While this statement is strictly true, tropospheric chlorine is irrelevant; it is stratospheric chlorine that affects ozone depletion. Greener Refrigerators Finally Legal in the U.S.", "GE Opening a Door to a Future of Cleaner Home Refrigeration", "Reducing abrupt climate change risk using the Montreal Protocol and other regulatory actions to complement cuts in CO, Proceedings of the National Academy of Sciences, "NOAA Study Shows Nitrous Oxide Now Top Ozone-Depleting Emission", Canada's SCISAT satellite explains 2006 ozone-layer depletion, SST emissions cut stratospheric ozone. [106], A 2005 IPCC review of ozone observations and model calculations concluded that the global amount of ozone has now approximately stabilized. As mentioned above, when such ozone-depleting chemicals reach the stratosphere, they are dissociated by ultraviolet light to release chlorine atoms. On peut trouver l'expression « potentiel de destruction d'ozone » pour PDO. According to simulation models, it is estimated the ozone layer will be mostly recovered by 2040. Ozone concentrations in the lower stratosphere over Antarctica will increase by 5–10 percent by 2020 and return to pre-1980 levels by about 2060–2075. [140] Also again in 2011 an ozone hole appeared over mountainous regions of Tibet, Xinjiang, Qinghai and the Hindu Kush, along with an unprecedented hole over the Arctic, though the Tibet one is far less intense than the ones over the Arctic or Antarctic. Monitoring of the ozone layer continues, and itâs finding that the recovery may not be as straightforward as hoped. The ozone layer outside the Polar regions is projected to recover to its pre-1980 levels some time before the middle of this century. [46] The sudden identification in 1985 that there was a substantial "hole" was widely reported in the press. Most of the biological impacts of ozone layer depletion are due to the damaging nature of UV-B radiations. Since the adoption and strengthening of the Montreal Protocol has led to reductions in the emissions of CFCs, atmospheric concentrations of the most-significant compounds have been declining.
Scientists have attributed ozone depletion to the increase of man-made (anthropogenic) halogen compounds from CFCs by combining observational data with computer models. Groundbreaking research in the 1970s and 1980s revealed signs of trouble. [1][22], Large volcanic eruptions have been shown to have substantial albeit uneven ozone-depleting effects, as observed with the 1991 eruption of Mt. The latter phenomenon is referred to as the ozone hole. INTRODUCTION. The ozone hole occurs during the Antarctic spring, from September to early December, as strong westerly winds start to circulate around the continent and create an atmospheric container. [78] In 1993, DuPont Canada closed its CFC facility. There are various areas of linkage between ozone depletion and global warming science: In 2019, NASA reported that there was no significant relation between size of the ozone hole and the climate change. Ozone shield or ozone hole were useful "easy-to-understand bridging metaphors". During the spring, however, the sun comes out, providing energy to drive photochemical reactions and melt the polar stratospheric clouds, releasing considerable ClO, which drives the hole mechanism. Ozone-depleting substances (chlorines and bromines) can be emitted from natural and anthropogenic (man-made) sources. A number of economically important species of plants, such as rice, depend on cyanobacteria residing on their roots for the retention of nitrogen. The growing awareness of the serious impacts of ODSs in the ozone layer led to the groundbreaking Montreal Protocol on Substances that Deplete the Ozone Layer which was signed in 1987. It has become general knowledge that chemicals called CFCs (chloroflourocarbons) are responsible for the hole in the ozone layer - in fact, scientists estimate 80% of CFCs in the stratosphere are derived from humans, not nature. (Bromine compounds and nitrogen oxides can also react with these cloud particles.) This is 10–25 years later than predicted in earlier assessments, because of revised estimates of atmospheric concentrations of ozone-depleting substances, including a larger predicted future usage in developing countries. [36], As explained above, the primary cause of ozone depletion is the presence of chlorine-containing source gases (primarily CFCs and related halocarbons). "Voluntary Initiatives to Reduce Greenhouse Gas Emissions", Sarma, K. Madhava "Compliance with the Multilateral Environmental Agreements to Protect the Ozone Layer" in Ulrich Beyerlin et al. However, 99% of its total mass lies in two regions within the first 50 kilometres above the Earth's surface. Ozone depletion only stops when the chlorine randomly reacts with another molecule to form a long-lived, stable substance. ClO is produced by the reaction of Cl with ozone—its observation thus demonstrated that Cl radicals not only were present in the stratosphere but also were actually involved in destroying ozone.
[46], After a 1976 report by the United States National Academy of Sciences concluded that credible scientific evidence supported the ozone depletion hypothesis[77] a few countries, including the United States, Canada, Sweden, Denmark, and Norway, moved to eliminate the use of CFCs in aerosol spray cans.
In the presence of UV light they produce free chlorine, fluorine and bromine which in contact with ozone break down their molecules. Ozone decreases of as much as 70% have been observed on a few days. It extends to about 6 to 17 kilometres above the Earth's surface and is thickest at the equator.
On the bright side, research and monitoring have shown that the ozone hole over Antarctica has started to heal. [71] "Despite mechanisms to reduce or repair the effects of increased ultraviolet radiation, plants have a limited ability to adapt to increased levels of UVB, therefore plant growth can be directly affected by UVB radiation.
Short-wavelength UV radiation splits an oxygen (O2) molecule into two oxygen (O) atoms, which then combine with other oxygen molecules to form ozone.
[138][139], As winters that are colder are more affected, at times there is an ozone hole over Tibet. By signing up for this email, you are agreeing to news, offers, and information from Encyclopaedia Britannica. [141], Research in 2012 showed that the same process that produces the ozone hole over Antarctica occurs over summer storm clouds in the United States, and thus may be destroying ozone there as well.[142][143]. One study showed that a 10 percent increase in UVB radiation was associated with a 19 percent increase in melanomas for men and 16 percent for women. As with Antarctica, large increases in concentrations in reactive chlorine have been measured in Arctic regions where high levels of ozone destruction occur. There, ultraviolet light breaks down the bond holding chlorine atoms to the CFC molecule, and the free chlorine atoms are able to participate in a number of chemical reactions, which destroy ozone.
[46] The ozone hole was seen as a "hot issue" and imminent risk[47] as laypeople feared severe personal consequences such as skin cancer, cataracts, damage to plants, and reduction of plankton populations in the ocean's photic zone. [53], Ozone, while a minority constituent in Earth's atmosphere, is responsible for most of the absorption of UVB radiation. Harry E. Preble Professor of Atmospheric Sciences, Department of Atmospheric Sciences, University of Illinois, Urbana, Illinois. Hydro chlorofluorocarbons (HCFCs) are one of the temporary replacements for CFCs, while Hydroflourocarbons (HFCs) are the main long term replacements for CFCs and HCFCs. While it is true that volcanoes and oceans release large amounts of chlorine, the chlorine from these sources dissolves in water so it washes out of the lower atmosphere in rain. In laboratory analytics (a former "essential" use) the ozone depleting substances can be replaced with various other solvents. They identify key chemical reactions and transport processes that bring CFC photolysis products into contact with ozone. We're all in this together. Cyanobacteria are sensitive to UV radiation and would be affected by its increase. Polar winters are dark, consisting of three months without solar radiation (sunlight). The thinning is most pronounced in the polar regions, especially over Antarctica. Reports have found that when plants are exposed to UVB radiation similar to stratospheric ozone depletion, there was no significant change in plant height or leaf mass, but showed to respond in shoot biomass and leaf area in a small decrease. Since more ozone loss occurred than could be explained by the supply of reactive chlorine available in the polar regions by known processes at that time, other hypotheses arose. “We’re going to do a bowling for dollars type thing, but it’ll be breakdancing on cardboard for yen for the ozone layer, so it’ll be called breaking in space.”, Author is a student at P. K Roy Memorial College, Dhanbad, Jharkhand, India. In 1976 the United States National Academy of Sciences released a report concluding that the ozone depletion hypothesis was strongly supported by the scientific evidence. Two bar graphs depicting the maximum ozone hole size and the minimum ozone coverage (in Dobson units) of the Southern Hemisphere ozone hole, 1979–2014. Atmospheric measurements have clearly corroborated theoretical studies showing that chlorine and bromine released from halocarbons in the stratosphere react with and destroy ozone. The reduction of ozone concentration in the stratosphere allows more UV-B radiations to penetrate into the Earth’s atmosphere. are continuing. Crutzen, Molina, and Rowland received the Nobel Prize for Chemistry in 1995 for their efforts.
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