سؤال قراءة ستيب محلول — فهم التفاصيل (تدريب 5887)

(1) Ozone is an undesirable secondary air pollutant in lower regions of the atmosphere. In contrast, ozone in the upper region of the atmosphere, called the stratosphere, has an important natural function. The ozone in the stratosphere screens Earth from harmful ultraviolet light. Ozone forms in the stratosphere through reactions of molecular oxygen that absorb radiation. (2) Ozone molecules formed in the stratosphere absorb more than 90% of ultraviolet radiation, including the higher energy ultraviolet light sometimes-called UV-C and UV-B radiation. In this way, ozone serves to shield the lower atmosphere and Earth’s surface from a significant fraction of high-energy UV light that is part of solar radiation. Ozone concentrations stay at relatively fixed levels because ultraviolet light also decomposes ozone molecules to reform oxygen molecules. (3) The formation and decomposition of ozone produce a steady-state concentration of ozone within the stratosphere. This layer or region of ozone is sometimes called the ozonosphere, and a maximum concentration of about 10 ppm (parts per million) occurs in the stratosphere from 25 to 30 km in altitude. (4) Recent observations of the ozone layer using rocket, high-flying aircraft, and satellites revealed that the ozone concentration is dramatically decreasing, especially in the polar regions. This decrease in concentration is called thinning of the ozone layer, and larger decreases are called “holes” in the layer. The “holes” in the polar regions of Earth seem to vary with the seasons. The hole over Antarctica is most widespread from September to November, a time that corresponds to springtime in the southern hemisphere, In Fall of 1998 the ozone hole over Antarctica was the largest ever observed up to that time, “Holes” have also been observed over the north pole and some industrialized regions of the northern hemisphere. (5) A decrease in the ozone concentration allows more ultraviolet radiation to reach the surface of Earth. This increase in radiation causes damage to humans, other animals, plants, bacteria, and microscopic marine organisms (phytoplankton and zooplankton). For each 1% decrease in the ozone layer, there is a 2% increase in the UV radiation reaching Earth. It is estimated that a 2% increase in radiation may result in a 4% to 10% increase in basal-cell skin cancer and an 8% to 20% increase in more serious squamous-cell skin cancer but the significance of these effects is not known. (6) The thinning of the ozone layer appears to be a result of human activity. The chemical culprits are synthetic compounds containing chlorine, fluorine, and carbon known as chlorofluorocarbons or CFCs (also known as Freons). CFCs were developed in the 1930s. Over the years, they found use as coolant gases in refrigerators and air conditioners, as propellant gases in aerosol cans, as industrial solvents, and as foaming agents in plastic products like Styrofoam and cushion materials. In the United States, they are no longer used in aerosol cans and other uses are being phased out. Halons, which are chemically similar to CFCs. find use in fire extinguishers and medical anesthetics. Halons also affect the ozone layer. (7) Most gaseous pollutants that enter the atmosphere have a natural sink. The term sink refers to a long-term repository in the environment. A sink is some place or chemical form in which a chemical ends up. Normally, a pollutant is changed by some chemical reaction and is washed from the atmosphere by the rain. Sulfur dioxide and nitrogen oxides, for example, are transformed to sulfuric acid and nitric acid, which are carried to Earth as acid rain. (8) One reason CFCs are used in products or as solvents is that they are very chemically inert. They are chemically stable, are nontoxic, do not support combustion, and are non-corrosive. Because CFCs are chemically inert and have no environmental sink, they are destructive to the ozone. What chemical is used in fire extinguishers?