11 Sep :The ozone molecule contains three atoms of oxygen and is mainly formed by the action of the ultraviolet rays of the sun on the diatomic oxygen molecules in the upper part of the earth’s atmosphere (called the stratosphere). Atmospheric pollution near the Earth’s surface can form localized areas of ozone.
The stratospheric ozone layer protects life on earth by absorbing most of the harmful ultraviolet radiation from the sun. In the mid 1970s it was discovered that some manmade products destroy ozone molecules in the stratosphere. This destruction can result in damage to ecosystems and to materials such as plastics. It may cause an increase in human diseases such as skin cancers and cataracts.
The discovery of the role of the synthetic ozone-depleting chemicals such as chlorofluorocarbons (CFCs) stimulated increased research and monitoring in this field. Computer models predicted a disaster if no action was taken to protect the ozone layer. Based on this research and monitoring, the nations of the world took action in 1985 with the Vienna Convention for the Protection of the Ozone Layer followed by the Montreal Protocol on Substances that deplete the Ozone Layer in 1987. The Convention and Protocol were amended and adjusted several times as new knowledge that was obtained. The Meetings of the Parties to the Montreal Protocol appointed three Assessment Panels to review the progress in scientific knowledge on their behalf. These panels are the Scientific Assessment Panel, the Technological and Economic Assessment Panel and the Environmental and Health Effects Assessment Panel. Each panel covers a designated area.
Effects of human activities on depletion of ozone layer and climate change
There is overwhelming evidence that human activities are influencing global phenomena. Natural environmental cycles often span thousands of years but most scientific measurements have been made only over the past 150 years. It is often not easy to accurately determine the influence of humans on any natural activity. In the case of the ozone layer, the depletion of the ozone over the Antarctica cannot be explained by natural cycles but is caused by the increase of synthetic chemicals in the stratosphere. The relationship between these chemicals (e.g. chlorofluorocarbons also known as CFCs) and ozone depletion has been proven by experiments in laboratories, numerical modelling studies and by direct measurements in the atmosphere. By absorbing the infrared radiation emitted by the earth, some gases control the way natural energy flows through the atmosphere. Such gases are known as greenhouse gases. Carbon dioxide, although only a tiny fraction of the atmosphere, is an important greenhouse gas.
Measurements show that its concentration has increased by almost 30% as a result of human activities since the beginning of the industrial revolution (around 1750), resulting in enhancement of the greenhouse effect.Methane and nitrous oxide emitted from agricultural activities, changes in land use, and other sources are also potent greenhouse gases. The increase in greenhouse gasses contributes to climate change in the form of increased temperatures on the earth and a rise in sea level. Carbon dioxide is produced when fossil fuels are used to generate energy and when forests are burned. Observations show that global temperatures rose by about 0.6 °C over the 20th century.
Relationship between ozone and solar ultraviolet radiation
There is an inverse relationship between the concentration of ozone and the amount of UV-Bradiation transmitted through the atmosphere. Stratospheric ozone is naturally formed in chemical reactions involving ultraviolet sunlight and oxygen molecules. These reactions occur continually wherever ultraviolet sunlight is present. The production of stratospheric ozone is balanced by its destruction in chemical reactions. Ozone reacts continually with a variety of natural and anthropogenic chemicals in the stratosphere. In the lower atmosphere ozone is produced by the chemical reactions between mainly nitrogen oxides and organic chemical pollutants produced by motor vehicle and industrial emissions. The ozone in both the troposphere and the atmosphere absorbs the UV radiation received at the surface. The radiation emitted by the sun contains an ultraviolet component. As the sunlight passes through the atmosphere, all the UV-C and approximately 90% of the UV-B are absorbed mainly by ozone and oxygen. UV-A radiation is less affected by the atmosphere. Therefore, the ultraviolet radiation reaching the Earth’s surface is composed of mainly UV-A with a small UV-B component. A decrease in the concentration of ozone in the atmosphere results in increased UV-B radiation at the surface of the earth. DNA and other biological macromolecules absorb UV-B and can be damaged in this process.
determinants of UV-B radiation at a specific place
The sun is the origin of the ultraviolet radiation reaching the earth. That radiation is partly absorbed by the components of the earth’s atmosphere. The amount of potentially harmful ultraviolet radiation that is absorbed by one of these components, ozone, depends on the length of the path of the sunlight through the atmosphere. The UV-B irradiation varies with the time of the day, geographic location and the season. The ultraviolet radiation that reaches the earth is greatest in the tropics and decreases towards the poles. For the same reason it is greatest near local noon and least near sunrise or sunset. Outside the tropics it is generally greater in the summer and least in the winter. Clouds, particulate matter, aerosols and air pollutants absorb and scatter some of the ultraviolet radiation and thereby diminish the amount reaching the earth’s surface. Under clear skies the maximum irradiation occurs when the sun is directly overhead. Locations at higher altitudes have less atmosphere overhead, as evidenced by the thinner air and lower atmospheric pressure therefore the radiation of the sun is less attenuated. This increase in UV radiation varies between 10% and 20% for each kilometre of height, depending on the specific wavelength, solar angle, reflections, and other local conditions. Frequently, other factors besides the thickness of the atmosphere cause even larger differences in UV radiation between different altitudes. Surface reflection, especially from snow, ice and sand increases the irradiation at a particular site because the reflected radiation is redirected towards the surface through scattering by particles in the atmosphere or on the ground. In some conditions clouds will have the same effect. Snow is more common at higher altitudes, and reflects as much as 90% of the ultraviolet radiation. Dry beach sand and sea foam reflects about 25% of UV-B radiation. Clouds also reflect an appreciable amount of radiation to the areas where they do not directly obscure the sunlight The ultraviolet irradiation to which an individual is exposed is determined by a combination of all these factors.
Effect of pollution of the lower atmosphere on UV-B irradiation
Pollutants emitted by human activities can absorb UV-B radiation near the surface, while particles may lead to enhancement by scattering. While most of the atmospheric ozone is formed in the stratosphere, some ozone is produced in the lower atmosphere by the chemical reactions between pollutants such as nitrogen oxides and hydrocarbons. This ozone is a component of the photochemical smog found in many polluted areas. Airborne particles (smoke, dust and sulphate aerosols) block UV radiation, but at the same time can increase the amount of scattered light (haze) and therefore increase the UV exposure of side-facing surfaces (e.g., face, eyes). Comparisons of measurements made in industrialized regions of the Northern Hemisphere (e.g., central Europe) and in very clean locations at similar latitudes in the Southern Hemisphere (e.g., New Zealand) indicate the importance of particulate and pollution-related UV-B reductions.
At any particular location there is a direct relationship between UV-B irradiation and the amount of ozone in the atmosphere. UV-B increases with ozone depletion in the stratosphere but decreases with ozone formation in the lower atmosphere. The natural UV-B variability (e.g., from time of day, or clouds) can be larger than the effect of pollution, but goes in both directions, up and down. The cumulative amounts will depend critically upon local conditions and are therefore difficult to model in a general way. Many detrimental effects of UV-B are proportional to the cumulative UV-B exposure.