Presentation on theme: "Chlorofluorocarbons, were developed in the 1930s. Man-made CFCs have been the main cause of ozone depletion in the stratosphere. CFCs have a lifetime."— Presentation transcript:
Chlorofluorocarbons, were developed in the 1930s. Man-made CFCs have been the main cause of ozone depletion in the stratosphere. CFCs have a lifetime of about 20-100 years and can therefore continue to destroy ozone for a long period. One CFC molecule can result in the loss of 100,000 ozone molecules. Chlorofluorocarbons are a family of compounds containing chlorine, fluorine and carbon. They are in general unreactive, having low flammability, toxicity and solubility in water.
The major sources of CFCs are aerosol propellants, cleaning solvents, refrigerants and plastic blowing agents
CFCs such as trichloromethane (CCl 3 F) and dichlorodifluoromethane (CCl 2 F 2 ) are normally packed with materials like paint, insecticide or cosmetic preparations in pressurised containers. Upon depressurisation by opening the valve, the propellant vaporises and expels the materials inside the can in the form of aerosol spray.
Freon is a series of CFCs. Dichlorodifluoromethane (CCl 2 F 2 ) is the most important one. Freons absorb heat of vaporisation on evaporation resulting in the cooling of the surroundings. they are widely used as refrigerants in refrigerators and air conditioning units.
In making foam plastic, a volatile CFC, trichlorofluoromethane (CCl 3 F) is incorperated in the plastic. The heat evolved during the polymerisation reaction vaporises the CFC which then forms tiny bubbles in the plastic.
Damage from increased U.V. at the surface includes : Increased skin cancer (especially malignant melanoma- a lethal form of skin cancer) Destruction of DNA in phytoplankton (microscopic plant-like organisms) in the ocean. This is a LARGE problem because these organisms produce about 30% of the oxygen we breathe and are the base of the Marine Food Pyramid! Increased cataracts (an eye disorder rendering the sufferer blind) in humans and herd animals grown for food. Lowered crop yeilds due to damage to the plant. It should be obvious what problems this could cause!
Increasing amounts of UV radiation will have an impact on plankton and other tiny organisms at the base of the marine food web. These organisms provide the original food source for all other living organisms in the oceans. Plankton- phytoplankton as well as zooplankton are highly sensitive to UV radiation, as they lack the protective UV-B-absorbing layers that higher forms of plants and animals have. (Phyto = plant. Zoo = animal). More UV-B radiation reduces the amount of food phytoplankton create through photosynthesis. Zooplankton, feeding off the phytoplankton, are also affected. UV-B also damages small fish, shrimp and crab larvae. It has been estimated that on shallow coastal shelves, a 16 percent reduction of the ozone layer would kill more than 50 percent of e.g. anchovy larvae, and cause a 5 percent drop in plankton numbers and a 6-9 percent drop in fish yield.
Ozone-layer depletion seems likely to increase the rate of greenhouse warming, by reducing the effectiveness of the carbon dioxide sink in the oceans. Phytoplankton in the oceans assimilates large amounts of atmospheric carbon dioxide. Increased UV radiation will reduce phytoplankton activity significantly. This means that large amounts of carbon dioxide will remain in the atmosphere. A 10 percent decrease in carbon dioxide uptake by the oceans would leave about the same amount of carbon dioxide in the atmosphere as is produced by fossil fuel burning.
Emissions of CFCs to date have accounted for roughly 80% of total stratospheric depletion. Whilst chlorine is a natural threat to ozone, CFCs which contain chlorine are a man-made problem. Although CFC molecules are several times heavier than air, winds mix the atmosphere to altitudes far above the top of the stratosphere much faster than molecules can settle according to their weight. CFCs are insoluble in water and relatively unreactive in the lower atmosphere but are quickly mixed and reach the stratosphere regardless of their weight. When UV radiation hits a CFC molecule it causes one chlorine atom to break away. The chlorine atom then hits an ozone molecule consisting of three oxygen atoms and takes one of the oxygen molecules, destroying the ozone molecule and turning it into oxygen. When an oxygen molecule hits the molecule of chlorine monoxide, the two oxygen atoms join and form an oxygen molecule. When this happens, the chlorine atom is free and can continue to destroy ozone. Naturally occurring chlorine has the same effect in the ozone layer, but has a shorter life span.
A high increase in UV radiation may disrupt many ecosystems on land. Rice production may be drastically reduced by the effects of UV-B on the nitrogen assimilating activities of micro-organisms. With a diminishing ozone layer, it is likely that the supply of natural nitrogen to ecosystems, such as tropical rice paddies, will be significantly reduced. Most plants (and trees) grow more slowly and become smaller and more stunted as adult plants when exposed to large amounts of UV- B. Increased UV-B inhibits pollen germination.
UV-B stimulates the formation of reactive radicals - molecules that react rapidly with other chemicals, forming new substances. The hydroxyl radicals, for example, stimulate the creation of tropospheric ozone and other harmful pollutants. Smog formation creates other oxidized organic chemicals, such as formaldehydes. These molecules can also produce reactive hydrogen radicals when they absorb UV-B. In urban areas, a 10 percent reduction of the ozone layer is likely to result in a 10-25 percent increase in tropospheric ozone.
Since 1995, emissions of new CFCs in the developed world have been completely phased out. As evidence accumulated that man-made CFCs were contributing to stratospheric ozone depletion, scientists urged nations to control the use of CFCs. In 1987, the Montreal Protocol on Substances that Deplete the Ozone Layer was negotiated and signed by 24 countries. The Protocol called for the parties to phase down the use of CFCs, as well as other ozone depleting chemicals such as halons and other man-made halocarbons. Although emissions of CFCs have fallen dramatically as a result of the Montreal Protocol, because each chlorine molecule remains in the atmosphere for such a long time, damage done to stratospheric ozone will persist for many years to come.