2. What is Ozone?
Ozone is a gas made up of three oxygen atoms
(O3
). It occurs naturally in small (trace) amounts
in the upper atmosphere (the stratosphere).
Ozone protects life on Earth from the Sun’s
ultraviolet (UV) radiation. In the lower atmosphere
(the troposphere) near the Earth’s surface, ozone
is created by chemical reactions between air
pollutants from vehicle exhaust, gasoline vapors,
and other emissions. At ground level, high
concentrations of ozone are toxic to people and
plants.
3. What is Stratospheric Ozone?
Ozone is comparatively rare in the atmosphere - there
are only 3 molecules of ozone for every ten million air
molecules. 90% of the planet's ozone is in the "ozone
layer" which exists in the lower level (20-25 kilometres
above sea level) of the stratosphere.
The stratosphere is the region of the atmosphere which
exists between 10 to 50 kilometres above the surface of
the earth. Solar wavelengths in the ultraviolet range
(180-240 nanometers) are absorbed by and break apart
oxygen molecules (which are made of two oxygen
atoms). Some of the resulting unattached pairs of
oxygen atoms then recombine into triplets to form
ozone. A different range of wavelengths of ultraviolet
(290-300 nm) are strongly absorbed by ozone, which
breaks down as a result and reforms into molecular
4. The higher up in the atmosphere, the thinner the air is,
and thus the less oxygen there is to absorb the 180-240
nm ultraviolet to form ozone. This means that ozone
amounts tend to decrease as you go higher. The lower
down in the atmosphere, the more oxygen the
ultraviolet has to pass through to get there, and the
greater the chances are that it has already been
absorbed to create ozone somewhere higher up. This
means that very low down, the ozone concentrations
tend to be lower. From roughly 12 to 30 km, the two
tendencies balance out, and the highest ozone
concentrations are found there, in what is called the
"ozone layer". This is why the ozone layer exists in the
lower part of the stratosphere.
.
5. The lower layer of the atmosphere that immediately
surrounds the earth is called the troposphere.
Stratospheric ozone is a naturally-occurring gas that
filters the sun's ultraviolet (UV) radiation. This is typically
regarded as 'good' ozone since it reduces the harmful
effects of ultraviolet (UV-B) radiation. A diminished ozone
layer allows more radiation to reach the Earth's surface.
Excessive exposure to UV-B at the surface of the earth
has been shown to cause harmful effects in plants and
animals. Absorption of UV-B by ozone in the
stratosphere reduces the amount of UV-B reaching the
earth's surface and also generates heat that plays a role
in maintaining the temperature structure of the
atmosphere.
6. Ozone that occurs in the troposphere is a
much smaller proportion of the total
planetary ozone and is regarded as 'bad'
ozone since it reacts easily with other
Molecules making it highly toxic to living
organisms. Tropospheric ozone is known to
have negative impacts on such things as
crop production, forest growth and human
health. Tropospheric ozone is a key
component of photochemical smog which
are observed in many cities.
7. Stratospheric “good” ozone
Ninety percent of the ozone in the atmosphere sits in the
stratosphere, the layer of atmosphere between about 10
and 50 kilometers altitude. The natural level of ozone in
the stratosphere is a result of a balance between
sunlight that creates ozone and chemical reactions that
destroy it. Ozone is created when the kind of oxygen we
breathe—O2
—is split apart by sunlight into single oxygen
atoms. Single oxygen atoms can re-join to make O2
, or
they can join with O2
molecules to make ozone (O3
).
Ozone is destroyed when it reacts with molecules
containing nitrogen, hydrogen, chlorine, or bromine.
Some of the molecules that destroy ozone occur
naturally, but people have created others.
8. The total mass of ozone in the
atmosphere is about 3 billion metric
tons. That may seem like a lot, but it is
only 0.00006 percent of the
atmosphere. The peak concentration of
ozone occurs at an altitude of roughly
32 kilometers above the surface of the
Earth. At that altitude, ozone
concentration can be as high as 15
parts per million (0.0015 percent).
9. The concentration of ozone varies with altitude. Peak
concentrations, an average of 8 molecules of ozone per
million molecules in the atmosphere, occur between an
altitude of 30 and 35 kilometers.
Ozone in the stratosphere absorbs most of the
ultraviolet radiation from the Sun. Without ozone, the
Sun’s intense UV radiation would sterilize the Earth’s
surface. Ozone screens all of the most energetic, UV-c,
radiation, and most of the UV-b radiation. Ozone only
screens about half of the UV-a radiation. Excessive UV-
b and UV-a radiation can cause sunburn and can lead
to skin cancer and eye damage .
10. Solar ultraviolet radiation is largely
absorbed by the ozone in the
atmosphere—especially the harmful,
high-energy UV-a and UV-b. more
energy.
11. Increased levels of human-produced gases
such as CFCs (chlorofluorocarbons) have
led to increased rates of ozone destruction,
upsetting the natural balance of ozone and
leading to reduced stratospheric ozone
levels. These reduced ozone levels have
increased the amount of harmful ultraviolet
radiation reaching the Earth’s surface.
When scientists talk about the ozone hole,
they are talking about the destruction of
stratospheric, “good,” ozone.
12. Tropospheric “bad” ozone
Although ozone high up in the stratosphere provides a shield to
protect life on Earth, direct contact with ozone is harmful to both
plants and animals (including humans). Ground-level, “bad,” ozone
forms when nitrogen oxide gases from vehicle and industrial
emissions react with volatile organic compounds (carbon-
containing chemicals that evaporate easily into the air, such as
paint thinners). In the troposphere near the Earth’s surface, the
natural concentration of ozone is about 10 parts per billion
(0.000001 percent). According to the Environmental Protection
Agency, exposure to ozone levels of greater than 70 parts per
billion for 8 hours or longer is unhealthy. Such concentrations
occur in or near cities during periods where the atmosphere is
warm and stable. The harmful effects can include throat and lung
irritation or aggravation of asthma or emphysema.
16. The polar winter leads to the formation of the polar vortex which
isolates the air within it. Cold temperatures form inside the
vortex; cold Enough for the formation of Polar Stratospheric
Clouds (PSCs). As the vortex air is isolated, the cold temperatures
and the PSCs persist.
18. Stratospheric Ozone Chemistry
Polar Stratospheric Clouds, the rest of the story
Recall the termination steps for the Chlorine catalytic
cycle:
ClO + NO2 + M →ClONO2 + M
Cl + CH4 →HCl + CH3
Once the PSCs form, heterogeneous reactions take place
and convert the inactive chlorine reservoirs to more
active forms of chlorine.
HCl + ClONO2→ HNO3 + Cl2
ClONO2 + H2O →HNO3 + HOCl
HCl + HOCl →H2O + Cl2
N2O5 + HCl→ HNO3 + ClONO
N2O5 + H2O→2 HNO3
19. No ozone loss occurs until sunlight returns
to the air inside the polar vortex and allows
the production of active chlorine and
initiates the catalytic ozone destruction
cycles. Ozone loss is rapid. The ozone hole
currently covers a geographic region a little
bigger than Antarctica and extends nearly
10km in altitude in the lower stratosphere.
20. .
PROOF OF PSC ROLE:
MT. PINATUBO ERUPTION
The largest eruption
of the 20th Century
20 million tons of SO2
emitted into the
stratosphere, which rapidly
forms H2SO4 aerosols
