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Stratospheric Ozone Depletion

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  1. Stratospheric Ozone Depletion • Results from large-scale industrial manufacture and release of synthetic compounds (chlorofluorocarbons, CFCs) in quantities that can interfere with chemical processes in the Earth’s atmosphere • Unanticipated - like acid rain, global warming, etc., effects of CFCs were not expected… only appreciated in hindsight • But, also might be an environmental “success” story…

  2. History • 1974: Molina & Rowland (1974) Nature 249, 810-812 • Paper calls attention to dangers of CFC’s in ozone breakdown • 1978: U.S. bans CFCs as propellants • 1987: Montreal Protocol calls for decrease in CFC to 50% of 1986 levels by 1999 • 1990: London Amendmentscall for complete CFC phaseout by 2000 • 1992: Copenhagen Amendmentsaccelerate phase out to 1996 • 1995: Molina & Rowland win Nobel Prize in Chemistry

  3. Ozone (O3) • Blue colored, strong smelling molecule • Absorbs UV radiation (more in a moment…) • Unstable: constantly breaks down, reforms in stratosphere • Breakdown can be accelerated by certain chemicals (catalysts) • Also a primary constituent of photochemical smog in the troposphere

  4. Atmospheric Pressure Altitude (km) 50 Stratosphere 40 Weather Balloons 36 32 U2 Spy Plane 28 24 20 16 Stealth Bomber Commercial Airliners Troposphere 12 Mt. Everest 8 4 Denver 0 0 0.2 0.4 0.6 0.8 1.0 Atmospheric Pressure (atm)

  5. Atmosphere Structure

  6. Atmosphere Structure • Troposphere • Lowest layer of the atmosphere: below 15 km altitude • Heated from below • From re-emitted IR wavelengths radiated from the surface • Convection drives vertical mixing • Stratosphere • From 15 km to 50 km altitude • Heated from above • UV absorbed by O2, O3 heats air • More intense UV at top of stratosphere, higher temperatures at top • No vertical mixing.. No convection possible • “Ozone layer” - contains most of atmospheric ozone • Mesosphere • 50 km up to 80 km • Heated from below • Radiant heat escaping from Stratosphere • Convection drives vertical mixing • Thermosphere • Above 80 km (out to ~120 km) • Heated from above • Short wavelength UV • Mostly ionized gases at high temperature

  7. Ozone Distribution in the Atmosphere Ozone partial pressure (mPa) 0 5 10 15 20 25 50 Stratosphere 40 36 32 “Ozone layer” Ozone concentration curve 28 24 Altitude (km) 20 16 Troposphere 12 8 Pressure gradient 4 0 0 0.2 0.4 0.6 0.8 1.0 Atmospheric Pressure (atm)

  8. Electromagnetic Radiation • Solar Radiation • Important to understand radiation, in order to understand climate controls • The Electromagnetic Spectrum • All radiant energy occurs in the form of electromagnetic (EM) radiation • The energy exists in particle units called photons, which travel at all times at the speed of light (300,000 km/s in a vacuum) • Photons have no mass, are bundles of energy • Photons as Waves • Photons vibrate as waves, and have a wavelength (l) and a frequency (n) • Wavelength • Distance crest to crest • Or trough to trough • Expressed in distance (e.g. nm) • Frequency • No. of waves that pass a point in a unit of time (e.g. 1 sec.) • Expressed in Hertz (waves/sec) wavelength

  9. Photon Energy • High Energy photons • Have short wavelengths (high frequencies) • Lower Energy photons • Have long wavelengths (low frequencies)

  10. Electromagnetic Spectrum

  11. Electromagnetic Spectrum

  12. Ozone Formation in the Atmosphere • Solar radiation striking the Earth’s atmosphere is absorbed by air molecules • O2strongly absorbs in the UV band • Absorption of UV by molecular oxygen splits the O=O bond, forming •O free radicals • These •O free radicals combine with molecular oxygen to form O3 (ozone)

  13. Ozone Absorption in the UV Band • UV radiation includes wavelengths from 200 to 400 nm • UV-A 320-400 nm • UV-B 200-320 nm • UV-C 200-290 nm • UV-C • Nearly all UV-C is absorbed in the upper atmosphere • UV-B • 90% of UV-B is absorbed by the atmosphere, mostly by O3 • UV-A • Not strongly absorbed by the atmosphere

  14. Ecological Damage from increased UV-B • Ecosystem-level effects • Plants • Experimental exposure of seedlings, young plants to enhanced UV-B negatively impacts rates of photosynthesis and growth • Complex, however… different plant groups respond differently, and adults can differ in response from juveniles • Oceanic Phytoplankton • Evidence is clearer that enhanced UV-B negatively affects marine phytoplankton • Reduced photosynthesis rates, reduced productivity, which could affect… • Fixed N production in marine ecosystems • Carbon fixation (CO2 drawdown) • 10% loss of phytoplankton productivity diminishes CO2 drawdown by ~5 Gt/yr (equivalent to total annual fossil fuel contribution today)

  15. Ozone Formation • In the stratosphere, UV intensity is sufficient to form O3 O2(g) + UV = 2 •O •O + O2(g) = O3(g) • UV radiation can also destroy ozone molecules… O3(g) + UV = •O + O2(g) O3(g) + •O = 2O2(g) -Chemists looked at these processes and predicted O3 much higher than what is actually observed.. Suggests that there is another O3 destruction process at work…

  16. Catalytic O3 Destruction! • O3 is unstable, but degrades slowly on its own • CATALYSTS can greatly accelerate the process • Catalyst = chemical species that helps a reaction along, but is not used up in the reaction • A.K.A. “chain carrier X”, X is the “divorce lawyer” X + O3(g) = OX + O2(g) OX + •O = X + O2(g) Net Reaction: O3(g) + •O = 2O2(g) • One molecule of X will continue to act as a catalyst in this reaction again and again, destroying O3 molecules until X itself is removed or destroyed WHAT IS X?

  17. What is X? • Hydroxyl Radical (OH·) • Responsible for up to 1/2 of all O3 destruction in lower stratosphere • Natural process, but perhaps accelerated by addition of CH4 (results in formation of more OH·) • Nitric Oxide (NOx·) • Produced in lower atmosphere via biomass burning, lightning, mostly oxidized to NO2/HNO3 & rained out before reaching stratosphere • Some does reach stratosphere, especially N2O, which is photolyzed, producing NO· • NO destroys O3, forms NO2 • NO2 reactions with other X molecules (OH·, Cl·), can store them and then release them later • Complex results: • Above 25 km NO accounts for >50% of O3 destruction • Below 25 km NO net effect is to slow O3 destruction

  18. Anthropogenic Effects on Stratospheric Ozone • Chlorine (Cl·) and Bromine (Br·) Radicals • Very efficient carrier molecules -- primary cause of ozone depletion in the stratosphere • Naturally low abundance • Anthropogenic Source: production of CFCs (hydrocarbons having at least one hydrogen replaced by a halogen) • CH4 (methane) CFCl3 (fluorotrichloromethane) • Uses: refrigerants, aerosol spray propellant, solvents, fire retardants • Nontoxic and nonreactive: not destroyed in troposphere, rise to stratosphere & broken apart by photons: R-Cl + UV photon = R· + Cl· • Eventually (long residence time in stratosphere!) destroyed,e.g. by methane molecules: Cl· + CH4 = HCl + CH3· • Now largely banned by Montreal Protocol (1987) and amendments, but CFC catalyzed O3 destruction will continue for decades

  19. Polar Ozone Destruction • “Ozone Hole”: term for regional, seasonal thinning of O3 layer over the poles • Cause: catalytic destruction of O3 by Cl·, but not because X = Cl· • Mechanism is complex: • Ice clouds form in frigid winter air, absorb HNO3, ClONO2, HCl • Surface reactions on ice convert these to reactive Cl2, HOCl, which accumulate, trapped in ice • Spring daylight returns, solar radiation converts Cl2 to Cl· and HOCl to HO· and Cl· • Sudden burst of Cl· reacts with O3, produces ClO· which forms ClO-OCl, which forms ClOO· and Cl· • Abundant Cl· destroys lots of ozone • Chain is brokenwhen sunlight evaporates polar clouds , releasing bound HNO3; NO2 reacts with ClO· and traps it again

  20. Source: CSIRO Atmospheric Research; Data NASA GSFC Code 916

  21. Area of the Antarctic Ozone Hole v. Time Climate Variability & Predictability - World Climate Research Programme (www.clivar.org)

  22. Measured and Projected Trends in Ozone-Depleting Gases Data & figure from NOAA (http://www.esrl.noaa.gov/gmd/odgi/)

  23. Contribution of Various CFCs to Ozone Depletion Data & figure from NOAA (http://www.esrl.noaa.gov/gmd/odgi/)

  24. PHOTOCHEMICAL SMOG Formation: Nitrogen oxides + volatile hydrocarbons • Enhanced by presence of inversion layer (cool air blows under warm air) • Enhanced by stagnant air masses Mechanism: O3 + NO = NO2 + O2 NO2 + sunlight = NO + O O + O2 = O3 Note: CANNOT build up more O3 than NO2 in this way Why? NO destroys the O3 (reaction goes backwards) Need another ingredient…”peroxyl radicals” formed from volatile organic hydrocarbons: O2 + VOC = RO2 RO2 + NO = NO2 + RO RxN is “reset” to form more O3 from NO2 without removal of O from NO!

  25. PHOTOCHEMICAL SMOG EMISSIONS CONTROL? Reduction of NOx is best • Hydrocarbons more abundant • More sources of hydrocarbons • Catalytic converters are very effective: convert NO back to N2, CO/hydrocarbons back to CO2 before release 1970-1990: 60% increase in miles driven in L.A., peak ozone levels cut in half!!