ENV 200 Chapter 6, Fall 2013

The Ozone Layer • Located in the stratosphere at 17 to 30 km above sea level • Contains much of the atmosphere’s small amount of ozone • Acts as a global sunscreen, keeping about 95% of the sun’s harmful UV radiation (UV-A and UV-B) away How is Ozone Formed? • Through the interaction of oxygen gas (O2) and ultraviolet radiation (UV) emitted by the sun 3 O2 + UV ↔ 2 O3 Ozone Depletion • Seasonal depletion (thinning) of ozone concentrations in the stratosphere above Antarctica and the Arctic • Lower overall thinning everywhere except over the tropics Ozone Depletion Global Average Total Ozone Values in the Stratosphere (1979–2005) Ozone Depletion • Each year, 40 to 50% of the ozone in the upper stratosphere over Antarctica (100% in some places) disappears in October and November. • When this seasonal thinning ends, huge masses of ozone-depleted air above Antarctica flow northward, lingering for a few weeks above Australia, New Zealand, South Africa and South America; raises damaging UV-B levels 3 – 20%!! Ozone Depletion Ozone Depletion • Observed loss of ozone referred to as “ozone hole” • More accurately “ozone thinning” as ozone depletion varies with altitude and location Ozone Depletion Massive Ozone Thinning over Antarctica in 2007 Ozone Depletion • Similar, usually less severe thinning (11 to 38%; compared to ca. 50% over Antarctica) over the Arctic from February to June Chlorofluorocarbons (CFCs) • First CFC discovered in 1930 by Thomas Midgley, Jr., a General Motors chemist • Similar compounds later developed by chemists to create a family of compounds known as CFCs or Freons Chlorofluorocarbons (CFCs) • Thought to be dream chemicals because they are – chemically un-reactive. – odorless. – nonflammable. – nontoxic. – noncorrosive. – inexpensive to manufacture. Chlorofluorocarbons (CFCs) • Uses of CFCs – Coolants in air conditioners and refrigerators – Propellants in aerosol spray cans – Cleaners for electronic parts such as computer chips – Fumigants for granaries and ship cargo holds – Also used to fill tiny bubbles in plastic foam used for packaging and insulation Ozone Depletion • In 1974, Sherwood Rowland and Mario Molina of the University of California, Irvine, indicated that chlorofluorocarbons (CFCs) and other ozone-depleting chemicals were lowering the average concentration of ozone in the stratosphere!! Ozone Depletion • Other ozone-depleting chemicals include – halons and hydrobromofluorocarbons (HBFCs) used in fire extinguishers. – methyl bromide, a widely used fumigant. – hydrogen chloride emitted into the stratosphere by space shuttles. – some cleaning solvents such as carbon tetrachloride. Ozone Depletion • Rowland and Molina 1974; Nobel Prize in Chemistry 1995 – CFCs are persistent in the atmosphere; once in the atmosphere, they remain there. – CFCs rise into the stratosphere over 11-20 years. – In the stratosphere, they break down under high- energy UV radiation; Halogens produced accelerate the breakdown of ozone to oxygen gas. – Each CFC molecule can last 65-385 years, depending on its type. Ozone Depletion Ozone Depletion Ozone Depletion • Arctic unlikely to develop the large-scale ozone thinning found over the Antarctic based on models • Ozone depletion over the Arctic and Antarctic predicted to be the worst between 2010 and 2019!! Ozone Depletion • Ozone depletion in the stratosphere poses a serious threat to – humans. – other animals. – some primary producers (mostly plants). Ozone Depletion Why should we worry about ozone depletion? • Biologically damaging UV-A and UV-B radiation will reach the earth, giving people more eye cataracts, worse skin burns and more skin cancer!! • UV radiation may impair or destroy phytoplankton, especially in Antarctic waters!! Simplified food web in the Antarctic Ozone Depletion Ozone Depletion Ozone Depletion What Shall We Do? • Immediately stop producing ozone- depleting chemicals • According to models, even with immediate and sustained action, 60 – 100 years for recovery of the ozone layer • Global warming in troposphere makes stratosphere cooler; slower rate of ozone repair Ozone Depletion What Shall We Do? • Montreal Protocol (1987; 36 countries): Cut emissions of CFCs but not other ozone- depleting chemicals by about 35% between 1989 and 2000 • Copenhagen Protocol (1992; 93 countries): Amendment that accelerates the phase-out of key ozone depleting chemicals • Ozone protocols now signed by 191 countries: – Ozone levels to return to 1980 levels by 2068 – Ozone levels to return to 1950 levels by 2108 – Prevention is the key Ozone Depletion What Shall We Do? • Substitutes for CFCs are available; more are being developed. • HCFC-22 – Most widely used substitute; causes some ozone depletion – Also warms the troposphere (10,000 times more than carbon dioxide per molecule) – To be phased out in 2020 in developed countries and 2030 in developing countries