OZONE -The life force

OZONE -The life force

We know that our atmosphere has mainly four layers. The stratosphere extends from about 15 km to 50 km. In the stratosphere temperature increases with altitude, due to the absorption of UV light by oxygen and ozone. This creates a global "inversion layer" which impedes vertical motion into and within the stratosphere - since warmer air lies above colder air, convection is inhibited. The word "stratosphere" is related to the word "stratification" or layering. The stratosphere is often compared to the "troposphere", which is the atmosphere below about 15 km. The boundary - called the "tropopause" - between these regions is quite sharp, but its precise location varies between ~9 and ~18 km, depending upon latitude and season. The prefix "tropo" refers to change: the troposphere is the part of the atmosphere in which weather occurs. This results in rapid mixing of tropospheric air. Above the stratosphere lie the "mesosphere", ranging from ~50 to ~100 km, in which temperature decreases with altitude; the "thermosphere", ~100-400 km, in which temperature increases with altitude again, and the "exosphere", beyond ~400 km, which fades into the background of interplanetary space The density of the air in the atmosphere depends upon altitude, and in a complicated way because the temperature also varies with altitude.

Ozone
Ozone is formed naturally in the upper stratosphere by shortwavelength ultraviolet radiation. Wavelengths less than ~240nanometers are absorbed by oxygen molecules (O₂), which dissociate to give O atoms. The O atoms combine with other oxygen molecules to make ozone: O₂ + hv -> O + O (wavelength <> O₃

Ozone depletion
Concern has grown about depletion of the ozone layer - particularly the ozone 'hole' which has been detected over Antarctica in spring, and can spread as far as South America and the Falkland Islands. An ozone hole is established when a large percentage of ozone is destroyed. Ozone depletion over the Arctic is not as severe. Depletion has also been detected over northern Europe. The extent of depletion is seasonal and dependent on weather conditions. In 2003 the Antarctic hole reached record size - 28 million km2, after reducing in 2002. Damage to the ozone layer is attributed to pollutants from human activity - the main culprits are chlorofluorocarbons (CFCs) and halons. Scientists estimate that it will be 2050 before efforts to reduce CFCs etc in the stratosphere will pay off.

Ozone Depleters
CFCs and Halons
Man-made CFCs are the main cause of stratospheric ozone depletion. CFCs have a lifetime of about 20 to 100 years, and consequently one free chlorine atom from a CFC molecule can do a lot of damage, destroying ozone molecules for a long time. Although emissions of CFCs around the developed world have largely ceased due to international control agreements, the damage to the stratospheric ozone layer will continue for a number of years to come.CFCs have been used as propellants in aerosols, refrigeration, air conditioning, foam packaging and insulating materials. Halons are used in fire extinguishers. When they break down in the atmosphere, CFCs release chlorine (Cl) and halons release bromine (Br). Both of these gases destroy ozone, for example - Cl + O₃ ¨ ClO + O₂
- the chlorine monoxide then reacts with an oxygen atom, liberating the chlorine -
ClO + O ¨ Cl + O₂
- which is then free to destroy another ozone molecule. Chlorine takes several years to reach the stratosphere, and remains there for many years. The pie chart below shows the uses of CFCs in various products before the 1987 Montreal Protocol, which required countries to phase out their usage to protect the ozone layer.


HCFCs
When it was confirmed that CFCs cause ozone destruction, manufacturers looked for alternatives. HCFCs were introduced as a CFC substitute. However, these also have ozone depleting properties. They do less damage in the long term, but act much more rapidly than CFCs.
Other ChemicalsThe industrial solvents carbon tetrachloride (used in medicines, pesticides and paints) and methyl chloroform (a cleaning solvent) are also ozone destroyers. Methyl bromide is a pesticide used to fumigate soils and stored crops and is produced naturally by seaweed. It releases bromine, which destroys up to 60 times as much ozone as CFCs and worldwide emissions are increasing by 5-6% a year.

Efforts to reduce production and use of these damaging chemicals will not have an immediate effect in restoring the ozone layer - in fact, ozone depletion may well increase before it improves as it takes time for gases released to reach the upper atmosphere. Scientists believe that ozone depleting gases have reached their limit in the upper atmosphere, but that it will take decades for concentrations to decline and the stratosphere to return to normal. Therefore it is crucial that nations stick to timetables for reduction in use agreed in the Montreal Protocol.

Hole over the Antarctic
Every winter and spring since the late 1970s, an ozone hole has formed in the stratosphere above the Antarctic continent. In recent years this hole has become both larger and deeper.
Man-made emissions of CFCs occur mainly in the northern hemisphere, with about 90% released in Europe, Russia, Japan, and North America. Gases such as CFCs that are insoluble in water and relatively unreactive are mixed throughout the lower atmosphere and rise from the lower atmosphere into the stratosphere; winds then move this air poleward.
Normally, chlorine and bromine is inactive, locked up in stable compounds, and does not destroy the ozone. However, during the Antarctic winter months (June to August) when the region receives no sunlight, the stratosphere becomes cold enough (-80°C) for high level [ice] clouds to form, called Polar Stratospheric Clouds (PSCs). These PSCs provide an ideal catalytic surface on which the chlorine can react with the ozone, thus destroying the ozone layer. This reaction requires sunlight, and therefore only begins when the Sun returns to Antarctica in spring (September to October), before the PSCs have had a chance to melt. The ozone hole disappears again when the Antarctic air warms up enough during late spring and summer. During the southern hemisphere winter, Antarctica is isolated from the rest of the world by a natural circulation of wind called the polar vortex. This prevents atmospheric mixing of stratospheric ozone, thus contributing to the depletion of ozone. Although some ozone depletion occurs over the Arctic, meteorological conditions there are very different to Antarctica and so far have prevented the formation of ozone holes as large as in the southern hemisphere.

Harmful effects of ozone
The increases in UV radiation associated with ozone depletion are likely to lead to increases in the incidence and/or severity of a variety of short-term and long-term health effects, if current exposure practices are not modified by changes in behavior

The most well-known effect of UV radiation is the slight reddening or burning of the skin in sunshine. This change of colour is caused by an expansion of the skin's blood vessels. For most people burning is followed by tanning within a couple of days. A permanent tan will occur when the UV radiation causes a pigment called melanin to form in the pigment cells of the skin. Over a period of years, exposure to radiation originating from the Sun causes damages in the skin's connective tissues, so-called photo-ageing. This shows itself as a thickening of the skin, as wrinkles and decreasing elasticity. Elastine and collagen fibres determining the firmness and elasticity of the skin are damaged. UV radiation increases the risk of getting skin cancer.
Australia, with high sunshine levels, has very high skin cancer rates. An estimated 2 out of every 3 people in most parts of the country will develop some form of skin cancer.
Strong UV radiation can cause inflammation of the cornea leading to photokeratosis or "snow blindness". Symptoms of this kind of an infection include the eyes becoming reddish, a sensitivity to light, enhanced excretion of tears, the feeling of having some dirt in one's eye, and pain. The trauma appears 3-12 hours after exposure. Thanks to the quick regeneration of the eye cells, symptoms will normally disappear within a few days. A long-term exposure to UV radiation may cause permanent damage to the cornea.
Adverse effects include depressed resistance to certain tumors and infectious diseases, potential impairment of vaccination responses, and possibly increased severity of some autoimmune and allergic responses. Beneficial effects could include decreases in the severity of certain immunologic diseases/conditions such as psoriasis and nickel allergy.

Effect on environment
The increase in UV radiation over the last couple of decades could cause disastrous effects for the environment. Amphibian populations all over the world are experiencing a sharp decline, and many scientist speculate that this could be because of increased UV-B radiation, as amphibian eggs are transparent and highly susceptible. Many plants have shown a decrease in photosynthetic activity when bombarded with increased levels of UV-B radiation. Photosynthesis is a integral part of the food cycle as plants can create sugar from water and sunlight. However, loss of this ability would prove disastrous.
Phytoplanktons are the most important biomass producers in aquatic systems. When increased levels o f UV-B radiation come down, phytoplankton experience a decrease in mobility and orientation. This decrease results in reduced survival rates for phytoplankton.


Montreal protocol
The depletion of the ozone layer is one of the most prominent issues today that is truly global. As such, it required the whole world to come together to work out some sort of action to be taken. Most efforts would be useless in the long run unless everyone cooperated. When the problem first arose, governments of the world came together to decide what sort of restrictions should and could be made to work towards a solution. The first meeting, the Montreal Protocol, occurred in 1987. Since then, countries throughout the world have come together and worked individually to solve the problem.

It first met in 1987, 24 countries came together to set standards for phasing out of ozone depleting chemicals. These chemicals, which are commonly used as refrigerants and propellants, cause serious damage to the ozone layer. A schedule was set up for technologically developed countries to set regulations for the amount of CFCs (chlorofluorocarbons) that could be produced each year, gradually reducing the amount each year until they had been reduced to 50% in the year 2000. For less-developed countries, they would have the same schedule but ten years later.
It quickly became evident that a faster phase-out schedule would be feasible. The Montreal Protocol met again in 1990, this time with 93 countries in attendance. Rates of allowed CFC production were drastically reduced across the schedule, and now CFCs would be completely out of production in developed nations by the year 2000.