Presentation on theme: "Ange Darnell. Ozone is a relatively unstable molecule made up of three oxygen atoms. The majority of this compound exists in the stratosphere (15."— Presentation transcript:
Ozone is a relatively unstable molecule made up of three oxygen atoms. The majority of this compound exists in the stratosphere (15 miles up) and shields the Earth from harmful UV radiation from the sun. Source: University of Alaska
Ozone also naturally exists at the Earth’s surface (troposphere); ozone is a natural component of the clean atmosphere. However, excess ozone formed in the troposphere by human activities is a harmful pollutant: dangerous to health, a major constituent of smog, and a greenhouse gas. This is known as “bad” ozone. Source: theozonehole.com
Ozone in the troposphere is produced when sunlight and heat react with nitrous oxides and volatile organic compounds, known as ozone precursors. These pollutants are emitted by vehicles, power plants, refineries, and chemical plants. The amounts of “good” and “bad” ozone in the atmosphere depend on a balance between processes that create and destroy ozone; an upset in this balance has serious consequences for life on Earth. Source: EPA
Halogen source gases are emitted at the Earth’s surface by human activities and natural processes These gases accumulate in the atmosphere because they are unreactive and do not dissolve readily in rain or snow, and are distributed throughout the lower atmosphere by winds The gases are then transported to the stratosphere by air motions They are then converted to reactive halogen gases in chemical reactions involving UV radiation from the sun These radicals catalyze the breakdown of ozone into O and O 2 A catalytic reaction cycle is a set of chemical reactions which result tin the destruction of many ozone molecules while the molecule that started the reaction is reformed to continue the process A single CFC molecule can destroy up to 100,000 ozone molecules The very thing that makes ozone good for filtering UV radiation makes it easily destroyed: it is very unstable Source: EPA and theozonehole.com
Chlorofluorocarbons were developed in the 1930s as a safe, non-toxic, non-flammable alternative to dangerous substances for refrigeration and spray can propellants. Very little chlorine exists naturally in the atmosphere and CFCs are an excellent way of introducing chlorine into the ozone layer. They have been used extensively as aerosol-spray propellants, refrigerants, solvents, and foam-blowing agents. CFCs have a lifetime in the atmosphere of 20 to 100 years, so their effect lasts for a long time, even after their use decreases. Source: theozonehole.com
Bromine is another important contributor to ozone depletion. Bromine is a “halon” (halogenated hydrocarbon gas) and is used for fire extinguishers, protection of large computers, military hardware and commercial aircraft. It is also used a s an agricultural fumigant. There are also natural sources of chlorine and other ozone destroying compounds, such as volcanic eruptions, and those emitted by oceanic and terrestrial ecosystems. However, only a fraction of these emissions reach the stratosphere because they are rapidly removed by rainfall and other natural processes. “Changes in the natural sources of chlorine and bromine since the middle of the 20 th century are not the cause of observed ozone depletion” Source: EPA
Although chlorine emitted from CFCs has a longer atmospheric lifetime than bromine sources, the bromine has a 7 to 16 times greater ozone depletion potential than chlorine. A common misconception is that CFCs and other halogen source gases will never reach the stratosphere because they are “heavier than air.” However, the distribution of gases in the troposphere and stratosphere are not controlled by the molecular weight of the gases because air is in continual motion as a result of winds and convection. Source: EPA
The ozone hole appears above Antarctica in the spring, from September to early December. Polar stratospheric clouds (PSCs) form during the winter, due to extreme cold. The extreme cold is due to the fact that there are 3 months without sunlight as well as the “polar vortex” trapping and chilling the air. These extremely low temperatures cause cloud particles to form that are composed of either nitric acid or ice, which provide surfaces for chemical reactions that lead to ozone destruction Sunlight is required for the chemical reactions to take place, which is why even though PSCs are most abundant during winter, the hole does not form until the spring. Source: theozonehole.com
While ozone is only a minority constituent of the atmosphere, it is responsible for the majority of UVB absorption. Without this absorption, UV can damage crops, plankton and human health. Many economically important species of plants, such as rice, depend of cyanobacteria which reside in their roots to retain nitrogen. These cyanobacteria are sensitive to UV radiation. Source: theozonehole.com and Wikipedia
The Southern Ocean is one of the world’s most productive marine ecosystems, and is a major supplier of nutrients carried to other parts of the world by undersea currents. The phytoplankton in these oceans capture the sun’s energy through photosynthesis, providing food for microscopic animals. They are then eaten by krill, which in turn sustain the Antarctic’s seals, penguins and whales. Plankton are extremely susceptible to effects of UV light, and a decrease means less food for these marine animals, which would eventually lead to a loss of approximately 7 million tons of fish per year. With the human food supply already strained because of increasing population, small reductions due to UV damage may be disastrous, especially to third world countries. Source: theozonehole.com and Wikipedia
UV radiation can produce radicals that are very reactive and cause damage by oxidizing biological molecules, such as DNA. Skin cancer rates increase and the number of deaths related to skin cancer also increase. “An estimated 10% reduction in the ozone layer will result in a 25% increase in skin cancer rates for temperate latitudes by the year 2050.” Skin damage could lead to other health issues because skin in the immune system’s largest defense, which would leave the body vulnerable to many diseased cause by bacteria and viruses entering through the skin. Increased exposure to UV has been linked to greater risk of the herpes virus, HIV-1, papilloma viruses, malaria, forms of TB, leprosy, dermatitis, and E. coli. In addition, since UV rays readily damage DNA, this could play another role in the mutation of existing disease bacteria and viruses, possibly producing totally new strains of pathogens UV rays can also harm the cornea Source: EPA and theozonehole.com
Although the releasing of ozone depleting chemicals into the atmosphere is at or near its peak, the stratosphere lags behind the surface by several years. Complete recovery of the ozone is not expected to occur until the late 21 st century, and this precludes a cessation of ozone depleting chemicals being released by human activities. Assuming full compliance with the Montreal Protocol, the ozone layer will begin to recover in future decades. Source: EPA and wikipedia
Although climate change is not the cause of the ozone hole (or vice versa), it could have future consequences regarding its recovery or further ozone depletion. Global warming from CO2 and other greenhouse gases (including tropospheric ozone) is expected to heat the troposphere and cool the stratosphere. This decrease in temperature will increase ozone depletion and the frequency of ozone holes. Conversely, ozone depletion is a radiative forcing of the climate system. Reduced ozone will cause the stratosphere to absorb less solar radiation, which further cools the stratosphere and heats the troposphere, further contributing to climate change.
Ozone and climate change are also linked indirectly because both ozone-depleting gases and substitute gases contribute to climate change. Chlorine-containing gases have a significant importance for ozone depletion and have a high global warming potential. Bromine-containing gases have a high ozone depleting potential and a relatively high global warming potential.
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