygen atom and wreak havoc on as many as 10,000 more ozone molecules fsource:
LICS]. From their findings, the chemists projected that after years of unrestrained CFC
production, the ozone would deplete significantly.
When scientists and popular media refer to a "hole" in the ozone, what they really mean is an
area with low DU, or where the vertical column of ozone layer (which, as we found, spans about
25 vertical miles) includes little ozone gas compared to the other areas. In one sense, the ozone
"hole" can be understood as a "thin" area of the layer because if we collected all the ozone in that
vertical area, it would be thinner than other places over the earth. Specifically, scientists are
worried that production of CFCs will lead to a "hole" over Antarctica.
Every year, the ozone levels over Antarctica sink drastically during the Southern Hemisphere's
spring. Scientists believe this began happening in the late 1970s as a result of CFCs. The hole
forms in the Antarctic because cold air becomes trapped there as a result of the polar vortex -strong, circulating winds. The cold temperatures allow the formation of polar stratospheric
clouds (PSCs), or ice clouds. These PSCs are conducive to the breakdown of chlorinecontaining compounds, which are there because of our production of CFCs. This makes the area
especially susceptible to ozone depletion. When sun hits PSCs in early spring, large amounts of
chlorine monoxide form from the substances that contain chlorine. Fortunately, by early summer,
ozone from other areas comes in to help fiil this hole [source: Fahe-vl. But because of our CFC
production, the hole returns each year