What is Ozone and where is it in the atmosphere ?
Ozone gas is naturally present in our atmosphere. Each molecule contains three atoms of Oxygen. Ozone is found primarily in two regions of the atmosphere. About 10% of atmospheric ozone is in the troposphere, the region closest to Earth (from the surface to about 10-16 kilometers). The remaining 90% Ozone resides in the Stratosphere, primarily between the top of the Troposphere and about 50 kilometers altitude. The large amount of Ozone in the stratosphere is often referred to as the “Ozone layer”.
How is Ozone formed in the atmosphere ?
Ozone is formed throughout the atmosphere in multi-step chemical processes that require sunlight. In the Stratosphere, the process begins with the breaking apart of an Oxygen molecule by ultraviolet radiation from the Sun. In the lower atmosphere (Troposphere), Ozone is formed in a different set of chemical reactions involving hydrocarbons and nitrogen-containing gases.
Why do we care about atmospheric Ozone?
Ozone in the Stratosphere, absorbs some of the Sun’s biologically harmful ultraviolet radiation. Because of this beneficial role, Stratospheric Ozone is considered “Good Ozone”. In contrast, Ozone at Earth’s surface that is formed from pollutants is considered “Bad Ozone” because it can be harmful to humans and plant and animal life. Some Ozone occurs naturally in the lower atmosphere where it is beneficial because Ozone helps remove pollutants from the atmosphere.
Is total Ozone uniform over the globe?
No, the total amount of Ozone above the surface of Earth varies with location on the time scales that range from daily to seasonal. The variations are caused by Stratospheric winds and chemical production and destruction of Ozone. Total Ozone is generally lowest at the equator and highest near the poles because of the seasonal wind patterns in the Stratosphere.
How is Ozone measured in the atmosphere?
The amount of Ozone in the atmosphere is measured by instruments on the ground and carried aloft in balloons, aircraft, and satellites. Some measurements involve drawing air into an instrument that contains a system for detecting Ozone. Other measurements are based on Ozone’s unique absorption of light in the atmosphere. In that case, sunlight or laser light is carefully measured after passing through a portion of the atmosphere containing Ozone.
What are the principal steps in stratospheric Ozone depletion caused by human?
The initial step in the depletion of stratospheric Ozone by human activities is the emission of Ozone-depleting gases containing Chlorine and Bromine at Earth’s surface. Most of these gases accumulate in the lower atmosphere because they do not react and do not dissolve readily in rain or snow. Eventually, the emitted gases are transported to the Stratosphere where they are converted to more reactive gases containing Chlorine and Bromine. These more reactive gases then participate in reactions that destroy Ozone.
Certain industrial processes and consumer products result in the atmospheric emission of “Halogen source gases”. These gases contain Chlorine and Bromine atoms, which are known to be harmful to the Ozone layer. For example, the Chlorofluorocarbons (CFCs) and Hydrochlorofluorocarbons (HCFCs), once used in almost all refrigeration and air conditioning systems, eventually reach the Stratosphere where they are broken apart to release Ozone-depleting chlorine atoms. Other examples of human-produced Ozone-depleting gases are the “Halons”, which are used in fire extinguishers and which contain Ozone-depleting Bromine atoms.
Why has an “Ozone hole” appeared over Antarctica when Ozone-depleting gases are present throughout the stratosphere?
Ozone-depleting gases are present throughout the Stratospheric Ozone layer because they are transported to great distances by atmospheric air motions. The severe depletion of the Antarctic Ozone layer known as the “Ozone hole” forms because of the special weather conditions that exist there and nowhere else on the globe. The very cold temperatures of the Antarctic Stratosphere create ice clouds called Polar Stratospheric Clouds (PSCs). Special reactions that occur on PSCs and the relative isolation of Polar Stratospheric air allows chlorine and Bromine reactions to produce the Ozone hole in Antarctic springtime.
Severe depletion of the Antarctic Ozone layer was first observed in the early 1980s. Antarctic Ozone depletion is seasonal, occurring primarily in late winter and spring (August-November).
Is there depletion of the Arctic Ozone layer?
Yes, significant depletion of the Arctic Ozone layer now occurs in the late winter/spring period (January-April). However, the maximum depletion is generally less severe than that observed in the Antarctic and is more variable from year to year. A large and recurrent “Ozone hole”, as found in the Antarctic Stratosphere, does not occur in the Arctic .
How large is the depletion of the global Ozone layer?
The Ozone layer has been depleted gradually since 1980 and now is about an average of 3% lower over the globe. The depletion, which exceeds the natural variations of the Ozone layer, is very small near the equator and increases towards the poles. The large average depletion in Polar Regions is primarily a result of the late winter/spring Ozone destruction that occurs there annually.
Are there regulations on the production of Ozone-depleting gases?
Yes, the production of Ozone-depleting gases is regulated under a 1987 international agreement known as the “Montreal Protocol on Substances that Deplete the Ozone Layer” and its subsequent Amendments and Adjustments. The Protocol, now ratified by over 191 Nations, establishes legally binding controls on the national production and consumption of Ozone depleting gases. Production and consumption of all principal Halogen-containing gases by developed and developing Nations will be significantly reduced or phased out before the middle of the 21st century.
As a result of the Montreal Protocol, the total abundance of Ozone-depleting gases in the atmosphere has begun to decrease in recent years. If the Nations of the world continue to follow the provisions of the Montreal Protocol, the decrease will continue throughout the 21st century.
Does depletion of the Ozone layer increase ground-level ultra-violet radiation?
Yes, ultra-violet radiation at Earth’s surface increases as the amount of overhead total ozone decreases, because ozone absorbs ultraviolet radiation from the Sun. Measurements by ground-based instruments and estimates made using satellite data have confirmed that surface ultraviolet radiation has increased in regions where ozone depletion is observed.
Which are the Ozone Depleting Substances (ODS) presently used in India ?
Chloroflourocarbons(CFCs)-12 for Refrigeration, Chillers and Metered Dose Inhalers.
Hydrochloroflourocarbons (HCFCs) - 22 for Air Conditioners.
Carbon Teterachloride (CTCs) as solvent process agent mainly in the metal cleaning and textile industries. It is also used as feedstock in the manufacture of CFCs and DV Acid Chloride.
Which ODS is no longer produced in India ?
Halons, which were earlier used in fire extinguishers. Halons continue to be used in Defence sector, which is exempt from Montreal Protocol.
*Inputs from Ministry of Environment & Forests