Aryan katariya PPT .ppt

Editor's Notes

• #4: Ozone: a gas composed of three atoms of oxygen and is a bluish gas that is harmful to breathe. While ground level ozone is a pollutant, stratospheric ozone is beneficial. Nearly 90% of the Earth's ozone is in the stratosphere and is referred to as the ozone layer. The ozone layer lies approximately 15-40 kilometers (10-25 miles) above the Earth's surface, in the stratosphere. The ozone layer prevents most UVB from reaching the ground. Ozone absorbs a band of ultraviolet radiation called UVB that is particularly harmful to living organisms. Depletion of this layer by ODS will lead to higher UVB levels, which in turn will cause increased skin cancers and cataracts and potential damage to some marine organisms, plants, and plastics. Ozone Depletion: Chemical destruction of the stratospheric ozone layer beyond natural reactions. Stratospheric ozone is constantly being created and destroyed through natural cycles. Various ozone-depleting substances (ODS), however, accelerate the destruction processes, resulting in lower than normal ozone levels. In 1974, F. Sherwood Rowland and Mario J. Molina pointed out that CFCs are tranported into the stratoshpher, where they photodissociate to release chlorine atoms. The chlorine atoms destroy ozone according to another catalytic cycle: Cl + O3 --- ClO + O2 ClO + O --- Cl + O2 net reaction: O3 + O --- 2O2 In this cycle , atomic chlorine (Cl) and chlorine monoxide radicals (ClO) are catalysts, since they promote the overall reaction, but are not consumed. Sources: Energy Demand and Energy Engineering Course Pack
• #5: http://www.al.noaa.gov/wwwhd/pubdocs/Assessment98/faq5.html The springtime Antarctic ozone hole is a new phenomenon that appeared in the early 1980s. The observed average amount of ozone during September, October, and November over the British Antarctic Survey station at Halley, Antarctica, first revealed notable decreases in the early 1980s, compared with the preceding data obtained starting in 1957. The ozone hole is formed each year when there is a sharp decline (currently up to 60%) in the total ozone over most of Antarctica for a period of about three months (September-November) during spring in the Southern Hemisphere. Late-summer (January-March) ozone amounts show no such sharp decline in the 1980s and 1990s. Observations from three other stations in Antarctica and from satellite-based instruments reveal similar decreases in springtime amounts of ozone overhead. Balloon borne ozone instruments show dramatic changes in the way ozone is distributed with altitude. As the figure below from the Syowa site shows, almost all of the ozone is now depleted at some altitudes as the ozone hole forms each springtime, compared to the normal ozone profile that existed before 1980. As explained in an earlier question, the ozone hole has been shown to result from destruction of stratospheric ozone by gases containing chlorine and bromine, whose sources are mainly human-produced halocarbon gases. Before the stratosphere was affected by human-produced chlorine and bromine, the naturally occurring springtime ozone levels over Antarctica were about 30-40% lower than springtime ozone levels over the Arctic. This natural difference between Antarctic and Arctic conditions was first observed in the late 1950s by Dobson. It stems from the exceptionally cold temperatures and different winter wind patterns within the Antarctic stratosphere as compared with the Arctic. This is not at all the same phenomenon as the marked downward trend in ozone over Antarctica in recent years. Changes in stratospheric meteorology cannot explain the ozone hole. Measurements show that wintertime Antarctic stratospheric temperatures of past decades had not changed prior to the development of the ozone hole each September. Ground, aircraft, and satellite measurements have provided, in contrast, clear evidence of the importance of the chemistry of chlorine and bromine originating from human-made compounds in depleting Antarctic ozone in recent years.
• #6: Antarctic Ozone Levels Fall, 1995 This graphic shows ozone levels over the Antarctic during Fall, 1995 in dobson units (DU). During the deepest ozone loss, the center of the hole (the red area) can drop below 100 DU. Since normal values are usually around 300, the worst holes can reach over 70% depletion. Although it was not visible on September 1, 1995, the Antarctic ozone hole was the red and purple area that appeared over Antarctica September 15. The ozone hole is defined as the area having less than 220 dobson units (DU) of ozone in the overhead column (i.e., between the ground and space). The ozone hole is a well-defined, large-scale destruction of the ozone layer over Antarctica that occurs each Antarctic spring. The word "hole" is a misnomer; the hole is really a significant thinning, or reduction in ozone concentrations, which results in the destruction of up to 70% of the ozone normally found over Antarctica. Unlike global ozone depletion, the ozone hole occurs only over Antarctica. Source: http://www.epa.gov/ozone/science/hole/holehome.html
• #7: 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. For over 50 years, chlorofluorocarbons (CFCs) were thought of as miracle substances. They are stable, nonflammable, low in toxicity, and inexpensive to produce. Over time, CFCs found uses as refrigerants, solvents, foam blowing agents, and in other smaller applications. 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. http://www.epa.gov/ozone/defns.html#cfc
• #8: Certain chemicals within this class of compounds are viewed by industry and the scientific community as acceptable alternatives to chlorofluorocarbons. The HCFCs have shorter atmospheric lifetimes than the CFCs and a much smaller capacity to deliver reactive chlorine to the stratosphere where the ozone layer is found. Consequently, it is expected that these chemicals will contribute much less to stratospheric ozone depletion than CFCs. Because they still contain chlorine and have the potential to destroy stratospheric ozone, they are viewed only as temporary replacements for the CFCs. Current international legislation has mandated production caps for HCFCs in the future; production in developed countries is prohibited after 2030. HCFCs are less stable than CFCs because HCFC molecules contain carbon-hydrogen bonds. Because the HFCs contain no chlorine they will not contribute to stratospheric ozone depletion. Furthermore, mechanisms for ozone destruction involving fragments produced as HFCs are decomposed within the atmosphere (CF3 radicals) have been shown to be insignificant. Like HCFCs, the HFCs contain hydrogen that is susceptible to attack by the hydroxyl radical. Oxidation of HFCs by the hydroxyl radical is believed to be the major destruction pathway for HFCs in the atmosphere. Atmospheric lifetimes of the most commonly used HFCs (HFC-134a and HFC-152a) are limited to less than 12 years because of this reaction. Although it is believed that HFCs will not deplete ozone within the stratosphere, it is also believed that this class of compounds has other adverse environmental effects (see the Chlorofluorcarbon Alternative Measurements Project info). Concern over these effects may make it necessary to regulate production and use of these compounds at some point in the future. Source: http://www.cmdl.noaa.gov/noah/flask/hcfc.html
• #9: The halons are used as fire extinguishing agents, both in built-in systems and in handheld portable fire extinguishers. Halon production in the U.S. ended on 12/31/93 because they contribute to ozone depletion. They cause ozone depletion because they contain bromine. Bromine is many times more effective at destroying ozone than chlorine. At the time the current U.S. tax code was adopted, the ozone depletion potentials of halon 1301 and halon 1211 were observed to be 10 and 3, respectively. These values are used for tax calculations. Recent scientific studies, however, indicate that the ODPs are at least 13 and 4, respectively. The current best estimate of the Ozone Depletion Potential (ODP) for methyl bromide is 0.4 (with a range of 0.2 to 0.5), as compared to an ODP of 0.6 (with a range of 0.3 to 0.9) estimated in the previous Assessment (1994). Source: http://www.epa.gov/ozone/defns.html#halon
• #10: 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. A table of all ozone-depleting substances shows their ODPs, GWPs, and CAS numbers,
• #12: Ultraviolet radiation is a portion of the electromagnetic spectrum with wavelengths shorter than visible light. The sun produces UV, which is commonly split into three bands: UVA, UVB, and UVC. UVA is not absorbed by ozone. UVB is mostly absorbed by ozone, although some reaches the Earth. UVC is completely absorbed by ozone and normal oxygen. UVC: a band of ultraviolet radiation with wavelengths shorter than 280 nanometers UVB: a band of ultraviolet radiation with wavelengths from 280-320 nanometers produced by the Sun UVB is a kind of ultraviolet light from the sun (and sun lamps) that has several harmful effects.particularly effective at damaging DNA. It is a cause of melanoma and other types of skin cancer. It has also been linked to damage to some materials, crops, and marine organisms. The ozone layer protects the Earth against most UVB coming from the sun. It is always important to protect oneself against UVB, even in the absence of ozone depletion, by wearing hats, sunglasses, and sunscreen. However, these precautions will become more important as ozone depletion worsens. UVC is extremely dangerous, but it is completely absorbed by ozone and normal oxygen (O2).
• #13: The Montreal Protocol is working. Global observations have shown that the combined abundance of anthropogenic chlorine-containing and bromine-containing ozone-depleting substances in the lower atmosphere peaked in 1994 and has now started to decline. One measure of success of the Montreal Protocol and its subsequent Amendments and Adjustments is the forecast of "the world that was avoided" by the Protocol: The abundance of ozone-depleting gases in 2050, the approximate time at which the ozone layer is now projected to recover to pre-1980 levels, would be at least 17 ppb of equivalent effective chlorine (this is based on the conservative assumption of a 3% per annum growth in ozone-depleting gases), which is about 5 times larger than today's value. Ozone depletion would be at least 50% at mid latitudes in the Northern Hemisphere and 70% at mid latitudes in the Southern Hemisphere, about 10 times larger than today. Surface UV-B radiation would at least double at mid latitudes in the Northern Hemisphere and quadruple at mid latitudes in the Southern Hemisphere compared with an unperturbed atmosphere. This compares to the current increases of 5% and 8% in the Northern and Southern Hemispheres, respectively, since 1980. Furthermore, all of the above impacts would have continued to grow in the years beyond 2050. It is important to note that, while the provisions of the original Montreal Protocol in 1987 would have lowered the above growth rates, recovery (i.e., an improving situation) would have been impossible without the Amendments and Adjustments (London, 1990; Copenhagen, 1992; and Vienna, 1995). Source: http://www.al.noaa.gov/wwwhd/pubdocs/Assessment98/executive-summary.html#A