Regions of Earth’s atmosphere:
Scientists divide the atmosphere into several different layers according to temperature variation and composition.
As far as visible events are concerned, the Troposphere is the most active layer (see fig.1).
All the dramatic events of weather-rain, lightning, hurricanes-occur in this region. It is the thinnest layer of the
atmosphere (10 km); yet it contains about 80 % of the total mass of air and practically all of the atmosphere’s water vapor.
Temperature decreases almost linearly with increasing altitude in this region. Above the troposphere is the
stratosphere, which consists of nitrogen, oxygen, and ozone. In the stratosphere, the air temperature
rises with altitude. This warming effect is the result of exothermic reactions triggered by UV radiation from the sun.
One of the products of this reaction sequence is ozone (O3), which, serves to prevent harmful UV rays
from reaching Earth’s surface. The concentration of ozone and other gases in the mesosphere above
the stratosphere is low, and the temperature there decreases with increasing altitude. The upper most layer of the
atmosphere is the thermosphere, which is also known as the ionosphere. The rise in
temperature in this region is the result of the bombardment of molecular oxygen and nitrogen and atomic species by
energetic particles, such as electrons and protons, from the sun. Typical reactions are:
Fig.1 The Earth’s Atmosphere (90kB)
(Gr. ozein, to smell), ozone is an allotropic form of oxygen having three atoms in each molecule, formula
O3. It is a pale blue, highly poisonous gas with a strong odor and it is an irritating, corrosive,
colorless gas with a smell something like burning electrical wiring. Ozone boils at -111.9° C, melts at -192.5° C,
and has a specific gravity of 2.144. Liquid ozone is a deep blue, and a strongly magnetic liquid. It is formed
when an electric spark is passed through oxygen. The presence of ozone causes a detectable odor near electrical
machinery. In fact, ozone is easily produced by any high-voltage electrical arc (spark plugs, Van de Graff generators,
Tesla coils, arc welders, as well as photo-copiers, laser printers, CRT-tubes as used in TV and PC-sets, etc).
The commercial method of preparation consists of passing cold, dry oxygen through a
silent electrical discharge. 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. Ozone is chemically much more
active than ordinary oxygen and is a better oxidizing agent. It is used in purifying water, sterilizing air, and
bleaching certain foods. Ozone formed in the lower troposhere originates from nitrogen oxides and organic gases
emitted by automobiles and industrial sources.
In the stratosphere at an altitude of 10 and 50 kilometers, the following reactions occur (fig.3):
Factors influencing ozone concentrations:
1. Stratospheric sulfate aerosols: large explosive volcanoes are able to place a significant amount
of aerosols into the lower stratosphere, as well as some chlorine. Because more than 90% of a volcanic plume
is water vapor most of the other compounds, including volcanic chlorine, get ''rained-out'' of the stratosphere.
The effects of a large volcano on global weather are nevertheless significant, which in turn can affect localized weather
patterns such as the Antarctic ozone hole. Many observations have linked the 1991 Mt. Pinatubo
eruption to a 20% increase in the ozone hole that following spring. The effects of a large volcanic eruption
on total global ozone are more modest (less than 3%) and last no more than 2-3 years (see fig.9).
Fig.9 Popocatepetl Nov. 10th 2000 (11kB)
2. Stratospheric winds: every 26 months the tropical winds in the lower stratophere change from
easterly to westerly and then back again, an event called the Quasi-biennial Oscillation (QBO).
The QBO causes ozone values at a particular latitude to expand and contract roughly 3%.
Since stratospheric winds move ozone, not destroy it, the loss of one latitude is the gain of another and
globally the effects cancel out.
3. Influences from the hydrosphere: As can be seen in fig.10, the polward movement of air is promoted
by the Hadley-cells in both hemispheres; the load of CFC's is transported along this stratospheric system of
conveyor belts towards the polar regions.
Both wind and water currents in the south polar region circulate
around the antarctic continent (in contrast, the north polar region is not encircled by a circumpolar
water current, because the landmass of Greenland efficiently blocks this pattern).
In the south polar region, this circulating pattern facilitate the formation of a polar vortex (fig.13). This enables
the high pressure system to suck very cold air from the upper layers of the atmosphere into the layers of the
4. Greenhouse gases: to the degree that greenhouse gases might heat the planet and alter weather patterns,
the magnitude of the stratospheric winds will certainly be affected. Some of the more popular scenarios of global
warming predict cooler stratospheric temperatures, leading to more polar stratospheric clouds and more active
chlorine in the area of the Antarctic ozone hole.
Fig.10 Atmospheric and hydrospheric influences aiding in Antarctic ozone depletion (160kB)
Stratospheric ozone has decreased by about 3 % since 1978:
Since measurement of atmospheric composition were first made, measurable decreases in stratospheric
ozone have been recorded. These changes are seasonal being most severe in the winter. They show
extreme variations with latitude: large reductions in the ozone layer over the Antartic were noticeable
as early as 1978. Satellite measurements confirmed that total ozone content in this part of the atmosphere
in 1987 was less than half of its usual value, and by 1994 it had dropped further, to less than one-third
of normal. Localized regions of the Antartic had no ozone layer above them at all:
an ozone hole was observed. Ozone amounts north of the artic Circle sank to 45 % of normal,
the lowest readings so far were recorded in the winter of 1996.
Video of Ozone levels '97/98 (1MB)
Stratospheric clouds, which can form only in the extreme cold of the polar regions,
appeared to correlate with ozone-hole formation (fig.13). Reduction in total ozone above the temperate regions
of the Northern Hemisphere currently average 3 %, approximately the worldwide mean, but 6 % depletion
is measured at 40°N latitude during the winter months. Epidemiological studies suggest 1-3 % increase
in skin cancers over the next decades. As a consequence, considerable effort has been made to identify the
causes of ozone depletion. Are CFC's the only responsible substances? Are other natural or manmade substances
contributing to it as well?
Fig.11 Antarctic O3 distribution (1995 - 110kB)
The Polar Vortex:
During the winter polar night, sunlight does not reach the south pole. A strong circumpolar wind
develops in the middle to lower stratosphere. These strong winds are known as the 'polar vortex'. This has the effect
of isolating the air over the polar region. Since there is no sunlight, the air within the polar vortex can get very cold.
So cold that special clouds can form once the air temperature gets to below about -80C (fig.2). These clouds are called
Polar Stratospheric Clouds (or PSCs for short) but they are not the clouds that you are used to seeing in the sky which
are composed of water droplets. PSCs first form as nitric acid trihydrate. As the temperature gets colder however,
larger droplets of water-ice with nitric acid dissolved in them can form. However, their exact composition is still the
subject of intense scientific scrutiny. These PSCs are crucial for ozone loss to occur. The main long-lived inorganic
carriers (reservoirs) of chlorine are hydrochloric acid (HCl) and chlorine nitrate (ClONO2).
These form from the breakdown products of the CFCs. Dinitrogen pentoxide (N2O5)
is a reservoir of oxides of nitrogen and also plays an important role in the chemistry. Nitric acid (HNO3)
is significant in that it sustains high levels of active chlorine. The central feature of this unusual chemistry is that the
chlorine reservoir species HCl and ClONO2 (and their bromine counterparts) are converted into more
active forms of chlorine on the surface of the polar stratospheric clouds. The most important reactions in the destruction
of ozone (fig.11,13).
Wind speeds around the vortex may reach 100 metres per second. The vortex establishes itself in the middle
to lower stratosphere. It's important because it isolates the very cold air within it.
When the sun rises after the long winter night, its light triggers the wholesale destruction of ozone by
chlorine monoxide. In the late winter and early spring of 1987 and 1991, the loss was as much as 40 %
of the ozone layer.
Fig.11 shows the Ozone distribution of the southern hemisphere for the years of Oct.1980 to 1991. The ozone
levels reached a min. of about 120 Dobson units, far below the 220 Dobson units typically seen over
Antartica before the hole forms.
Fig.12 Antarctic Ozone distribution (1980-91, 70kB)
Fig.13 The polar vortex (120kB)
The Situation in the Northern Hemisphere:
The situation in the North, near the Arctic Circle, was considered to be less severe because its polar
vortex is not as well defined and the Arctic stratosphere is warmer than its Antarctic counterpart.
Studies showed that ozone levels in this region had decline between 4 and 8 % in the past decade.
However, measurements in 1992 revealed a surprising high level of ClO over the northermost parts of
the USA, Canada, and Europe (see fig.14). This evidence implies that a large ozone hole,
like the one over Antarctica, will probably open up near the North pole in the years to come. What caused
this sudden change? The culprint is believed to be the tiny particles and sulfuric acid aerosols from
volcanic eruptions, the most recent of which was at Mount Pinatubo in the Philippines in 1991.
Apparently, these particles can perform the same catalytic function as the ice crystals at the South Pole.
Fig.14 South Pole image (100kB)
Ozone "Microholes" affects Chile and Argentina. The hole in the atmosphere's
ozone layer over Antarctica has spread north into southern Chile and lasts longer each year,
it has spread as far as the south-central Chilean city of Puerto Montt (see fig.14, 16, and video).
Earlier studies showed the ozone hole used to appear for only days or weeks during the southern
hemisphere spring; it now appears in September and lasts until November. That study found unusually
high UV radiation levels in central Chile during the weeks of heaviest ozone depletion in Antarctica.
A huge gap in the Earth’s ozone layer opened over major cities for the first time this month when
the atmosphere’s protective covering against ultra-violet radiation thinned out across the whole
continent of South America, from Santiago de Chile on the Pacific coast to Buenos Aires, the
capital city of Argentina on the Atlantic.
Video of Chilenian Microhole
Global efforts to protect the ozone layer:
50% reduction in CFC production worldwide by 2000 now signed by over 90 nations, revised to require the
virtual phase out of CFC production by 1996.
- In 1978 the USA banned the use of CFCs in hair sprays and other aerosols. International conference
to control these chemicals signed by U.S. and 22 other countries. Limited production and use of CFC's.
- In 1987, an international treaty, The Montreal protocol, was signed by most industrialized nations;
in which it sets targets for cutbacks in CFC production and the complete elimination of these substances
by the year 2000. While some progress has been made in this respect, it is doubtful that poorer nations
such as China and India can strictly abide the treaty because of the importance of CFCs to their economies.
- The Vienna Convention for the Protection of the Ozone Layer. In 1981 the Governing Council set up a
working group to prepare a global framework convention for the protection of the Ozone Layer. Its aim was
to secure a general treaty to tackle ozone depletion. First, a general treaty resolved in principle to tackle a
problem; then the parties got down to the more difficult task of agreeing protocols that established specific controls.
Fig.15 North Pole image (190kB)
Fig.16 Global ozone image (100kB)
- 1990 In U.S. "Clean Air Act Amendment" passed.
- CFCs gone by January 1, 1996.
- May 15th, 1993 Warning labels are printed on all products with ozone-depleting substances.
- Recycling could play a significant supplementary role in preventing CFCs already in appliances from
escaping into the atmosphere.
- There are efforts to find substitutes that are not harmful to the ozone layer. One is the
hydro-chloro-fluorocarbon-123 or HCFC-123 (CF3CHCl2).
The presence of the hydrogen atom makes the compound more susceptible to oxidation in the lower atmosphere,
so that it never reaches the stratosphere. Unfortunately, the hydrogen also makes the compound more active
biologically than the CFCs. Laboratory tests have shown the HCFC-123 can cause tumors in rats,
although its toxic effect on humans is not known.
- By reducing the Cl atoms, some chemists suggested sending a fleet of planes to spray 50,000 tons of
ethane (C2H6) or propane (C3H8) high over the
South Pole in an attempt to heal the hole in the ozone layer. Been reactive species, the chlorine atom would
react with the hydrocarbons:
The products of these reactions would not affect the ozone concentration.
- A less reactive plan is to rejuvenate the ozone layer by producing large quantities of ozone and releasing
it into the stratosphere from airplanes. Technically this solution is feasible, but it would be very costly and it would
require the collaboration of many nations.
- As proofed by Greenpeace's alternative refrigerating technology, there are several alternatives; e.g. greenfreeze,
a mixture of propane (R290) and isobutane (R600a).
Cl + C2H6 → HCl + C2H5
Cl + C3H8 → HCl + C3H7