Chemicals that React with Stratospheric Ozone

5 Chemicals that React with Stratospheric Ozone  

The previous seetion mentions the harmful effects of ground-level ozone. The stratosphere, loeated about 6 to 30 miles (10 to 50 km) above the ground. 

Over the past 3 decades, scientists have concluded that this protective shield has been damaged. Each year, an ozone hole forms over Antarctica. Ozone levels there can fall to 60% below normal. Even over the United States, stratospheric ozone levels are about 3% below normal in siunmer and 5% below normal in winter. 

In the 1970s, scientists linked several man-made substances to ozone depletion, including carbon tetrachloride (CCE), chlorofluorocarbons (CFCs), halons, methyl bromide, and methyl chloroform. These chemicals leak from air conditioners, refrigerators, insulating foam, and some industrial processes. Winds carry them through the lower atmosphere into the stratosphere, where they react with strong solar radiation to release chlorine and bromine atoms. These atoms initiate chain reactions that consiune ozone. Scientists estimate that a single chlorine atom can destroy 100,000 ozone molecules. 

The 1998 and 2002 Scientific Assessments of Stratospheric Ozone firmly established the link between decreased ozone and increased UV-B radiation. In humans, UV-B is linked to skin cancer. It also contributes to cataracts and suppression of the immune system. The effects of UV-B on plant and aquatic ecosystems are not well understood. However, the growth of certain plants can be slowed by excessive UV-B. Some scientists suggest that marine phytoplankton, which are the foundation of the ocean food chain, are already under stress from UV-B. If true, this could adversely affect supplies of food from the oceans. 

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Just as in the stratosphere, the balance between the rates at which ozone is formed and destroyed will determine the ozone level. The rate constants in Table 11.2 show that reaction ll.lO.c is much faster than reaction ll.lO.b. Thus, if no other chemicals present react with NO, ozone is not likely to form in the... 

Ozone is of major interest to tropospheric chemists for two reasons It leads to the production of hydroxyl radicals, and it is a greenhouse gas. For many years, tropospheric ozone was believed to be chemically inert. Scientists held that the ozone present in the troposphere was formed initially in the stratosphere — where ultraviolet radiation is of high enough energy to dissociate oxygen — and mixed down into the troposphere. It was postulated further that ozone was removed from the atmosphere primarily by reacting with Earth s surface. 

Atmospheric N20 was discovered in 1938 by Adel via infrared absorption features in the solar spectrum. For the next 30 yr, N20 aroused little interest, presumably because it is neither a hazardous pollutant nor does it display any particular chemical activity. In fact, there are no gas-phase reactions that remove it from the troposphere as far as we know. In the stratosphere (see Chapter 3), N20 undergoes photodecomposition, and it reacts with O( D). The second reaction is the major source of higher nitrogen oxides in that region, and since these reduce ozone catalytically via chain reactions, N20 is an important agent in controlling the stratospheric ozone balance. The recognition of this relationship by Crutzen (1970, 1971) and McElroy... 

Without stratospheric ozone (O3), harmful solar radiation would cause gene alterations. Ozone forms when O2 breaks and each O atom reacts with another O2 molecule. It is destroyed by reaction with Cl atoms that are formed when the C—Cl bond in synthetic chemicals breaks. Find the wavelengths of light that can break the C—Cl bond and the bond in O2. 

The ozone hole today Scientists have also learned that the ozone hole forms each year over Antarctica during the spring. Stratospheric ice clouds form over Antarctica when temperatures there drop below —78°C. These clouds produce changes that promote the production of chemically active chlorine and bromine. When temperatures begin to warm in the spring, this chemically active chlorine and bromine react with ozone, causing ozone depletion. This ozone depletion causes the ozone hole to form over Antarctica. Some ozone depletion also occurs over the Arctic, but temperatures do not remain low for as long, which means less ozone depletion in the Arctic. 

FIGURE 7.5 A simplified summary of how chlorofluorocarbons and other chlorine-containing compounds can destroy ozone in the stratosphere faster than it is formed. Note that chlorine atoms are continuously regenerated as they react with ozone. Thus, they act as catalysts, chemicals that speed up chemical reactions without being used up by the reaction. Bromine atoms released from bromine-containing compounds that reach the stratosphere destroy ozone by a similar mechanism. 

It was estimated that the eruption of the Mount Pinatubo volcano resulted in the injection of 20 million metric tons of SO2 into the atmosphere. Most of this SO2 underwent oxidation to SO3, which reacts with atmospheric water to form an aerosol. (a) Write chemical equations for the processes leading to formation of the aerosol (b) The aerosols caused a 0.5-0.6 C drop in surface temperature in the northern hemisphere. What is the mechanism by which this occurs (c) The sulfate aerosols, as they are called, also cause loss of ozone from the stratosphere. How might this occur ... 

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