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

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

1 Dobson Unit (DU) is defined to be 0.01 mm thickness at STP Latest satellite ozone data: THE OZONE LAYER

ABSORPTION OF SOLAR UV RADIATION BY OZONE Solar UV radiation spectrum at different altitudes

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

SOLAR SPECTRUM AND ABSORPTION X-SECTIONS O 2 +hv O 3 +hv

ENERGY STATES OF THE O ATOM (1s 2 2s 2 2p 4 ) multiplicity total electronic orbital angular momentum number 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 Energy O( 1 S) O( 1 D) O( 3 P) determined by the arrangement of the four electrons in the 2p orbitals O. :. 94 kJ/,ole

CHAPMAN MECHANISM FOR STRATOSPHERIC OZONE (1930) O O 3 O2O2 slow fast Odd oxygen family [O x ] = [O 3 ] + [O] R2 R3 R4 R1

STEADY-STATE ANALYSIS OF CHAPMAN MECHANISM Lifetime of O atoms: …is sufficiently short to assume steady state for O: …so the budget of O 3 is controlled by the budget of O x. Lifetime of O x : Steady state for O x : τ Ox

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

CHAPMAN MECHANISM vs. OBSERVATION -3 shape determined by k 1 n O2 Chapman mechanism reproduces shape, but is too high by factor 2-3  missing sink!

EVOLUTION OF O 2 AND O 3 IN EARTH’S ATMOSPHERE

Questions 1. Show that the loss of ozone in the Chapman mechanism depends quadratically on the ozone concentration, i.e., L(O 3 ) ~ [O 3 ] 2 2. The production of ozone by photolysis of O 2, P(O 3 ) = k 1 [O 2 ], appears to depend linearly on the O 2 concentration but the dependence is in fact much weaker than linear. Explain why.

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

WATER VAPOR IN STRATOSPHERE Source: transport from troposphere, oxidation of methane (CH 4 ) H 2 O mixing ratio

Initiation: Propagation: Termination: OH HO 2 H2OH2O slow fast HO x radical family Ozone loss catalyzed by hydrogen oxide (HO x ≡ H + OH + HO 2 ) radicals

Supersonic aircraft (Concorde) cruising at 60,000’

Questions 1, A sink for HO x radicals in the stratosphere is formation of hydrogen peroxide (H 2 O 2 ): H 2 O can then go on to either photolyze or react with OH: Is this an effective termination pathway for HO x -catalyzed ozone loss? 2. Write a catalytic cycle of propagation reactions starting with the reaction and based on the reactions we have seen so far. Does your cycle destroy ozone or is it a null cycle?

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.” It is not necessarily the slowest reaction in the sequence!

NITROUS OXIDE IN THE STRATOSPHERE H 2 O mixing ratio

ATMOSPHERIC CYCLING OF NO x AND NO y

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

STRATOSPHERIC DISTRIBUTION OF CF 2 Cl 2 (CFC-12)

ATMOSPHERIC CYCLING OF ClO x AND Cl y

SOURCE GAS CONTRIBUTIONS TO STRATOSPHERIC CHLORINE (2004)

CHLORINE PARTITIONING IN STRATOSPHERE

Decrease of Cl-containing gases following Montreal protocol  = 45 years  = 100 years  = 26 years  = 5 years Original Montreal protocol (1987): cap production rates at 1980s levels London (1990), Copenhagen (1992) amendments: phase-out in developed world Beijing (1999): worldwide ban on production

Questions 1. It has been argued that a fleet of supersonic aircraft releasing NO x in the lower stratosphere would decrease chlorine-catalyzed ozone loss. Why? [Hint: think of the chlorine reservoirs] 2. Peroxynitric acid (HNO 4 ) is produced and removed in the stratosphere by HO 2 + NO 2 + M → HNO 4 + M HNO 4 + OH → H 2 O + NO 2 + O 2 What is the effect on stratospheric ozone? Think of the effects on both the NO x and HO x budgets. 3. Photochemical model calculations for the stratosphere including only the Chapman mechanism overestimate observed ozone levels by a factor of 3. However, in a budget calculation constrained by ozone observations we find that the O 3 + O reaction accounts for only 10% of the O x sink. Can you reconcile these two results?

OZONE TREND AT HALLEY BAY, ANTARCTICA (OCTOBER) Farman et al. paper published in Nature 1 Dobson Unit (DU) = 0.01 mm O 3 STP = 2.69x10 16 molecules cm -2

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

THE OZONE HOLE IS A SPRINGTIME PHENOMENON Movie of the 2013 ozone hole

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

Sep. 2, 1987 Sep km altitude High ClO in polar vortex Sept ER-2 aircraft measurements at 20 km altitude south of Punta Arenas ClO O3O3 O3O3 Edge of Polar vortex Measurements by Jim Anderson’s group (Harvard)

SATELLITE OBSERVATIONS OF ClO IN THE SOUTHERN HEMISPHERE STRATOSPHERE

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

PSC FORMATION AT COLD TEMPERATURES PSC formation Frost point of water

Seasonal PSCs in the Antarctic stratosphere

HOW DO PSCs START FORMING AT 195K? HNO 3 -H 2 O PHASE DIAGRAM Antarctic vortex conditions PSCs are not water but nitric acid trihydrate (NAT) clouds

DENITRIFICATION IN THE POLAR VORTEX: SEDIMENTATION OF PSCs

CHRONOLOGY OF ANTARCTIC OZONE HOLE

Chronology of 2013 ozone hole

Increasing CO 2 cools the stratosphere 15  m (220 K) Add CO 2 to stratosphere (T 2 ). At 15  m: fT14fT14 T 1 = 220 K fT24fT24 fT24fT24 Net heating = f  (T T 2 4 ) < 0

Greenhouse gases warm the surface but cool the stratosphere

Questions 1. What ratio of HCl to ClNO 3 concentrations in Antarctic fall will lead to the largest ozone depletion the following spring? 2. Satellite observations of ClO in the Antarctic stratosphere in the middle of winter show a "collar" of maximum values around 60 degrees S. Why isn't ClO highest over the South Pole, where temperatures are lowest?

Correlation of Arctic ozone loss with temperature

Unusually cold Arctic stratosphere in spring 2011

2011 Arctic ozone hole

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