THE NATURAL OZONE LAYER

1. Based on ozonesonde observations in the 1970s Ozone concentrations in units of 1012 molecules cm-3 THE NATURAL OZONE LAYER

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

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

4. Absorption cross-section s photon is not absorbed photon is absorbed Molecular cross-section A CALCULATION OF PHOTOLYSIS RATES k is the photolysis rate constant (also called photolysis frequency or J-value) actinic flux quantum yield absorption x-section Probability of absorption for incoming photons = σ/A

5. CALCULATION OF 3-BODY REACTION RATES A and B are reactants; AB* is the activated product; AB is the stable product; M is the “third body” (N2, O2 ) General solution: Low-pressure limit (Rate(2) >> Rate (3)): High-pressure limit (Rate(2) << Rate (3)):

6. ENERGY STATES OF THE O ATOM (1s22s22p4) determined by the arrangement of the four electrons in the 2p orbitals total electronic orbital angular momentum number multiplicity Multiplicity = 2S+1, where S is the spin. The spin of an electron is (+/‐) 1/2. Hund’s Rule: lowest-lying energy state is the one of maximum multiplicity O(1 S) O(1D) O(3P) Energy

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: Ox Steady state for Ox:

9. PHOTOLYSIS RATE CONSTANTS: VERTICAL DEPENDENCE quantum yield absorption X-section actinic flux In simplest case of overhead Sun and no scatter:

10. 1. Consider harmful UV radiation for which the ozone layer has an optical depth of 10. The ozone layer has thinned by 6% since 1970. What is the resulting percent increase in the flux of this UV radiation at the surface of the Earth? 2.  The original Chapman mechanism included a fifth reaction:                                                                O + O + M -> O2 + MWhat is the effect of this reaction on ozone? Is it more important in the lower or in the upper stratosphere? 3. Based on the Chapman mechanism, would you expect O and O3 concentrations in the stratosphere to vary with time of day, and if so how? QUESTIONS

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

12. 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

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

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

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

16. 1. A persistent mystery in atmospheric chemistry is why the stratosphere is so dry (3-5 ppmv H2O). Based on water vapor concentrations observed just below the tropopause, one would expect the air entering the stratosphere to be moister, One theory is that very strong thunderstorms piercing through the tropopause can act as a “cold finger” for condensation of water and thereby remove water from the lower stratosphere. Explain how this would work. 2. We saw that a chain termination pathway for HOx- -catalyzed ozone loss is the self-reaction of HO2 radicals producing H2O2 (hydrogen peroxide). But H2O2 has a lifetime of only about 1 day against photolysis (50%) and oxidation by OH (50%). How will each of these two sinks of H2O2 affect further HOx -catalyzed ozone loss? QUESTIONS

17. Methane originates from the surface, has an atmospheric lifetime ~ 10 years against oxidation (ppm) Satellite Climatology (CLAES +HALOE) METHANE AS TRACER OF STRATOSPHERIC TRANSPORT

18. BREWER-DOBSON CIRCULATION OF STRATOSPHERE Net loss Net loss Net production Ozone number density Ozone mixing ratio

19. H2O mixing ratio NITROUS OXIDE IN THE STRATOSPHERE

20. ATMOSPHERIC CYCLING OF NOx AND NOy

21. NOy = NO + NO2 + HNO3 + ClNO3 + … Keim et al., JGR 102 13193, 1997

22. NH4++3/2O2 NO2+ H2O + 2 H+ NO3+ Org-C  N2 + … N2O N2O: LOW-YIELD PRODUCT OF BACTERIAL NITRIFICATION AND DENITRIFICATION IPCC [2007]

23. PRESENT-DAY GLOBAL BUDGET OF ATMOSPHERIC N2O IPCC [2001] Although a closed budget can be constructed, uncertainties in sources are large! (N2O atm mass = 5.13 1018 kg x 3.1 10-7 x28/29 = 1535 Tg N)

24. 1. A minor branch for NO3 photolysis is What is its effect on stratospheric ozone? 2. What is the effect on stratospheric ozone of the reaction QUESTIONS 3. What is the effect on stratospheric ozone of the reaction 4. What is the effect on stratospheric ozone of production and loss of peroxynitric acid, 5. Show that N2O5 hydrolysis is a source of HOx .

25. STRATOSPHERIC DISTRIBUTION OF CF2Cl2 (CFC-12)

26. ATMOSPHERIC CYCLING OF ClOx AND Cly

27. SOURCE GAS CONTRIBUTIONS TOSTRATOSPHERIC CHLORINE (2004)

28. CHLORINE PARTITIONING IN STRATOSPHERE

30. 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

31. 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

32. THE POLAR VORTEX (Sep-Oct 2006)

33. THE OZONE HOLE IS A SPRINGTIME PHENOMENON

34. THE 2010 OZONE HOLE SEASON

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

36. 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)

37. SATELLITE OBSERVATIONS OF ClO IN THE SOUTHERN HEMISPHERE STRATOSPHERE

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

39. QUESTIONS • We saw that the Chapman mechanism overestimates the amount of ozone in the stratosphere by a factor of 3. Yet the diagram of the contribution of different mechanisms to ozone loss (“stratospheric ozone budget”) shows the Chapman mechanism accounting for only about 10% of total ozone loss. Why isn’t it 33%? • It has been suggested that “now is the time” for a supersonic aircraft fleet as it would help slow down the standard mechanism for chlorine-catalyzed ozone loss. How is that? • However, a supersonic aircraft fleet could also aggravate the Antarctic ozone hole. Explain why.

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

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

42. DENITRIFICATION IN THE POLAR VORTEX:SEDIMENTATION OF PSCs

43. CHRONOLOGY OF ANTARCTIC OZONE HOLE

44. TRENDS IN GLOBAL OZONE Mt. Pinatubo

45. STRATOSPHERIC SULFATE AEROSOL condensation OH, O(1D) OH H2SO4•H2O H2SO4 COS SO2 aerosol stratosphere troposphere OH COS volcanic eruptions vegetation oceans

47. SKIN CANCER EPIDEMIOLOGY PREDICTIONS

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

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

50. OZONE LOSS IN ARCTIC STRATOSPHERE vs. PSC PROCESSING Rex et al. [GRL 2006]