Ozone (O3)
- A form of oxygen. It is naturally created in the Stratosphere when ultraviolet radiation breaks down the oxygen molecule (O2) into two individual oxygen atoms. Each of these atoms combines with an O2 molecule to form ozone. Essential in one part (stratosphere) and a pollutant in another part of the atmosphere (troposphere)
Ozone shield -
- The stratosphere (layer above the troposphere) is the location of ozone shield. Ozone in stratosphere forms naturally when oxygen reacts with incoming ultraviolet (UV) radiation (from sun). Stratospheric ozone blocks much of the UV from penetrating to Earth's surface - reflects it out; approximately 99% of all ultraviolet solar radiation is absorbed or screened out in the ozone layer
- Ozone is constantly being produced and destroyed in a natural cycle, as shown in the above picture. However, the overall amount of ozone is essentially stable. This balance can be thought of as a stream's depth at a particular location. Although individual water molecules are moving past the observer, the total depeth remains constant. Similarly, while ozone production and destruction are balanced, ozone levels remain stable. This was the situation until the past several decades.
- The Ozone Layer protects us from the harmful effects of certain wavelengths of ultra-violet (UV) light from the sun, specifically UV-B. Any significant decrease in ozone in the stratosphere would result in an increase of UV-B radiation reaching the earth surface. Increases in levels of UV-B radiation can result in the increase in skin cancers, suppress the immune system, exacerbate eye disorders including cataracts and affect plants, animals and plastic materials.
Ozone-Depleting Substance(s) (ODS)
- A compound that contributes to stratospheric ozone depletion. ODS include CFCs, HCFCs, halons, methyl bromide, carbon tetrachloride, and methyl chloroform. ODS are generally very stable in the troposphere and only degrade under intense ultraviolet light in the stratosphere. When they break down, they release chlorine or bromine atoms, which then deplete ozone.
- Chlorofluorocarbons (CFCs) invented in 1928 found many uses in aerosols, foams, refrigeration, air conditioners, solvents, fire extinguishers etc. These CFCs are long lived; their emissions reach the stratosphere and cause ozone depletion. This ozone depletion has been dramatically confirmed through the Antarctic "Ozone Hole" discovered in 1985 and observations, since then, of ozone depletion in the middle and higher latitudes by the satellite Nimbus-7.
Carbon Tetrachloride (CCl4)
- A compound consisting of one carbon atom and four chlorine atoms.Carbon tetrachloride was widely used as a raw material in many industrial uses, including the production of CFCs, and as a solvent. Solvent use ended when it was discovered to be carcinogenic. It is also used as a catalyst to deliver chlorine ions to certain processes. Its ozone depletion potential is 1.2.
Chlorofluorocarbon (CFC)
- A compound consisting of chlorine, fluorine, and carbon CFCs are very stable in the troposphere. They are broken down by strong ultraviolet light in the stratosphere and release chlorine atoms that then deplete the ozone layer. CFCs are commonly used as refrigerants, solvents, and foam blowing agents. The most common CFCs are CFC-11, CFC-12, CFC-113, CFC-114, and CFC-115. The ozone depletion potential (ODP) for each CFC is, respectively, 1, 1, 0.8, 1, and 0.6.
Ozone Depletion Potential (ODP)
- A number that refers to the amount of ozone depletion caused by a substance. The ODP is the ratio of the impact on ozone of a chemical compared to the impact of a similar mass of CFC-11. Thus, the ODP of CFC-11 is defined to be 1.0. Other CFCs and HCFCs have ODPs that range from 0.01 to 1.0. The halons have ODPs ranging up to 10. Carbon tetrachloride has an ODP of 1.2, and methyl chloroform's ODP is 0.11. HFCs have zero ODP because they do not contain chlorine.
Dobson Unit (DU)
- A measurement of column ozone levels. If 100 DU of ozone were brought to the Earth's surface, it would form a layer 1 millimeter thick. In the tropics, ozone levels are typically between 250 and 300 DU year-round. In temperate regions, seasonal variations can produce large swings in ozone levels.
Ozone depletion in the stratosphere The Cause
- Ozone is broken down by human-made pollutants. Primary cause is chlorofluorocarbons (CFCs) which are normally used as propellants in aerosol cans, as coolants in air conditioner (AC). and refrigerators, as foam for insulation and packaging, as sterilizers for hospitals.
- Also caused by halons (in many fire extinguishers), methyl chloroform (used to degrease metals), and carbon tetrachloride (used to manufacture pesticides and dyes) contain chlorine
- Oxides of NOx released by super sonic transport system (SST)
The ozone depletion process begins when CFCs and other ozone-depleting substances (ODS) are emitted into the atmosphere
- Winds efficiently mix the troposphere and evenly distribute the gases. CFCs are extremely stable, and they do not dissolve in rain. After a period of several years, ODS molecules reach the stratosphere, about 10 kilometers above the Earth's surface
- Strong UV light breaks apart the ODS molecule. CFCs release chlorine atoms, and halons release bromine atoms
- It is these atoms that actually destroy ozone, not the intact ODS molecule. It is estimated that one chlorine atom can destroy over 100,000 ozone molecules before finally being removed from the stratosphere
- Large increases in stratospheric chlorine and bromine, however, have upset that balance. In effect, they have added a siphon downstream, removing ozone faster than natural ozone creation reactions can keep up. Therefore, ozone levels fall.
- Since ozone filters out harmful UVB radiation, less ozone means higher UVB levels at the surface. The more depletion, the larger the increase in incoming UVB. UVB has been linked to skin cancer, cataracts, damage to materials like plastics, and harm to certain crops and marine organisms. Although some UVB reaches the surface even without ozone depletion, its harmful effects will increase as a result of this problem
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