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pgeog251 ch17 af

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Education

Published on February 20, 2008

Author: Demetrio

Source: authorstream.com

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Slide1:  Chapter 17 Ozone Depletion Slide2:  Ozone Depletion: Introduction the primary reason why ozone is so important is its absorption of UV radiation UV radiation can be harmful to plants and animals In the paleoclimate chapter we discussed how the rise of oxygen and ozone in the atmosphere allowed for different life forms to flourish Here we discuss UV radiation’s biological effects, ozone’s radiative effects, and the chemistry of ozone, in more detail Slide3:  Ozone Depletion: Problems with Ozone Ozone problem #1: stratospheric ozone is being depleted. Ozone in the stratosphere is a good thing. This is the subject of this chapter Ozone problem #2: ozone in the lower troposphere, where people live, is a secondary pollutant caused by chemical reactions of products of fossil fuel combustion in the presence of sunlight and high temperatures. This is a bad thing. Not the subject of this chapter. Additional ozone impact: in the troposphere ozone oxidizes many pollutants, which is good for air quality. Not the subject of this chapter Slide4:  UV A, B, and C and Ozone Absorption UV A 320-400 relatively harmless UV B 290-320 harmful UV C 200-290 very harmful Ozone Absorption Slide5:  UVB and its Biological Effects Erythemal action spectrum: describes the dose rate leading to sunburn in humans Shorter wavelengths (higher frequencies) have more energy, and are more harmful Doses above the erythemal action spectrum can cause cancer, cataract, retina damage. harms plant growth, and aquatic life Slide6:  UVB and the vertical distribution of Ozone Both theoretical and observational evidence suggest that the transmissivity of the atmosphere to UVB radiation is proportional to ozone concentrations Slide7:  UVB and the vertical distribution of Ozone Both theoretical and observational evidence suggests that the transmissivity of the atmosphere to UVB radiation is proportional to ozone concentrations ozone concentrations peak in the stratosphere (ozone absorption is responsible for higher temperatures in the stratosphere) a few ppm Slide8:  The Spatial Distribution of Ozone Dobson Units (DU) is the typical unit of measurement of atmospheric ozone concentrations. 1 DU = a layer of pure ozone 0.001 m thick at 1 atm pressure How many millimeters is 0.001 m? Latitudinal / seasonal distribution of ozone prior to ozone hole Slide9:  The Atmospheric Chemistry of Ozone The production and maintenance of ozone in the stratosphere is caused by a complicated series of chemical reactions The Chapman Mechanism is the simplified description of the chemistry leading to the production and maintenance of ozone in the stratosphere The Chapman Mechanism includes a series of four chemical reactions O2 + photon O + O O + O2 + M  O3 + M O3 + photon  O + O2 O + O3  2 O2 Slide10:  The Chapman Mechanism O2 + photon O + O Production of ozone O + O2 + M  O3 + M O3 + photon  O + O2 Destruction of ozone O + O3  2 O2 The production of ozone (reactions 1 and 2) (reaction 1) Oxygen is split apart by photolysis. Requires high frequency UVC radiation. Since all radiation of this frequency is absorbed above 20km, the stratosphere and the ozone layer exist above the troposphere (also see reaction 3) (reaction 2) Atomic Oxygen combines with Oxygen to form ozone. Requires the presence of a third molecule “M” Slide11:  The Chapman Mechanism O2 + photon O + O O + O2 + M  O3 + M O3 + photon  O + O2 O + O3  2 O2 The destruction of ozone (reactions 3 and 4) (reaction 3) ozone is split into O2 and O by photolysis. Does not require high energy radiation; requires visible light. (reaction 4) ozone combines with atomic oxygen to form O2. The Chapman cycle by itself would result in 30% more ozone than actually exists. There must be other chemical reactions occurring. Slide12:  Catalytic reactions that hasten the destruction of Ozone: Nitrogen, Chlorine, and Bromine O2 + photon O + O O + O2 + M  O3 + M O3 + photon  O + O2 O + O3  2 O2 Catalytic Reaction: A reaction in which a molecule that increases the rate of a chemical reaction, but is itself unaffected by the reaction. That molecule is called the Catalyst Catalytic reactions involving Nitrogen and Chlorine act to hasten reaction 4, thus depleting ozone more quickly than if they did not exist. Slide13:  Catalytic reactions that hasten the destruction of Ozone: Nitrogen, Chlorine, and Bromine O2 + photon O + O O + O2 + M  O3 + M O3 + photon  O + O2 O + O3  2 O2 Catalytic Nitrogen Reaction (Nitric Oxide): NO + O3  NO2 + O2 NO2 + O  NO + O2 ____________________ O3 + O  2O2 Catalytic Chlorine Reaction: Cl + O3  ClO + O2 ClO + O  Cl + O2 ____________________ O3 + O  2O2 Slide14:  Source of NO in the Stratosphere: the odd Nitrogen (i.e. fixed Nitrogen) cycle Nitrous oxide: microbial activity in soils (denitrification by bacteria) photolysis Nitric acid Slide15:  Source of Cl in the Stratosphere: the Chlorine cycle Chlorofluorocarbons (CFCs): freon-11 (CCl3F), freon-12 (CCl2F2) chlorocarbons also photolysis Hydrogen Chloride Slide16:  F-11 F-12 Carbon tetrachloride Methyl chloroform Source of Cl in the Stratosphere: the Chlorine cycle Why have some of these Chlorine sources decreased more than others? Slide17:  The Antarctic Ozone Hole A drop in ozone concentrations over Antarctica and surrounding regions in Austral Spring (October) There is a northern hemisphere equivalent, but it is much weaker and less consistent Formation of the ozone hole involves chemistry , atmospheric dynamics, and radiation Slide18:  The Antarctic Ozone Hole Homogeneous Reactions: All reactants in the same phase (in the atmosphere, gaseous) Heterogeneous Reactions: Reactants are in different phases (occur on a solid or liquid surface) Antarctic Ozone Hole chemistry involves heterogeneous reactions that occur on droplets in Polar Stratospheric Clouds (PSCs): They affect the odd-nitrogen and chlorine cycles They provide sites for heterogeneous reactions that affect ozone Slide19:  The Antarctic Ozone Hole Antarctic Ozone Hole chemistry involves heterogeneous reactions that occur on droplets in Polar Stratospheric Clouds (PSCs): They affect the odd-nitrogen and chlorine cycles Link between nitrogen and chlorine cycles: ClO + NO2 + M  ClONO2 + M The presence of NO2 results in the removal of ClO. ClONO2 is not reactive, so the removal of ozone by Cl is inhibited In winter, NO2 is removed from atmosphere and incorporated into the PSC droplets, resulting in less Cl removal Slide20:  The Antarctic Ozone Hole Antarctic Ozone Hole chemistry involves heterogeneous reactions that occur on droplets in Polar Stratospheric Clouds (PSCs): They provide sites for heterogeneous reactions that affect ozone Heterogeneous reactions that result in more reactive Cl and therefor less Ozone: ClONO2 + HCl  Cl2 + HNO3 Cl2 + photon  Cl + Cl Sunlight is required for this reaction Slide21:  The Antarctic Ozone Hole Polar Vortex: Winter stratospheric circulation is westerly In the southern hemisphere, with fewer geographic obstructions, westerly flow is stronger than in the northern hemisphere This inhibits mixing between mid- and high-latitudes Depleted chemicals (e.g. NO2) are not replenished Before the breakdown of the vortex, springtime sunlight allows the Cl reaction to proceed, depleting ozone Slide22:  Mid-latitude Ozone Depletion Although Antarctic springtime depletion is the most dramatic, annual mean values across the mid- and high-latitudes have been decreasing. The reasons for this are not completely understood Slide23:  Halting ozone depletion: International Agreements Beginning with the 1987 Montreal Protocol, and under subsequent treaties in which the emission of chemicals that provide Cl to the atmosphere, Cl abundances in the atmosphere have leveled off, and are expected to decrease over the next century The chemicals include CFCs (freons) Why will Cl concentrations, and ozone levels, only start rebounding decades after emissions have ceased? Why were ozone agreements reached more quickly than climate change agreements? Slide24:  Halting ozone depletion: Freon Substitutes HCFCs replace CFCs Gets removed from the atmosphere by chemical reactions (the OH radical), so it has a short atmospheric life time HFCs replace CFCs No chlorine, and short atmospheric life time Potential health and environmental effects of these must be considered

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