1. NATS 101 Ozone Depletion

2. Last air pollution topic we’ll cover is the ozone hole. Is stratospheric ozone good or bad? Why?

3. inversion isothermal 6.5 o C/km Recall that the warming in the stratosphere occurs due to the photodissociation of O 3 and O 2. Photodissociation occurs because of absorption of the sun’s UV rays.

4. Ultraviolet (UV) Absorption A reduction in O 3 would increase the UV that penetrates to the surface IR Ahrens, p 36 UV Visible

5. Possible Harm from O 3 Loss UV damages DNA (genetic material) Damage to plants (including crops) Reduction in ocean phytoplankton-base of food chain and major sink of CO 2 Human impacts include… Skin cancers Eye cataracts Immunity suppression

6. Natural Balance of Ozone Danielson et al, Fig 2.28 Disassociation of O2 absorbs UV < 0.2  m O2 + UV  O + O O3 forms when O2 and O molecules collide O2 + O  O3 Disassociation of O3 absorbs 0.2-0.3  m UV O3 + UV  O2 + O Balance exists between O3 creation-destruction CFC’s disrupts balance

7. Ozone Production-Destruction Moran and Morgan, Fig 2.18 Disassociation of O2 absorbs UV < 0.2  m O2 + UV  O + O O3 forms when O2 and O molecules collide O2 + O  O3 Disassociation of O3 absorbs 0.2-0.3  m UV O3 + UV  O2 + O Balance exists between O3 creation-destruction CFC’s disrupts balance

8. Sources of Chlorofluorocarbons CFC’s make up many important products Refrigerants Insulation Materials Aerosol Propellants Cleaning Solvents

9. Commonly Used CFC’s NameFormulaPrimary Use Residence Time (50% decrease) CFC-11CCl 3 FPropellant~55 years CFC-12CCl 2 F 2 Refrigerant~100 years CFC-113C 2 Cl 3 F 3 Cleaning Solvent~65 years It would take ~10 years for CFC levels to start falling if all production ceased today owing to leakage of CFC’s from old appliances, etc.

10. Chronology of Ozone Depletion 1881Discovery of ozone layer in stratosphere 1928Synthesis of CFC’s for use as a refrigerant 1950sRapid increase in use of CFC’s 1974Description of ozone loss chemical reactions 1979Ban of CFC use in most aerosol cans in U.S. 1980sGrowth of CFC use worldwide 1985Discovery of Antarctic ozone hole Montreal Protocol 1987Adoption of Montreal Protocol calling for a 50% reduction in use of CFC’s by 1998

11. Chronology of Ozone Depletion 1989Confirmation of ozone declines in mid-latitudes of Northern Hemisphere and in the Arctic Montreal Protocol 1990Montreal Protocol amended to require a complete phase out of all ozone depleting chemicals by 2000 1990U.S. requirement for recycling of CFC’s 1992Discovery of high levels of ClO over middle and high latitudes of Northern Hemisphere Montreal Protocol 1992Further amendment of Montreal Protocol calling for an accelerated phase out by ozone depleting chemicals 2100Time needed until ozone layer heals completely?

12. How O 3 is Measured: Dobson Unit Ozone can be measured by compressing all ozone in column to sea-level pressure. One Dobson Unit (DU) corresponds to a 0.01 mm depth at sea-level temperature and pressure The ozone layer is very thin in Dobson units. There are only a few millimeters (few hundred Dobsons) of total ozone in a column of air

13. Mean October Total Ozone O 3 evolution over Antarctica during the period 1979-2008. Ozone < 220 DU Considered Hole

14. Ingredient #1 Polar Vortex Conditions over Antarctica promote ozone loss. Circumpolar vortex keeps air over Antarctica from mixing with warmer air from middle latitudes. Temperatures drop to below -85 o C in stratosphere. Chemical reactions unique to extreme cold occur in air isolated inside vortex. Williams, The Weather Book

15. Ingredient #2 Polar-Stratospheric Clouds June: Winter begins Polar vortex strengthens and temperatures begin to fall. July-August: Temperatures fall to below -85 o C Ice clouds form from water vapor and nitric acid. Chemical reactions that can occur on HNO 3 ice crystals, but not in air, free chlorine atoms from the CFC. nacreous cloud

16. How Ozone is Destroyed Sept: As sunlight returns in early Spring, stratospheric temperatures begin to rise. Clouds then evaporate, releasing chlorine atoms into air that were ice locked. Free chlorine atoms begin destroying ozone. Oct: Lowest levels of ozone are detected in early spring. Nov: Vortex weakens and breaks down, allowing ozone poor air to spread. Danielson et al, Fig 2.29

17. Chemistry of the Ozone Hole Chlorine atoms can be freed from CFC’s by UV reaction CCl 3 F + UV  CCl 2 F + Cl CCl 2 F 2 + UV  CClF 2 + Cl C 2 Cl 3 F 3 + UV  C 2 Cl 2 F 3 + Cl Once a chlorine atom is freed, it can destroy thousands of ozone molecules before its sequestering from the air Cl + O 3  O 2 + ClO ClO + O  O 2 + Cl Chlorine acts as a catalyst Moran and Morgan, Fig 2.19 CFC-11

18. Annual Cycle of Ozone over SP Ozone time-height cross-section http://www.esrl.noaa.gov/gmd/dv/spo_oz/

19. Arctic O 3 “Mini-Hole”? Trend of about 10% per decade?

20. Northern Hemisphere Ozone Loss Partial Skin Damage Risk About a 5% decrease in mid-latitudes Erythema: reddening of skin from ionizing radiation http://www.esrl.noaa.gov/gmd/about/ozone.html

21. Key Points: Ozone Hole Chlorofluorocarbons (CFCs) disrupt the natural balance of O 3 in S.H. stratosphere CFCs responsible for the ozone hole over SP! Responsible for lesser reductions worldwide. Special conditions exist in stratosphere over Antarctica that promote ozone destruction: Air cools to -85 o C inside circumpolar vortex Allows formation of HNO3 nacreous clouds

22. Key Points: Ozone Hole CFCs stay in atmosphere for ~100 years One freed chlorine atom destroys thousands of O 3 molecules before leaving stratosphere Montreal Protocol mandated total phase out of ozone depleting substances by 2000. Even with a complete phase out, O 3 levels Would not decrease for another 10 years Would not completely recover for ~100 years

23. Anthropogenic Damage And a Greater World Promise Ozone HoleThe Ozone Hole showed unequivocally that humans and industrialization can damage the environment on a large scale. Montreal ProtocolThe Montreal Protocol showed that the world community can come together to solve major global issues. A SUCCESS STORY

24. A UNEQUIVOCAL SUCCESS STORY Montreal Protocol is But… Corrective Measures Were Relatively Painless

25. Assignment for Next Lecture Climate Variability, Climate Change Reading - Ahrens 4th: 324-327, 373-399 5th: 334-337, 383-409 Homework11 - D2L Due Friday May 7 4th-Pg. 399: 14.5, 6, 7, 10 5th-Pg. 412: 14.5, 6, 7, 10 Do Not Submit Homework on D2L