NATS 101 Lecture 32 Ozone Depletion

1. NATS 101Lecture 32Ozone Depletion

2. Supplemental References for Today’s Lecture Danielson, E. W., J. Levin and E. Abrams, 1998: Meteorology. 462 pp. McGraw-Hill. (ISBN 0-697-21711-6) Moran, J. M., and M. D. Morgan, 1997: Meteorology, The Atmosphere and the Science of Weather, 5th Ed. 530 pp. Prentice Hall. (ISBN 0-13-266701-0)

3. Review: Ultraviolet (UV) Absorption UV Visible IR O2 and O3 absorb UV (shorter than 0.3 m) Therefore, reductions in the level of O3 would increase the amount of UV radiation that penetrates to the surface Ahrens, p 36

4. Hazards of Increased UV • Damage to crops and animals Reduction in ocean phytoplankton • Increase number of cases of skin cancers Increase in eye cataracts and sun burning Suppression of human immune system (But at least you have a tanned body!)

5. 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 Natural Balance of Ozone Danielson et al, Fig 2.28

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

7. Commonly Used CFC’s Name Formula Primary Use Residence Time (50% decrease) CFC-11 CCl3F Propellant ~55 years CFC-12 CCl2F2 Refrigerant ~100 years CFC-113 C2Cl3F3 Cleaning Solvent ~65 years It would take 10-20 years for CFC levels to start falling if all production ceased today owing to leakage of CFC’s from old appliances, etc.

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

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

10. How O3 is Measured: Dobson Unit • Ozone can be measured by the depth of ozone if all ozone in a column of atmosphere is brought to sea-level temperature and pressure. • One Dobson unit 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.

11. Mean Monthly Total Ozone Huge decrease in O3 over Antarctica during the period 1979-92.

12. 40 Year Springtime Ozone over SP http://www.cmdl.noaa.gov/ozwv/ozsondes/spo/ozdob.html

13. Conditions over Antarctica promote ozone loss. Circumpolar vortex keeps air over Antarctica from mixing with warmer air from middle latitudes. Temperatures drop to below -85oC in stratosphere. Chemical reactions unique to extreme cold occur in air isolated inside vortex. Setting the Stage Williams, The Weather Book

14. June: Winter begins. Polar vortex strengthens and temperatures begin to fall. July-August: The temperatures fall to below -85oC. Ice clouds form from water vapor and nitric acid. Chemical reactions that can occur on ice crystals, but not in air, free chlorine atoms from the CFC. How Ozone is Destroyed Williams, The Weather Book

15. 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. How Ozone is Destroyed Danielson et al, Fig 2.29

16. Chlorine atoms can be freed from CFC’s by UV reaction CCl3F + UV  CCl2F + Cl CCl2F2 + UV  CClF2 + Cl C2Cl3F3 + UV  C2Cl2F3 + Cl Once a chlorine atom is freed, it can destroy thousands of ozone molecules before being removed from the air Cl + O3 O2 + ClO ClO + O  O2 + Cl Chlorine acts as a catalyst Chemistry of the Ozone Hole CFC-11 Moran and Morgan, Fig 2.19

17. Annual Cycle of Ozone over SP http://www.cmdl.noaa.gov/ozwv/ozsondes/spo/index.html http://www.cmdl.noaa.gov/ozwv/ozsondes/images/ozone_anim2001.avi

18. Mean Monthly Total Ozone Ozone “Dent” over N.H. during the period 1979 to 1993.

19. Key Points: Ozone Hole • Chlorofluorocarbons (CFCs) disrupt the natural balance of O3 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 trapped inside circumpolar vortex Cold temperatures fall to below -85oC

20. Key Points: Ozone Hole • CFCs stay in atmosphere for ~100 years One freed chlorine atom destroys thousands of O3 molecules before leaving stratosphere • Montreal Protocol mandated total phase out of ozone depleting substances by 2000. • Even with a complete phase out, O3 levels Would not increase for another 10-20 years Would not completely recover for ~100 years

21. Assignment for Next Lecture • Topic - Natural Climate Variability • Reading - Ahrens, pg 373-390 • Problems - 14.5, 14.6, 14.7, 14.10