AA-CD-1 9fl93 Attachment 10 •u''a MM A (NASA-cR-190953) REPORTS TO THE N93--16438 NATION ON OUR CHANGING PLANET. FALL 1992, NO. 2: OUR OZONE SHIELD Semiannual Report, Apr. - Sep. 1992 Unclas (University Corp. for Atmospheric s •rch) 22 p G3/45 0126301 ii1UGNAL Our Ozone Shweeld Ozone and Humankind ................................................... U A Problem Arises: The Early 1970s ................ Stratospheric Ozone: The First Decade ......................... D The Ozone Years: 1985-1989 .................... D Our Ozone Layer: Present and Future ................ oi1 .... C C PS'1C: ,.. arCIa 1 Oceanic and . Repnsn r CLan 1992 Orbiting alx)ve the Eapib, an asim- )!(111i can look ilo,,,i on our home (1/11/ '(( II)(, 1/)!!! /)/l! ,.i/)/)()il //)(1/ our rims plane!. Thai iransparew blanket—our atmucphere—rnaks 4[e x)ssibIe. It provides the air u Imatbe and regulates our global temperature. And it contains a special ingredient called ozone that filters deadly solar radiation. 4 - Life as we know it is possible in part because of the protection afforded by the ozone lar. Gradually, it has become clear to scientists and to governments alike that human activities are threatening our ozone shield. Behind this envirnnmentalprnblem lies a tale of twin cbalknges: the scientific quest to understand our shield and the debate among governments OZOPk' over bow to liest prntecl it. lien.' is the story. -. iLt yh: . ) 7/ 711 In 1 I Our Ozone Sbield ZONE AND HUMANKIND. For nearly a billion years, ozone molecules in the atmosphere have safeguarded life on this planet. But over the past half century, humans have placed the ozone layer in jeopardy. We have unwittingly polluted the air with chemicals that threaten to eat away the life-protecting shield surrounding our world. Although ozone molecules play such a vital role in the atmosphere, they are exceedingly rare; in every million molecules of air, fewer than ten are ozone. Nitrogen and oxygen make up the vast proportion of the molecules in the air we breathe. In this way, ozone resembles a critical spice in a pot of soup. Using just a few grains of a particular herb, a chef can season the whole pot with a distinctive flavor. Ozone molecules show different character traits depending on where they exist in the atmosphere. About 90 percent of the ozone resides in a layer between 10 and 40 kilometers (6 and 25 miles) above the Earth's surface in a region of the atmosphere called the strato- sphere. Ozone there plays a beneficial role by absorbing dangerous ultraviolet radiation from the sun. This is the ozone threatened by some of the chemical pollutants that we have released into the atmosphere. Close to the planet's surface, however, ozone displays a destructive \ side. Because it reacts strongly with other molecules, it can severely • damage the living tissue of plants and animals. Low-lying ozone is a key component of the smog that hangs over many major cities across th world, and governments are attempting to decrease its levels. Ozoriein the region below the stratosphere—called the troposphere— '-can also contribute to greenhouse warming. Reports to the Nation • Fa111992 N Nitrogen Herurn. r'ecp Krypton. Xenon \\\ Argon Ozone Water Vapo 4 y Hyd Carbon Dioxide L : ) Other Although smog ozone and stratospheric ozone Even as the sun's energy produces new ozone, are the same molecule, they represent separate these gas molecules are continuously destroyed by environmental issues, controlled by different forces natural compounds containing nitrogen, hydrogen, in the atmosphere. This monograph will focus on and chlorine. Such chemicals were all present in the stratospheric ozone layer and the world's the stratosphere—in small amounts—long before attempts to protect it. humans began polluting the air. Nitrogen comes What is ozone and where does it originate? The from soils and the oceans, hydrogen comes term itself comes from the Greek word meaning mainly from atmospheric water vapor, and chlo- "smell, a reference to ozone's distinctiv'ly pungent rine comes from the oceans. odor. Each molecule contains three oxygen atoms The stratospheric concentration of ozone there- bonded together in the shape of a wide triangle. In fore represents a balance, established over the the stratosphere, new ozone molecules are con- aeons, between creative and destructive forces. stantly created in chemical reactions fueled by The total level of ozone in the stratosphere remains power from the sun. fairly constant, an arrangement resembling a tank The recipe for making ozone starts off with with open drains. As long as the amount of water oxygen molecules (0). When struck by the pouring in equals the amount flowing out the drain sun's rays, the molecules split apart into single holes, the water level in the tank stays the same. - , oxygen atoms (0), which are exceedingly In the stratosphere, the concentration of ozone reactive. Within a fraction of a second, does vary slightly, reflecting small shifts in the the atoms bond with nearby oxygen balance between creation and destruction. These molecules to form thatoniic mol- fluctuations result from many natural processes ecules of ozone (0). such as the seasonal cycle, volcanic eruptions, and changes in the sun's intensity. Solar rays make ozone For about a billion years, the natural ozone system worked smoothly, but now human beings have upset the delicate balance. By polluting the atmosphere with. C) ditional chlorine-containing che R eactiv enit n Pr c C) destroys ozone . The amount of ozone in the Earths stratosphere is a balance between continuous production and loss. Ozone is produced by -1111111111111110- IQ"," ^C the sun's rays. It is removed by chemical reactions. But humans have added to the amount of reactive chlorine compounds in the Redaecstitvroey csh olozroinnee ._iuul!IIIIIlII1 Reactive hydrogen stratosphere. Since the loss of ozone is now greater than the destroys ozone production of ozone, rare thinning qur protective shield Reports to the Nation • Fall 1992 does SUCh a Sma/ip,. 14011' huge rolO cats, we have enhanced the forces that destroy accelerate the natural chemical destruction of ozone—a situation that leads to lower ozone ozone, causing ozone levels to drop. concentrations in the stratosphere. The addition of In 1974, news of another possible threat to the these chemicals is the same as drilling a larger ozone layer made national headlines. This time "chlorine" drain in the tank, causing the level to scientists implicated a widely used class of chemi- drop. cals known as chlorofluorocarbons (CFCs), which were most commonly known as the aerosol propel- A Problem Arises: The Early 1970s lant in spray cans. Invented in the late 1920s, CFCs No one dreamed human activity would threaten contain chlorine, fluorine, and carbon atoms ar- the ozone layer until the early to mid-1970s, when ranged in an extremely stable structure. scientists discovered two potential problems: ultra- Through decades of use, CFCs proved fast passenger planes and spray cans. themselves to be ideal compounds for many The plane threat surfaced first, after the inven- purposes. They are nontoxic, noncorrosive, tion of a new breed of commercial aircraft called nonflammable, and unreactive with most other supersonic transport (SST). These planes could fly substances. Because of their special properties, faster than the speed of sound and promised to they make excellent coolants for refrigerators trim hours off long journeys. In the 1970s, the and air conditioners. CFCs also trap heat well, so United States and other nations began considering manufacturers put them into foam whether to build large fleets of such ultrafast jets. As scientists such as Harold Johnston and Paul Crutzen looked at the SST issue, they grew concerned about the effects such planes might have on the stratosphere. SSTs are unusual because they must fly high up in the atmo- SatOspilelic sphere—where the air is thin—to achieve their fast speeds. Several researchers suspected that the reactive nitrogen . C:I1L compounds from SST exhaust might Most of the Earths ozone is high in the upper part of the atmosphere—thp stratosphere. This good" ozone serves as our shield against incoming solar ultraviolet radiation. The "bad" ozone in the lower part of the atmosphere— the troposphere—adds to greenhouse warming and is a major part of smog in cities. VA COLOR PHOTOGRAPH products such as cups and insulation for houses. harded by the sun's high-energy radiation. P CFCs Most scientists had not worried about how CFCs therefore carried millions of tons of extra chlorine would affect the atmosphere. But two chemists, F. atoms into the stratosphere, adding much more Sherwood Rowland and Mario Molina, began con- than the amount of chlorine supplied naturally by sidering these wonder compounds, and they un- the oceans in the form of methyl chloride. covered something disturbing. Because CFCs were Rowland and Molina hypothesized that the chlo- extremely stable in the lower atmo- rine buildup from CFCs would spell severe trouble sphere, they could drift up for the ozone layer. According to their predictions, kk^to the stratosphere, each chlorine atom could destroy 100,000 ozone where they would molecules, meaning that decades of CFC use could cause substantial declines in the concentration of stratospheric ozone. Any drop in ozone levels, whether from