ArgentinaLecture2

Global Change Workshop - Buenos Aires 3/27-30/2007Lecture 2Stratospheric Ozone: Chapman Chemistry Basic Chemical KineticsCatalytic Ozone DestructionAntarctic Ozone Hole: 

Global Change Workshop - Buenos Aires 3/27-30/2007 Lecture 2 Stratospheric Ozone: Chapman Chemistry Basic Chemical Kinetics Catalytic Ozone Destruction Antarctic Ozone Hole

Solar Irradiance at different altitudes (UV and visible): 

Solar Irradiance at different altitudes (UV and visible)

O2 Absorption Cross-section: 

O2 Absorption Cross-section Ref. Brasseur and Solomon, 1995

O3 Absorption Cross-section: 

O3 Absorption Cross-section Ref. Brasseur and Solomon, 1995

O3 Absorption Cross-section: 

O3 Absorption Cross-section Ref. Brasseur and Solomon, 1995

J O3(O1D): 

J O3(O1D)

J NO2: 

J NO2

Calculate J by quick TUVhttp://www.acd.ucar.edu/TUV/: 

Calculate J by quick TUV http://www.acd.ucar.edu/TUV/

5. Energy Flux and Actinic Flux: 

5. Energy Flux and Actinic Flux Actinic Flux: Irradiance:

Stratospheric Ozone: A brief history: 

Stratospheric Ozone: A brief history Surface ozone discovered in mid 1800s – regular measurements started soon after Late 1800s – study of solar spectra implied layer of ozone above troposphere 1921 - Oxford scientists (Dobson and Lindeman) discover temperature increases in stratosphere with respect to troposphere  Radiative processes Now measured routinely from space and sondes

Spectrum of Solar Radiation vs. Altitude: 

Spectrum of Solar Radiation vs. Altitude O3 + hn O2 + hn

Why is there an ozone layer in the stratosphere? - I: 

Start with: O2 + h  O + O ( < 240 nm) Highly reactive O(3P) Lots of energy needed Why is there an ozone layer in the stratosphere? - I (R1) Reaction rate (molec/cm3/s) Photolysis rate Coefficient (/s) Where concentration (number density) is denoted by: [X] = nX = CX . na

Why is there an ozone layer in the stratosphere? - II: 

O2 + h  O + O ( < 240 nm) What Happens to O (O + O + M  O2 + M very slow) O + O2 + M  O3 + M generally faster (R1) Combination reactions require a “third body” (M) to carry off excess energy (R2) Why is there an ozone layer in the stratosphere? - II

Kinetics of 3-body reactions: 

Kinetics of 3-body reactions O + O2 + M  O3 + M (R2) O + O2  O3* O3*  O + O2 O3* + M  O3 + M* M*  M + heat (R2a) (R2b) (R2c) (R2d) Elementary reaction steps Low-pressure limit High-pressure limit

Slide16: 

(R1) O2 + h  O + O ( < 240 nm) O + O2 + M  O3 + M O3 + h  O2 + O ( < 320 nm) O3 + h  O2 + O(1D) O(1D) + M  O + M O3 + O  2O2 (R2) (R3) 4 Elementary Reaction Steps Why is there an ozone layer in the stratosphere? - III O3 Production O3 bonds rel. weak O3 Loss (R4)

Kinetics of 2-body reactions: 

Kinetics of 2-body reactions A + B  C + D Reaction rate (molec/cm3/s) Second-order rate constant (cm3/molec/s)

Why is there an ozone layer in the stratosphere? - IV: 

Chemical mechanism proposed by Sydney Chapman (1930): O2 + h  O + O ( < 240 nm) (O + O + M  O2 + M very slow) O + O2 + M  O3 + M O3 + h  O2 + O ( < 320 nm) O3 + h  O2 + O(1D) O(1D) + M  O + M O3 + O  2O2 O3 Loss FAST SLOW SLOW (R1) (R4) Why is there an ozone layer in the stratosphere? - IV (R2) (R3)

Steady state solution: 

Steady state solution O O3 O2 slow slow fast Odd oxygen family Ox = O + O3 Chapman mechanism Chemical steady-state [s.s.] assumed for species IF: production and loss rate are (nearly) constant over lifetime Shortest-lived species: O = [O] / (k2[O][O2][M]) = 1 / (k2[O2][M])  secs  s.s. for [O] between fast R3 and R2 (& neglecting slow R1 and R4) R2 R3 R4 R1

Slide20: 

Odd-oxygen: What have we learned? [O3] controlled by slow production and loss by R1 and R4 (NOT fast production and loss of O3 from R2 and R3) Effective O3 lifetime  Ox: Ox = [Ox]/(2k4[O][O3])  1/ (2k4[O]) [Ox] = [O] + [O3]  [O3]

CHAPMAN CYCLE provides qualitative agreement with observations: 

CHAPMAN CYCLE provides qualitative agreement with observations Maximum  Ox production = 2 J1[O2] Lower stratosphere: s.s. not expected because of long Ox ALSO DYNAMICS Ox (Jacob, Fig. 10-5)

Slide22: 

CATALYTIC OZONE DESTRUCTION

Chapman got it qualitatively right…Catalytic Cycles For Ozone Loss: General Idea: 

Chapman got it qualitatively right… Catalytic Cycles For Ozone Loss: General Idea O3 + X  XO + O2 O + XO  X + O2 Net: O3 + O  2 O2 X is a catalyst The catalyst is neither created nor destroyed…but the rate for the catalytic cycle [Ox removal in this case] depends on catalyst concentrations

Water Vapor in the Stratosphere: 

Water Vapor in the Stratosphere High-altitude source from the oxidation of methane (CH4)

Hydrogen oxide (HOx) radicals (HOx = H + OH + HO2): 

Hydrogen oxide (HOx) radicals (HOx = H + OH + HO2) First identified as an important loss mechanism for mesospheric ozone by Bates and Nicolet (1950) Calculations in 1950s and 1960s found that HOx cycles are a significant Ox sink, but insufficient to reconcile O3 budget O(1D) quenching much too slow so HOx formation rate was too fast

Hydrogen oxide (HOx) radicals (HOx = H + OH + HO2): 

Hydrogen oxide (HOx) radicals (HOx = H + OH + HO2) Quenching O(1D) + M  O(3P) + M Initiation O(1D) + H2O  2OH Propagation through cycling of HOx radical family (example): OH + O3  HO2 + O2 HO2 + O3  OH + 2O2 Net: 2O3  3O2 Termination (example): OH + HO2  H2O + O2 HOx is a catalyst for O3 loss but not the key one…

Hydrogen oxide (HOx) radicals (HOx = H + OH + HO2): 

Hydrogen oxide (HOx) radicals (HOx = H + OH + HO2) Sources: H2O + O(1D)  2OH CH4 + O(1D)  CH3 + OH H2 + O(1D)  H + OH But O(1D) is also rapidly quenched

Nitrous Oxide in the Stratosphere: 

Nitrous Oxide in the Stratosphere from CLAES satellite instrument

Nitrogen oxide (NOx)radicals (NOx = NO + NO2): 

Nitrogen oxide (NOx)radicals (NOx = NO + NO2) Second catalytic cycle recognized for O3 loss (Crutzen, 1970; Johnston, 1971) In the late 1960s, the U.S. and other countries considered a supersonic aircraft fleet flying in the stratosphere Atmospheric chemists needed to figure out effects on O3 layer Aircraft exhaust: N2 + O2  2 NO At the same time natural source of NO proposed from N2O + O(1D) (McElroy & McConnell, 1971)

Nitrogen oxide (NOx)radicals (NOx = NO + NO2) : 

Nitrogen oxide (NOx)radicals (NOx = NO + NO2) Initiation N2O + O(1D)  2NO Propagation NO + O3  NO2 + O2 NO + O3  NO2 + O2 NO2 + h  NO + O NO2 + O  NO + O2 O + O2 + M  O3 + M Null cycle Net O3 + O  2O2 Termination Recycling NO2 + OH + M  HNO3 + M HNO3 + h  NO2 + OH O3 loss rate: NOx reservoir species: HNO3 (~weeks), N2O5 (~hours-days)

Atmospheric Cycling of NOx and NOy: 

Atmospheric Cycling of NOx and NOy

Nitrogen oxide (NOx)radicals (NOx = NO + NO2): 

Nitrogen oxide (NOx)radicals (NOx = NO + NO2) Because of environmental/economic concerns, U.S. decided not to build the supersonic fleet (Europe still built a few Concordes) However, there remains a natural source of NOx in the stratosphere N2O is a stable low-yield product of nitrification and denitrification in the biosphere N2O + O(1D)  2 NO (5%) N2O + O(1D)  N2 + O2 N2O + h  O(1D) + N2 The natural NOx catalytic cycle accounts for the remaining missing sink in the Chapman mechanism } (95%)

What have we learned about NOy?: 

What have we learned about NOy? Production Natural NOy source by N2O + O(1D) - well understood source Loss Dominant sink is deposition to troposphere. Residence time for air is 1-2 years. Loss rate well constrained Cycling O3 loss related to NOx/NOy ratio. Under most conditions s.s. between NOy species is a good approximation NOx catalytic cycle reconciled Chapman theory with observations…1995 Nobel Prize

Slide34: 

Wikipedia Rules! Ozone, the first allotrope of a chemical element to be described by science, was discovered by Christian Friedrich Schönbein in 1840, who named it after the Greek word for smell (ozein), from the peculiar odour in lightning storms.[1] The odour from a lightning strike is from ions produced during the rapid chemical changes, not the ozone itself. Allotropy (Gr. allos, other, and tropos, manner) is a behaviour exhibited by certain chemical elements: these elements can exist in two or more different forms, known as allotropes of that element. In each different allotrope, the element's atoms are bonded together in a different manner. Oxygen was first discovered by the Swedish pharmacist Carl Wilhelm Scheele some time before 1773, but Priestley who published first on August 1, 1774, called the gas dephlogisticated air. Scheele called the gas 'fire air' because it was the only known supporter of combustion. It was later called 'vital air' because it was and is vital for the existence of animal life. The gas was finally named by Antoine Laurent Lavoisier from Greek roots meaning "acid-former".

Slide35: 

STRATOSPHERIC OZONE HOLE

Slide36: 

Where we have been the last 2 days. 1. 4.6 Billion [109] years ago it all started on earth 2. At least 700 Million [106] years ago we had O2, O3, Chapman chemistry and NOx catalytic destruction via N2O, a minor natural waste product of life. 3. In the early 1970’s we became serious about understanding the chemistry, transport and our accidental modification of the earth’s ozone layer.

Nitrogen oxide (NOx)radicals (NOx = NO + NO2): 

Nitrogen oxide (NOx)radicals (NOx = NO + NO2) Second catalytic cycle recognized for O3 loss (Crutzen, 1970; Johnston, 1971) In the late 1960s, the U.S. and other countries considered a supersonic aircraft fleet [600-700 planes] flying in the lower stratosphere Atmospheric chemists needed to figure out effects on O3 layer Aircraft exhaust: N2 + O2  2 NO Because of environmental/economic concerns, U.S. decided not to build the supersonic fleet (Europe still built a few Concordes a flew them at a loss of ~$1,000,000,000/yr) At the same time natural source of NO proposed from N2O + O(1D) (McElroy & McConnell, 1971)

Gas Phase Catalytic Cycle for Ozone Loss:Stolarski and Cicerone(1974) developed Cl – ClO catalytic cycle for NASA rocket exhaustMolina and Rowland(1974) identified CFC’s as major Cl source in StratosphereMcElroy and Wofsy were also involved [HCl reservoir]All 3 groups published on CFC’s in 1974, but Molina and Rowland were first http://www.ciesin.org/docs/011-464/011-464.html - ozone wars: 

Gas Phase Catalytic Cycle for Ozone Loss: Stolarski and Cicerone(1974) developed Cl – ClO catalytic cycle for NASA rocket exhaust Molina and Rowland(1974) identified CFC’s as major Cl source in Stratosphere McElroy and Wofsy were also involved [HCl reservoir] All 3 groups published on CFC’s in 1974, but Molina and Rowland were first http://www.ciesin.org/docs/011-464/011-464.html - ozone wars Initiation: CF2Cl2 + hn g CF2Cl + Cl Destruction: Cl + O3 g ClO + O2 ClO + O g Cl + O2 Net: O3 + O g 2O2 Termination: Recycling: Cl + CH4 g HCl + CH3 HCl + OH gCl + H2O ClO + NO2 + M g ClNO3 + M ClNO3 + hn gClO + NO2 O3 loss rate: ClOx reservoir species: HCl (~month), ClNO3 (~hours BUT it slows both the natural and the CFC destructions)

Atmospheric Cycling of ClOx and Cly: 

Atmospheric Cycling of ClOx and Cly

Obvserved Chlorine Partitioning in Stratosphere: 

Obvserved Chlorine Partitioning in Stratosphere

Slide41: 

Predicted chlorine loading in the atmosphere based on protocol revisions

Antarctic Ozone Hole: 

Antarctic Ozone Hole In 1985, theory suggested that future build-up of CFCs could lead to substantial ozone depletion (but that current levels posed little risk) Ozone depletion was predicted to occur mainly in the tropical upper stratosphere (where the most Cly was present as active ClOx) WHY IN THE UPPER TROPICAL STRATOSPHERE? This understanding led to the development of the Montreal Protocol in 1987 SURPRISE! In 1985 scientists from the British Antarctic Survey at Halley Bay, Antarctica reported ~30% total ozone loss during austral spring (Farman et al., Nature) WHY WAS THIS A SURPRISE? Later it turned out that global satellite monitoring instruments measured this depletion earlier, but rejected the low O3 values as “unreasonable”

Ozone Trend At Halley Bay, Antarctica (October): 

Ozone Trend At Halley Bay, Antarctica (October) Farman et al. paper published in Nature,1985 1986

Vertical Structure Of The Ozone Hole: 

Vertical Structure Of The Ozone Hole Near-total depletion in lower stratosphere

Ozone Hole is a Spring Phenomenon: 

Ozone Hole is a Spring Phenomenon

PSCs Over Kiruna, Sweden (SOLVE Mission): 

PSCs Over Kiruna, Sweden (SOLVE Mission)

Gas Phase Catalytic Cycle for Ozone Loss:(1974 -1985 wisdom): 

Gas Phase Catalytic Cycle for Ozone Loss: (1974 -1985 wisdom) Initiation: CF2Cl2 + hn g CF2Cl + Cl Destruction: Cl + O3 g ClO + O2 ClO + O g Cl + O2 Net: O3 + O g 2O2 Termination: Recycling: Cl + CH4 g HCl + CH3 HCl + OH gCl + H2O ClO + NO2 + M g ClNO3 + M ClNO3 + hn gClO + NO2 O3 loss rate: ClOx reservoir species: HCl (~month), ClNO3 (~hours)

Slide48: 

In the absence of clouds, chlorine is trapped in inert chemical reservoirs

Reactions on Polar Stratospheric Clouds (PSCs): 

Reactions on Polar Stratospheric Clouds (PSCs) N2O5 (g) + H2O (s)  2 HNO3 (s) ClONO2 (g) + H2O (s)  HOCl (g) + HNO3 (s) ClONO2 (g) + HCl (s)  Cl2 (g) + HNO3 (s) N2O5 (g) + HCl (s)  ClNO2 (g) + HNO3 (s) HOCl (g) + HCl (s)  Cl2 (g) + H2O (s) These reactions: convert Cly reservoir species to easily photolyzed species (e.g., Cl2) tie up NOy as HNO3 on PSCs ( “denitrification”?)

Polar Ozone Catalytic Loss Cycles: 

Polar Ozone Catalytic Loss Cycles 2 (Cl + O3  ClO + O2) ClO + ClO + M  ClOOCl + M ClOOCl + hn  Cl + ClOO ClOO + M  Cl + O2 + M Net: 2 O3 + hn  3 O2 Key step: ClOOCl + hn  Cl + ClOO (Instead of: ClOOCl + hn  ClO + ClO) WHY IS THIS CRITICAL? Br + O3 g BrO + O2 Cl + O3 g ClO + O2 BrO + ClO g Br + Cl + O2 Net: 2O3 g 2O2 Major loss cycle Molina and Molina (1987) O3 loss rate: Extra Destruction

Slide51: 

In the presence of clouds and (weak) sunlight, reactive chlorine is released and catalytic ozone destruction cycle proceeds PSC sedimentation (“denitrification”) O3 “recovers” when vortex breaks up in November  Filling in by mid-latitude air

The “Smoking Gun”: 

The “Smoking Gun” Before Polar Sunrise After Polar Sunrise High ClO in vortex, but no O3 depletion yet Rapid O3 depletion correlated with high ClO Anderson et al. (Science, 1991)

What About the Arctic?: 

What About the Arctic? Vortex not as cold as Antarctic Temperature occasionally fall enough to allow PSC formation, but not as persistently as in Antarctic Not much PSC sedimentation (denitrification) Less stable vortex tends to break up earlier, before catalytic loss process has much time to operate Some Arctic ozone depletion observed

Trends In Polar Ozone:Could greenhouse-induced cooling of stratosphereproduce an Arctic ozone hole over the next decade?How about big volcanic eruptions?: 

Trends In Polar Ozone: Could greenhouse-induced cooling of stratosphere produce an Arctic ozone hole over the next decade? How about big volcanic eruptions?

Slide55: 

Will The Ozone Layer Protect Us In The Future? Stratospheric Aerosols. a. Very cold temperatures produce ice clouds and Greenhouse Gases are cooling the lower stratosphere. b. Strong volcanoes can be an excellent source of sulfate aerosol, though they also slow tropospheric warming. 2. Stratospheric Cl or Br a. The old CFC’s are going down slowly. b. The new HCFC’s are rising rapidly, but are still at very low levels c. Br could be controlled relatively easily. 3. My Answer: Yes, If we are careful, but we may have been quite lucky this time. Old CFC’s last a long time. We may have stopped emitting them just in time.

ATMOSPHERIC TRENDS OF CFCs: 

ATMOSPHERIC TRENDS OF CFCs

Slide58: 

Predicted chlorine loading in the atmosphere based on protocol revisions BUT, REMEMBER THIS PICTURE!

THE END OF THE STRATOSPHER LECTURES,BUT NOT THE STRATOSPHERIC OZONE LAYER: 

THE END OF THE STRATOSPHER LECTURES, BUT NOT THE STRATOSPHERIC OZONE LAYER

The Natural Ozone Layer: 

The Natural Ozone Layer Zonally averaged ozone concentrations (1012 molecules cm-3) at the equinox, based on springtime measurements taken in the 1960s. From Wayne, R.P., Chemistry of Atmospheres, Oxford, 1991.

Slide61: 

~Tropopause ~3,000 ppb <100 ppb

Slide62: 

[O]/[O3] = J3/ (k2[M][O2]) [O]/[O3] <<1 O3 O Observations agree closely with Chapman [Ox] = [O] + [O3]  [O3] “Odd-oxygen”

Slide63: 

J1(z) and J3(z) are photolysis rate constants (not reaction rate constants) J =  X() fX ()Id  I = I, e-t/cos t = z(O2 [O2] + O3 [O3])dz Actinic flux Solar zenith angle Optical depth (Jacob, Fig. 10-5)

Slide64: 

Upper Stratosphere Ox short enough that s.s. can be assumed: P(Ox) = L(Ox) 2J1[O2] = 2k4[O][O3] (not a straightforward relationship) Values of Ox < 1 day in the upper stratosphere several years in lower stratosphere WHY? Ox (Jacob, Fig. 10-5) CO2=0.209