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Stratospheric Chemistry EPS 133 28 March – 04 April 2011

Stratospheric Chemistry EPS 133 28 March – 04 April 2011. Polar Stratospheric Clouds. ATMOSPHERIC ATTENUATION OF SOLAR RADIATION. Solar UV radiation reaching the top of the atmosphere is absorbed by ozone. Based on ozonesonde observations in the 1970s. THE NATURAL OZONE LAYER.

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Stratospheric Chemistry EPS 133 28 March – 04 April 2011

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  1. Stratospheric ChemistryEPS 133 28 March – 04 April 2011 Polar Stratospheric Clouds

  2. ATMOSPHERIC ATTENUATION OF SOLAR RADIATION Solar UV radiation reaching the top of the atmosphere is absorbed by ozone

  3. Based on ozonesonde observations in the 1970s THE NATURAL OZONE LAYER

  4. 1 Dobson Unit (DU) is defined to be 0.01 mm thickness at stp; the ozone layer over Labrador is ~300 DU. Mean ratio, column O3: air = 5 x 10-7

  5. Ozone mixing ratio in parts per million

  6. SOLAR SPECTRUM AND ABSORPTION X-SECTIONS O3+hv O2+hv

  7. CHAPMAN MECHANISM FOR STRATOSPHERIC OZONE (1930) Odd oxygen family [Ox] = [O3] + [O] slow R2 R1 O2 O O3 fast R3 R4 slow

  8. Lifetime of O atoms: STEADY-STATE ANALYSIS OF CHAPMAN MECHANISM …is sufficiently short to assume steady state for O: …so the budget of O3 is controlled by the budget of Ox. Lifetime of Ox: τOx Steady state for Ox:

  9. PHOTOLYSIS RATE CONSTANTS: VERTICAL DEPENDENCE quantum yield absorption X-section photon flux

  10. shape determined by k1nO2 CHAPMAN MECHANISM vs. OBSERVATION -3 Chapman mechanism reproduces shape, but is too high by factor 2-3 e missing sink!

  11. non-radical radical + radical RADICAL REACTION CHAINS IN THE ATMOSPHERE photolysis thermolysis oxidation by O(1D) Initiation: bimolecular redox reactions Propagation: radical + non-radical non-radical + radical radical redox reaction Termination: non-radical + non-radical radical + radical non-radical + M 3-body recombination radical + radical + M

  12. H2O mixing ratio WATER VAPOR IN STRATOSPHERE Source: transport from troposphere, oxidation of methane (CH4)

  13. Ozone loss catalyzed by hydrogen oxide (HOx ≡ H + OH + HO2) radicals Initiation: Propagation: Termination: slow H2O OH HO2 fast HOx radical family slow

  14. Rate limiting step: Example OH + O3 -> HO2+ + O2k1 HO2 + O3 -> OH + O2k2 HO2 + NO ->->-> OH + NO + O3k3 { + O2 + hν … } d[OH] / dt = -d[HO2] / dt = - k1[OH][O3] + k2[O3][HO2] + k3*[NO][HO2] ≈ 0 A d[O3] / dt = -k1[OH][O3] – k2[HO2][O3] + k3*[NO][HO2] B To B,add (-1)xA ≈ 0 d[O3] / dt = - 2 k2 [HO2][O3] OH + O3 HO2 + O3 HO2 + NO Rate limiting step for removal of ozone by Reactions 1, 2, 3

  15. STRATOSPHERIC OZONE BUDGET FOR MIDLATITUDES CONSTRAINED FROM 1980s SPACE SHUTTLE OBSERVATIONS

  16. H2O mixing ratio NITROUS OXIDE IN THE STRATOSPHERE

  17. ATMOSPHERIC CYCLING OF NOx AND NOy

  18. Rate limiting step, NOx: Example NO + O3 -> NO2+ + O2k1 NO2 + hν -> NO + O -> O3k2 NO2 + O -> NO + O2k3 d[NO] / dt = -d[NO2] / dt = - k1[NO][O3] + k2[NO2] + k3[NO2][O] ≈ 0 A d[O3] / dt = -k1[NO][O3] + k2[NO2] - k3[NO2][O] B To B,add (-1)xA ≈ 0 d[O3] / dt = - 2 k3 [NO2][O] NO + O3 NO2 + O NO2 + hv Rate limiting step for removal of ozone by Reactions 1, 2, 3

  19. STRATOSPHERIC DISTRIBUTION OF CF2Cl2 (CFC-12)

  20. ATMOSPHERIC CYCLING OF ClOx AND Cly

  21. SOURCE GAS CONTRIBUTIONS TOSTRATOSPHERIC CHLORINE (2004)

  22. CHLORINE PARTITIONING IN STRATOSPHERE

  23. WHAT IS A RATE-LIMITING STEP? • From IUPAC: “A rate-controlling (rate-determining or rate-limiting) step in a reaction occurring by a composite reaction sequence is an elementary reaction the rate constant for which exerts a strong effect — stronger than that of any other rate constant — on the overall rate.”

  24. altitude Latitude Latitude http://ccmc.gsfc.nasa.gov/modelweb/atmos/msise.html ftp://hanna.ccmc.gsfc.nasa.gov/pub/modelweb/atmospheric/msis/msise90/

  25. altitude Latitude Latitude

  26. Stratospheric Circulation Prof. James R. Holton

  27. OZONE TREND AT HALLEY BAY, ANTARCTICA (OCTOBER) Farman et al. paper published in Nature 1 Dobson Unit (DU) = 0.01 mm O3 STP = 2.69x1016 molecules cm-2

  28. SPATIAL EXTENT OF THE OZONE HOLE Mean October data Isolated concentric region around Antarctic continent is called the polar vortex. Strong westerly winds, little meridional transport

  29. THE POLAR VORTEX (Sep-Oct 2006)

  30. THE OZONE HOLE IS A SPRINGTIME PHENOMENON

  31. VERTICAL STRUCTURE OF THE OZONE HOLE:near-total depletion in lower stratosphere Argentine Antarctic station southern tip of S. America

  32. ASSOCIATION OF ANTARCTIC OZONE HOLEWITH HIGH LEVELS OF CLO Sept. 1987 ER-2 aircraft measurements at 20 km altitude south of Punta Arenas O3 ClO O3 Sep. 16 Edge of Polar vortex ClO Sep. 2, 1987 20 km altitude Measurements by Jim Anderson’s group (Harvard)

  33. SATELLITE OBSERVATIONS OF ClO IN THE SOUTHERN HEMISPHERE STRATOSPHERE

  34. WHY THE HIGH ClO IN ANTARCTIC VORTEX?Release of chlorine radicals from reactions of reservoir species in polar stratospheric clouds (PSCs)

  35. PSC FORMATION AT COLD TEMPERATURES PSC formation Frost point of water

  36. HOW DO PSCs START FORMING AT 195K?HNO3-H2O PHASE DIAGRAM Antarctic vortex conditions PSCs are not water but nitric acid trihydrate (NAT) clouds

  37. DENITRIFICATION IN THE POLAR VORTEX:SEDIMENTATION OF PSCs

  38. CHRONOLOGY OF ANTARCTIC OZONE HOLE

  39. TRENDS IN GLOBAL OZONE Mt. Pinatubo

  40. LONG-TERM COOLING OF THE STRATOSPHERE Sep 21-30, 25 km, 65-75˚S Increasing CO2 is expected to cool the stratosphere

  41. TRENDS IN POLAR OZONECould greenhouse-induced cooling of stratosphereproduce an Arctic ozone hole over the next decade? Race between chlorine decrease and climate change

  42. SKIN CANCER EPIDEMIOLOGY PREDICTIONS

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