1
Ozone – In Nature, Laboratory and
Environment
K. Chandrasekharan
1.1 OZONE IN NATURE
Gaseous ozone, formed photochemically in the earth’s atmosphere by radiation from the sun, is
a normal constituent of the earth’s atmosphere which is important in shielding the earth from
cancer-causing ultraviolet radiation emitted by the sun. Large scale generation of ozone is
extremely important commercially, however, due to its strong oxidizing abilities.
O3(g) + 2H+ + 2e– <====> O2(g) +H2O {Eo = 2.075 V}
High in the atmosphere the ozone layer is created by the action of the harmful ultraviolet
radiation from the sun on the oxygen atoms. At ground level the easily identified fresh smell in
the air following a thunderstorm is ozone created by the effect of the lightning discharge on the
oxygen atoms. Ozone is also created naturally as a result of photochemical reactions due to the
pollutants such as NOX and SOX that are found in the atmospheres of cities and industrial
areas.
The word ozone comes from the Greek word “ozein” which means “to smell” since ozone
was first noticed because of its characteristic pungent odor (1). The odor is detectable in air at
levels of about 0.1 parts per million, and exposure to ozone
becomes fatal to humans at around levels of 100 ppm for
10,000 minutes or 10,000 ppm for 30 seconds. Ozone, O3, is a
blue-colored gas at ambient temperatures, but this color is not
1.278 Å
noticed at the low concentrations at which it is usually
generated. In the liquid and solid states, ozone is dark blue. [–]
[–]
116.8°
Liquid ozone boils at –111.3 °C and solid ozone melts at
–192.5 °C. Ozone, which is toxic, is an unstable gas and an Fig. 1.1 (a) The structure of Ozone
explosive liquid. The ozone molecule is a bent molecule with showing bond angle and length.
an O-O bond length of 1.278 Å and a bond angle of 116.8° as
shown in the diagram.
The ozone molecule is usually represented by two resonance structures in the top of Fig. 1.1.
In reality the two terminal oxygen atoms are equivalent and the hybrid structure is drawn on the
+
2
OZONE REACTOR TECHNOLOGY
right with a charge of –1/2 on both oxygen atoms and partial double bonds. Comparison of
other structures are also shown below: (Also see Ch. 4 Fig. 4.1)
+
+
–
–
+
—
—
—
–1/2
–1/2
—
—
—
+
H2C —
— C — CH2
H
+
H2C — C —
— CH2
H
—
—
—
H2C
+
CH
CH2
Fig. 1.1 (b) Examples of resonance - ozone, benzene and the alkyl cation.
The Formation of Ozone in the atmosphere is considered by J.H. Seinfeld and others based on the
chemical reactions involving volatile organic compounds oxides of Nitrogen and biogenic
hydrocarbons. Oxides of nitrogen are two: the Nitric oxide and the Nitrogen peroxide – NO and
NO2. The other Nitrous oxide (N2O) though produced in the troposphere by Biological processes
on inorganic fertilizer decomposition is unert to the radiation available there. However in the
upper layer or stratosphere, the ultra violet light available is able mat leads to decomposition of
the N2O and products thereby Ozone to decompose.
The Sun emits radiations of varying wavelengths known as the electromagnetic spectrum.
Ultraviolet radiation is one form of radiant energy coming out from the Sun. The various forms of
energy, or radiation, are classified according to wavelength (measured in nanometres where one
nm is a millionth of a millimetre). The shorter the wavelength, the more energetic the radiation. In
order of decreasing energy, the principal forms of radiation are gamma rays, x-rays, ultraviolet
radiation (UV) rays, visible light, infrared rays, microwaves, and radio waves. Ultraviolet
radiation, which is invisible, is so named because of its wavelengths are less than those of visible
violet radiations.
The total overhead amount of atmospheric ozone at any location is usually expressed in
terms of Dobson units (DU); one Dobson unit is equivalent to 0.01mm thickness of pure ozone at
the density it would possess if it were brought to ground level pressure (1atm) and 0°C
temperature. The normal amount of overhead ozone at temperate latitudes is about 350DU, so if
all the ozone were to be brought down to ground level, the layer of pure ozone would be only
3.5mm thick.
The formation of ozone from oxygen exposed to UV light at 140-190 nm was first reported by
Lenard in 1900 and fully assessed by Goldstein in 1903. It was soon recognized that the active
wavelengths for technical generation are below 200 nm.
Of these, UV-B and C being highly energetic are dangerous to life on earth. UV-A being less
energetic is not dangerous. Fortunately, UV-C is absorbed strongly by oxygen and also by ozone
in the upper atmosphere. UV–B is also absorbed by ozone layer in the Stratosphere and only
OZONE - IN NATURE, LABORATORY AND ENVIRONMENT
3
2-3% of it reaches the earth’s surface. The ozone layer, therefore, is highly beneficial to plant and
animal life on earth by filtering out the dangerous part of sun’s radiation and allowing only the
beneficial part to reach earth. Any disturbance or depletion of this layer would result in an
increase of UV–B and UV–C radiation reaching the earth’s surface leading to dangerous
consequences. Detailed discussion on stratosphers ozone and various reactions taking place
there may be found in Environmental Chemistry by P.S. Sindhu (8).
Ozone absorbs ultraviolet radiation in three main regions known as the Hartley band, the
Huggins band and the Chappuis bands. The Hartley band is a wide bell-shaped peak from 200310nm. The Huggins band is a structured region from 310-350nm, and the Chappuis bands are
found between 450 and 850nm. Therefore ozone absorbs all of the sun’s UV light in the UV-C
region (200-280nm), however it can only absorb a fraction of the sunlight in the UV-B region
(280-320nm), so 10% to 30% (depending on latitude), penetrates to the Earth’s surface. Radiation
in the UV-A range (320-400nm) is the least biologically harmful type of ultraviolet light. This is
not absorbed by ozone and therefore penetrates to the Earth’s surface.
1.2 OZONE LAYER IN STRATOSPHERE
The several regions of the atmosphere having different characteristics are as under: (Fig.1.2)
1. Troposphere: This lowest region (0-15km) has a decreasing temperature with altitude.
This region is having direct contact with the urban sources and sinks of chemicals. Wind
velocity and surface friction influence their mixing.
2. Stratosphere: (<50km) After the troposphere, there is small gap of 10-15km called the
Tropopause. Beyond this pause region, the temperature increases with altitude in the
stratosphere, due to UV light absorbed from sun’s rays. 210-270K is the range of
temperatures in stratosphere.
3. Mesosphere: Here the region is 50–85km, and temperature is decreasing with altitude.
There is less ozone there to absorb the UV radiation and so the temperature is
decreasing. The mixing of any gases here could be rapid.
4. The mesopause is the region above 85km but within 90 km. There the temperature is the
lowest (160K).
5. The thermosphere is the region where altitudes are over 90km and there the temperature
rises above 160K to larger values (below 270K any way).
~0%
– 1000
– 200
– 100
– 50
Stratopause
1% 10% -
Cirrus clouds
Stratosphere
Tropopause
– 10
–5
50% Cumulus rain
clouds
100%
– 20
Mt.
Everest
–2
–1
–0
Fig. 1.2 The atmosphere regions and altitudes and air percentage.
Altitude (km)
Percent of atmosphere
– 500
4
OZONE REACTOR TECHNOLOGY
The ozone layer refers to the ozone within stratosphere, where over 90% of the earth’s ozone
resides. Ozone is an irritating, corrosive, colorless gas with a smell something like burning
electrical wiring. In fact, ozone is easily produced by any high-voltage electrical arc (spark plugs,
Van de Graaff generators, Tesla coils, Xerox machines, arc welders). Each molecule of ozone has
three oxygen atoms and is produced when oxygen molecules (O2) are broken up by energetic
electrons or high energy radiation. Variations in temperature and pressure divide the earths
atmosphere into layers, shown below, and mixing of gases between the layers happens very
slowly.
The altitudes on the diagram are logarithmic. This fact might give us a better idea of the
relative thicknesses of these layers.
Notice that the lowest 10% of the atmosphere holds 90% of the air. This is because gases are
compressible. In a huge pile of feathers the bottom-most feathers become compressed under the
weight of the feathers above them. Likewise the lower levels of the atmosphere are filled with
compressed air while the upper levels, such as the stratosphere, contain very ‘thin’
uncompressed air. Although the stratosphere layer is over four times thicker than the lower
atmosphere, the stratosphere holds so little gas that ozone is still considered one of the minor
trace-gases of the overall atmosphere. Its maximum concentration, at a height of about 20-25
kilometers, is only ten parts per million.
The ozone layer absorbs 97-99% of the sun’s high frequency ultraviolet light, light which is
potentially damaging to life on earth (Fig. 1.4). Every 1% decrease in the earths ozone shield is
projected to increases in the amount of UV light exposure to the lower atmosphere by 2%.
Because this would cause more ozone to form in the lower atmosphere, it is uncertain how much
of UV light would actually reach the earth’s surface. Recent UV measurements from around the
northern hemisphere indicate small UV increases in rural areas and almost no increase in areas
near large cities.
80 –
Mesosphere
Altitude
60 –
80 km
Stratosphere
Mesosphere
-
40 –
50 km 20 –
-Stratosphere
Troposphere
20 –
10 km Troposphere
10+2
0
50
Ozone partial
100 150 Pressure
Temperature
Fig. 1.3 Showing temperature versus altitude and the regions called Troposphere,
Stratosphere and Mesosphere.
OZONE - IN NATURE, LABORATORY AND ENVIRONMENT
UV–B
UV–C
10
o(cm 2 molecule –1)
10
10
10
10
10
10
10
5
UV–A
–17
–18
O3
–19
Window at
185-210 mm
–20
Schumann
Range
bands
–21
–22
O2
–23
–24
120
160
200
240
280
320
360
Wavelength (nm)
Fig. 1.4. (a) Showing the intensity of sun’s U.V. radiation in space and on earth. Also shows the UV ranges
A, B and C. The insert figure is the contour of ozone level. The ozone hole is shown.
UV-C
UV-B
Ultraviolet radiaton
Intensity of
the sun's radiation
X-Rays
Visible
light
UV-A
Intensity in space
Intensity on earth
l
l
l
l
100
200
300
400
UNEP Scientific Assessment of Ozone
Depletion 1994
The Ozone layer is the Earth's sunscreen
Wavelength (nanometres)
Fig. 1.4. (b) Shows intensity on earth shown.
At any given time, ozone molecules constantly formed and destroyed in the stratosphere. The
total amount, however, remains relatively stable. The concentration of the ozone layer can be
thought of as a stream’s depth at a particular location. Although water is constantly flowing in
and out, the depth remains constant.
While ozone concentrations vary naturally with sunspots, the seasons, and latitude, these
processes are well understood and predictable. Scientists have established records spanning
several decades that detail normal ozone levels during these natural cycles. Each natural
6
OZONE REACTOR TECHNOLOGY
reduction in ozone levels has been followed by a recovery. Recently, however, convincing
scientific evidence has shown that the ozone shield is being depleted well beyond changes due
to natural processes. Ozone depletion occurs when the natural balance between the production
and destruction of stratospheric ozone is tipped in favor of destruction. An upset in this balance
can have serious consequences for life on the Earth, and scientists are finding evidence that the
balance has really changed.
In the stratosphere most of the oxygen exists as O2 rather than atomic oxygen. Because of
these relative concentrations the most likely fate of any oxygen atoms produced by photochemical
decomposition of O2 is their subsequent collision with diatomic oxygen molecules. This results in
the production of ozone, and is the source of all the ozone in the stratosphere. Ozone in the
stratosphere is produced in a cycle. The formation of ozone requires UV-C photons from sunlight,
and during daylight ozone is constantly being formed by this process. The heat generated by this
reaction in the stratosphere leads to a temperature inversion in the atmosphere, where the air at
a given altitude is warmer than that below it. The characteristic of ozone that makes it so
valuable, it’s ability to absorb ultraviolet radiation, also leads to it’s destruction. Ozone efficiently
absorbs UV light with wavelengths shorter than 320nm, and the excited state produced by this
absorption undergoes a dissociation reaction. The oxygen atom and O2 molecule produced by
this dissociation may be in a number of different electronically excited states.
Thus ozone is responsible for absorbing ultraviolet (UV) radiation from the sun. In the
absence of the ozone, more UV radiation is allowed to reach the Earth. This UV radiation is very
damaging to the skin.
The O* atom in the cycle refers to an oxygen atom that is said to be “electronically
excited”. As one can see from the ozone cycle, any substances other than oxygen that combine
with O* will remove the O* from the ozone cycle and decrease the volume of ozone being
produced. When this happens, more UV radiation passes through the ozone and is allowed to
reach the Earth’s surface.
The stratospheric ozone layer shields life on Earth from the Sun’s harmful ultraviolet
radiation. Chemicals that destroy ozone are formed by industrial and natural processes. With the
exception of volcanic injection and aircraft exhaust, these chemicals are carried up into the
stratosphere by strong upward-moving air currents in the tropics. Methane (CH4),
chlorofluorocarbons (CFCs), nitrous oxide (N2O) and water are injected into the stratosphere
through towering tropical cumulus clouds. These compounds are broken down by the ultraviolet
radiation in the stratosphere. Byproducts of the breakdown of these chemicals form “radicals”—
such as nitrogen dioxide (NO2) and chlorine monoxide (ClO)—that play an active role in ozone
destruction. Aerosols and clouds can accelerate ozone loss through reactions on cloud surfaces.
Thus, volcanic clouds and polar stratospheric clouds can indirectly contribute to ozone loss.
It is necessary therefore to determine definitely why the levels of the gases (CH4, N2O, O3,
CFCl3, CF2Cl2) have an increasing trend as well as how their radiation effects could change
climate globally.
They make measurements by flask samples and also satellite remote observations through
spectroscopy.
Methane gas emits from rice paddy; but the quantum of such gas is in parts per trillion (ppt)
and about 10 teragrams per annum would be the calculated value based on field measurements.
However, what we actually observe in the atmosphere a global emission of CH4 of 500 teragrams
per annum. Hence the rice paddy has but a 2% contribution to this value. Mathews and Fung
(1991) have worked to find methane emission from natural wet lands.
OZONE - IN NATURE, LABORATORY AND ENVIRONMENT
7
Concentration of Ozone within the protective ozone shield is decreasing, while levels in the
air we breathe are increasing. When very stable man-made chemicals containing chlorine and
bromine enter into the atmosphere, and reach the stratosphere, these chemicals are broken down
by the high energy solar UV radiation and release extremely reactive chlorine or bromine atoms.
These undergo a complex series of catalytic reactions leading to destruction of ozone.
The formation of ozone requires UV-C photons from sunlight, and during daylight ozone is
constantly being formed by this process. The heat generated by this reaction in the stratosphere
leads to a temperature inversion in the atmosphere, where the air at a given altitude is warmer
than that below it. There is an anti-correlation between temperature and ozone concentration. In
a similar manner, there are seasonal variations in upper stratospheric ozone concentrations that
are driven by the seasonal change in temperature.
The driving force in creating seasonal temperature variations at the surface and throughout
the atmosphere, including the upper stratosphere, is the seasonal change in the elevation angle
of the sun. The mechanisms differ, though, depending on altitude and latitude.
Earth’s surface absorbs the shortwave visible radiation from the sun that passes through the
atmosphere mostly uninterrupted. This energy is then reemitted as thermal long wave (infrared)
radiation. The lower atmosphere is warmed in turn by the absorption and reemission of this
infrared (IR) radiation. The progressive upward reemission of IR radiation warms the
atmosphere, but as air density drops off, the efficiency of heat transfer by reemission decreases,
so less warming occurs and temperature falls off with altitude. This fall off in temperature is
called the lapse rate and depending on the particular value of it, the atmosphere is said to be
stable, neutral, or unstable. Convective instability occurs when the lapse rate is especially steep
(so that the potential temperature falls off with height), that turbulent mixing results as large air
parcels, overturn and mix.
In the upper stratosphere, absorption of solar UV radiation by ozone itself provides the
source of heating to the atmosphere. In summer, when the sun is nearly overhead, this heating is
maximized, leading to a temperature maximum. The opposite occurs in winter when the sun is
much lower in the sky. The maximum temperature occurs near the summer solstice (June 21 or
day number 172) and the minimum temperature occurs near the winter solstice (December 21).
Ozone amount is inversely related to the temperature: it reaches a minimum when the
temperature is at a maximum, and it reaches a maximum when the temperature is at a minimum.
This inverse relationship is due simply to the photochemical loss reactions, which are
temperature dependent decreasing as temperature increases.
Radiative Equilibrium: Another aspect of upper stratospheric warming due to ozone is the
radiative equilibrium temperature. Radiative equilibrium is established when the temperature of
an air parcel is determined by a balance between heating from absorption of solar energy and
cooling to space. It is also determined by any heating from adjacent warmer air parcels and
cooling to adjacent cooler air parcels. In the case of the upper stratosphere, the heating is
primarily from absorption of UV radiation by ozone. The cooling occurs through infrared
emission by carbon dioxide with a small contribution from water vapor. The cooling by carbon
dioxide at these altitudes is through infrared radiation remission to space. Interestingly, this
means that as carbon dioxide increases and the surface is expected to warm through the
atmospheric greenhouse effect, the stratosphere is actually expected to cool.
The presence of ozone results in a maximum temperature that is 5-10K cooler than the
corresponding radiative equilibrium temperature, while the minimum temperature is 5-10K warmer.
Mid Altitude Ozone Variability: In addition to displaying altitude variations, ozone varies by
latitude. The variations in latitude arise from the fact that ozone is created in the tropics, whence
8
OZONE REACTOR TECHNOLOGY
it undergoes an equator to pole circulation. The circulation is driven by temperature differences.
In the hemisphere where it is winter, bigger temperature differences lead to a stronger ozone
circulation. This seasonal equator-to-pole circulation is known as the Brewer-Dobson circulation.
In the mid-altitudes ozone in the upper stratosphere has a regular seasonal variation with a
minimum in the early summer and a maximum in the early winter. The opposite of that for
temperature. This is associated with the same temperature dependent chemistry in this inverse
relationship. The magnitude of the seasonal variation of ozone and temperature in the upper
stratosphere is made somewhat smaller than it would otherwise be because of a negative
feedback effect. In summer when the temperature is maximum, less ozone is available to absorb
solar radiation.
Equatorial Ozone Variability: Equatorial latitude seasonal variations of ozone in the upper
stratosphere are more complicated. The Sun passes overhead twice during the year at the
equator. This would lead us to expect a semiannual oscillation (two peaks during the year) in the
temperature and hence in ozone amounts. However, the semiannual oscillation of tropical
temperatures is not symmetrical and the timing is not quite in phase with the passage of the Sun.
This is an indication of the importance of stratospheric dynamics in modifying the temperatures
which would occur if the atmosphere were in simple radiative equilibrium.
Chapman Reactions: Sidney Chapman produced a mechanism after his name in 1930, where at
wavelengths shorter than 242nm, oxygen is the absorber which gets dissociated into single atom
O and Ozone O3. M can be another substance or catalyst.
O2 + hν (λ<242nm) → O + O
…(1.1)
O + O2 + M
→ O3 + M
…(1.2)
O3 + hν
→ O + O2
…(1.3)
O + O3
→ 2O2
…(1.4)
Volatile organic compounds present are benzene, ethane, isobutane, propene, 1,2,4 trimethyl
benzene, formaldehyde, cyclopropane and so on. While methane is having no reaction, these
compounds are called as “Reactive organic Gases”.
Simple reactions like Nitrogen peroxide decomposing under UV radiation to yield Nitric Oxide
and oxygen:
…(1.5 a)
NO2 + hν → NO + O
The Oxygen can combine in presence of another substance “M” to form ozone
O2 + O + M → O3 + M
…(1.5 b)
The reverse reaction is also present recombining with Nitric oxide.
O3 + NO → O2 + NO2
…(1.5 c)
The above three equations contain NO, NO2 and Ozone. How much ozone will be present at any
moment depends on the incident radiation in (a) and the reverse reaction (c) depends on the ratio
of NO to NO2. When NO2 is more, the reverse reaction is slowed down. Then, if radiation hv
increases, equation (a) produces more of atomic oxygen and thereby the reaction (b) yields more
ozone. This is Philip Leighton’s contribution giving the value of ozone concentration as
[O3] = ka/kc .[NO2]/[NO]
where the suffices a and c denote the reactions (1.5a) and (1.5c) above.
OZONE - IN NATURE, LABORATORY AND ENVIRONMENT
9
1.3 CHEMICAL FAMILIES CAUSING CATALYTIC CYCLES WITH OZONE
The three following are the families which are responsible:
1. Odd Nitrogen
2. Odd Hydrogen
3. Odd Chlorine.
Ozone is destroyed by reactions with chlorine, bromine, nitrogen and hydrogen gases. Reactions
with these gases typically occurs through catalytic processes. A catalytic reaction cycle is a set of
chemical reactions which result in the destruction of many ozone molecules while the molecule
that started the reaction is re-formed to continue the process.
Out of the several possible reactions with the above, let us only describe the most important
ones involving chlorine. Because of catalytic reactions, an individual chlorine atom can, on
average, destroy nearly a thousand ozone molecules before it is converted into a form harmless to
ozone. When ultraviolet light waves (UV) strike CFC* (CFCl3) molecules in the upper atmosphere,
a carbon-chlorine bond breaks, producing a chlorine (Cl) atom. The chlorine atom then reacts
with an ozone (O3) molecule breaking it apart and so destroying the ozone. This forms an
ordinary oxygen molecule(O2) and a chlorine monoxide (ClO) molecule. Then a free oxygen**
atom breaks up the chlorine monoxide. The chlorine is free to repeat the process of destroying
more ozone molecules. A single CFC molecule can destroy 100,000 ozone molecules.
* CFC - chlorofluorocarbon: it contains chlorine, fluorine and carbon atoms.
** UV radiation breaks oxygen molecules (O2) into single oxygen atoms.
CFCl3 + UV Light ==> CFCl2 + Cl
...(1.6)
...(1.7)
Cl + O3 ==> ClO + O2
...(1.8)
ClO + O ==> Cl + O2
The free chlorine atom is then free to attack another ozone molecule
Cl + O3 ==> ClO + O2
...(1.9)
...(1.10)
ClO + O ==> Cl + O2
and again …
Cl + O3 ==> ClO + O2
...(1.11)
ClO + O ==> Cl + O2
...(1.12)
and again... for thousands of times (Fig. 1.5).
Fig. 1.5 Ozone cycle for its depletion.
1.4 THE OZONE LAYER DEPLETION
In the lowest portion of the atomosphere, ozone represents less than one part in 100 million.
The concentration of ozone in the stratosphere is up to 10 ppm: this corresponds to about 12
ozone molecules per million air molecules with a maximum ozone concentration between 20 and
27 kilometers, which corresponds to absolute values on the order of 2.5–5.1×1012 molecules per
cubic centimeter, or relative values of 4–8 parts per million (ppm), corresponding to 4–8
molecules of ozone for one million air molecules.
OZONE REACTOR TECHNOLOGY
The radiation breaks down oxygen molecules, releasing free atoms, some of which bond with
other oxygen molecules to form ozone. About 90 per cent of all ozone formed in this way lies
between 15 and 55 kilometers above the Earth’s surface – the part of the atmosphere called the
stratosphere. Hence, this is known as the ‘ozone layer’. Even in the ozone layer, ozone is present
in very small quantities; its maximum concentration, at a height of about 20-25 kilometers, is only
ten parts per million.
Ozone is an unstable molecule. High-energy radiation from the Sun not only creates it, but
also breaks it down again, recreating molecular oxygen and free oxygen atoms. The concentration
of ozone in the atmosphere depends on a dynamic balance between how fast it is created and
how fast it is destroyed.
Stratospheric air temperatures in both polar (at stratosphere levels) regions reach minimum
values in the lower stratosphere in the winter season. Average minimum values over Antarctica
are as low as –90°C in July and August in a typical year. Over the Arctic, average minimum
values are near –80°C in January and February. Polar stratospheric clouds (PSCs) are formed
when winter minimum temperatures fall below the formation temperature (about –78°C). This
occurs on average for 1 to 2 months over the Arctic and 5 to 6 months over Antarctica (see heavy
red and blue lines). Reactions on PSCs cause the highly reactive chlorine gas ClO to be formed,
which increases the destruction of ozone. The range of winter minimum temperatures found in
the Arctic is much greater than in the Antarctic. In some years, PSC formation temperatures are
not reached in the Arctic, and significant ozone depletion does not occur. In the Antarctic, PSCs
are present for many months, and severe ozone depletion now occurs Fig. 1.6.
As the stratospheric O3 concentration decreases, so does the protection it offers from harmful
ultraviolet radiation. Annual polar ozone depletion has been well documented, particularly in
the southern hemisphere. As the polar depletion worsens annually, regions of lower latitude are
increasingly affected, in both the southern and northern hemispheres, resulting in a growing
Minimum air temperatures in the polar lower stratosphere
Nov
Dec
Arctic winter
Jan
Feb
March
April
–50
–60
–55
Average winter values
Arctic 1978-79 to 2005-06
Antarctic 1979 to 2005
Temperature (degrees celsius)
–60
–65
–70
–80
–90
–70
–100
Arctic
–75
–110
–80
–120
–85
Antarctic
Temperature (degrees fahrenheit)
40° to 90° Latitude
Range of values
PSC formation temperature
–130
–90
–140
–95
May
June
July
August
Antarctic winter
Sep
Oct
Fig. 1.6 Showing temperature profile in the earth regions causal to Ozone in each winter season.