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Copyright © 2010 R. R. Dickerson & Z.Q. Li 1 AOSC 620 The Ozone Hole R. Dickerson
Copyright © 2010 R. R. Dickerson & Z.Q. Li 2 Recap: The general for of a catalytic ozone destruction cycle is: X + O ₃ → XO + O ₂ XO + O → X + O ₂ O + O ₃ → 2O ₂ NET Where X is OH, NO, Cl, or Br. But nobody saw the Ozone Hole coming!
Copyright © 2010 R. R. Dickerson & Z.Q. Li 3 October 24, 2009 From NASA http://ozonewatch.gsfc.nasa.gov/index.html
Copyright © 2012 R. R. Dickerson & Z.Q. Li 4 Antarctic Ozone Hole In the Antarctic winter there is no sunlight and even in the spring there is too little UV to generate enough O atoms to destroy ozone. The annual loss of ozone over Antarctica is driven by heterogeneous chemistry and visible radiation. A good review is provided by Solomon Rev. Geophys., 1999, and “Scientific Assessment of Ozone Depletion: 2006” (WMO). The destruction of ozone is usually moderated by the production of chlorine nitrate, an important reservoir species. NO ₂ + ClO + M → ClONO ₂ + M † In the Antarctic winter, heterogeneous reactions liberate chlorine from long-lived HCl and ClONO 2 and sedimentation of polar stratospheric clouds “denitrifies” the stratosphere (Solomon et al., Nature, 1986; McElroy et al., Nature, 1986; Toon et al., GRL, 1986). HCl + ClONO 2 Cl 2 (gas) + HNO 3 (aqueous) Cl 2 + h 2Cl HNO 3 (aqueous) sediments (falls) out of stratosphere, removing a potential source of NO x. ice
Copyright © 2010 R. R. Dickerson & Z.Q. Li 5 Molina and Molina (1987) 2(Cl + O ₃ → O ₂ + ClO) ClO + ClO + M → (ClO) ₂ + M (ClO) ₂ + hv → Cl + ClOO ClOO + M → Cl + O ₂ + M 2O ₃ → 3O ₂ NET Two types of Polar Stratospheric Clouds (PSC’s) exist. Type I = HNO ₃ ● 3H ₂ O Nitric acid trihydrate, formed at T ≤ 195K Type II = H ₂ O Water ice formed at T ≤ 190K They move NO y species from the vapor phase to the condensed phase as HNO ₃. They move chlorine from the reservoir species HCl and ClONO ₂ to ClOx.
Copyright © 2010 R. R. Dickerson & Z.Q. Li 6 Molina and Molina (1987) 2(Cl + O ₃ → O ₂ + ClO) ClO + ClO + M → (ClO) ₂ + M (ClO) ₂ + hv → Cl + ClOO ClOO + M → Cl + O ₂ + M 2O ₃ → 3O ₂ NET McElroy, Salawitch, et al. (1986) Cl + O ₃ → ClO + O ₂ Br + O ₃ → BrO + O ₂ ClO + BrO → Cl + Br + O ₂ 2O ₃ → 3O ₂ NET
Copyright © 2010 R. R. Dickerson & Z.Q. Li 7 Airborne Antarctic Ozone Expedition: Punta Arenas, Chile,1987 Anderson et al., Science, 1991
Copyright © 2010 R. R. Dickerson & Z.Q. Li 8 THE ANTARCTIC OZONE HOLE DU Southern Hemisphere ozone column seen from TOMS, October 1 Dobson Unit (DU) = 0.01 mm O 3 STP = 2.69x10 16 molecules cm -2
Copyright © 2010 R. R. Dickerson & Z.Q. Li 9 Polar Stratospheric Clouds (PSCs)
Copyright © 2010 R. R. Dickerson & Z.Q. Li 10
Copyright © 2010 R. R. Dickerson & Z.Q. Li 11
Copyright © 2010 R. R. Dickerson & Z.Q. Li 12 World Production of CFCs
Copyright © 2010 R. R. Dickerson & Z.Q. Li 13 Multiphase Processes Gas-Surface Reactions Key to ozone hole formation are reactions of gaseous species with PSC’s. Multiphase reactions are important for tropospheric chemistry as well – the formation of HONO appears to take place predominantly on organic surfaces. The probability of a molecule A striking a unit area is Z = ¼ M[A] A Where M is the molecular number density and [A] is the mixing ratio of A, and A is the rms velocity of molecules A (see Kinetics Lecture). The number of collisions with a single spherical particle of radius r is proportional to the area of the particle, 4 r 2. For an ensemble of particles with total area S tot (units area of particles per volume of air or cm 2 cm -3 ) the total collision rate is R = ¼ S tot M[A] A
Copyright © 2010 R. R. Dickerson & Z.Q. Li 14
Copyright © 2010 R. R. Dickerson & Z.Q. Li 15 From Farman et al., Nature 1985.
Copyright © 2010 University of Maryland. This material may not be reproduced or redistributed, in whole or in part, without written permission from Ross Salawitch or Tim Canty. 16 TOMS OMI Ground Based Antarctic Ozone Loss: Hole cannot get wider or deeper. After Farman et al., Nature, 315, 207, 1985 Models now provide good overall simulation of Antarctic ozone loss. Scientific understanding of polar ozone depletion led to international ban of CFC production
Copyright © 2010 R. R. Dickerson & Z.Q. Li 17 ALTITUDE (km) 0 5 10 15 20 25 30 35 D. Hofmann, NOAA CMDL 0 5 10 15 OZONE ABUNDANCE (PARTIAL PRESSURE, mPa) OCTOBER AVERAGE 1967 - 1971 282 DU SEP 29, 1999 90 DU Ozone Hole Update, II O ZONE P ROFILES, S OUTH P OLE: U PDATE
Copyright © 2010 R. R. Dickerson & Z.Q. Li 18 Accommodation Coefficients Condensed phase has lower entropy than gas phase. Accommodation coefficients (reaction probabilities) should be greater at lower temperatures.
Copyright © 2010 R. R. Dickerson & Z.Q. Li 19 Heterogeneous Chemistry: Faster at low temperatures In all cases, must be measured in the laboratory (thanks, RJS 2010) Reaction probabilities given for various surface types, with formulations of various degrees of complexity, in Section 5 of the JPL Data Evaluation. Atmospheric Chemistry and Physics by Seinfeld and Pandis provides extensive treatment of aqueous phase chemistry, properties of atmospheric aerosol, organic aerosols, etc.
Ozone hole 2013 Copyright © 2013 R. R. Dickerson http://www.esrl.noaa.gov/gmd/odgi/ 20
Copyright © 2010 R. R. Dickerson & Z.Q. Li 21 Summary of Ozone Hole Formation Threat to DNA-based life forms. Not predicted by any models First observed by Farman et al., (Nature 1985). Ozone destruction nearly complete. Halogen (Cl & Br) reactions are responsible. Polar stratospheric Clouds play a central role. Multiphase (heterogeneous) reactions denitrify stratosphere. Reaction rates depend on accommodation coefficients, f(T). Replacement of CFC’s should heal ozone hole.
Copyright © 2010 R. R. Dickerson & Z.Q. Li 22 Summary of Ozone Hole Formation Threat to DNA-based life forms. Not predicted by any models First observed by Farman et al., (Nature 1985). Ozone destruction nearly complete. Halogen (Cl & Br) reactions are responsible. Polar stratospheric Clouds play a central role. Multiphase (heterogeneous) reactions denitrify stratosphere. Reaction rates depend on accommodation coefficients, f(T). Replacement of CFC’s should heal ozone hole.
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Copyright © 2010 R. R. Dickerson & Z.Q. Li 1 AOSC 620 Ozone in the Stratosphere R. Dickerson Note material on layers in general will be covered in AOSC.
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