NATS 101 Lecture 32 Ozone Depletion

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

Review: Ultraviolet (UV) Absorption O 2 and O 3 absorb UV (shorter than 0.3  m) Therefore, reductions in the level of O 3 would increase the amount of UV radiation that penetrates to the surface IR Ahrens, p 36 UV Visible

Hazards of Increased UV Increase number of cases of skin cancers Increase in eye cataracts and sun burning Suppression of human immune system Damage to crops and animals Reduction in ocean phytoplankton

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  m UV O3 + UV  O2 + O Balance exists between O3 creation-destruction CFC’s disrupts balance

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  m UV O3 + UV  O2 + O Balance exists between O3 creation-destruction CFC’s disrupts balance

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

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 years for CFC levels to start falling if all production ended today due to leakage of CFC’s from old appliances, etc.

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 1987Adoption of Montreal Protocol calling for a 50% reduction in use of CFC’s by 1998

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

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.

Mean Monthly Total Ozone Huge decrease in O 3 over Antarctica during the period

Setting the Stage 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

How Ozone is Destroyed June: Winter begins. Polar vortex strengthens and temperatures begin to fall. July-August: The temperatures fall to below -85 o C. 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. Williams, The Weather Book

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

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 being removed from the air Cl + O 3  O 2 + ClO ClO + O  O 2 + Cl Moran and Morgan, Fig 2.19 CFC-11

Annual Cycle of Ozone over SP

Mean Monthly Total Ozone Decrease in O 3 over N.H. during the period 1979 to 1993.

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 trapped inside circumpolar vortex Cold temperatures fall to below -85 o C

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 Even with a complete phase out, O 3 levels Would not increase for another years Would not completely recover for ~100 years

Assignment for Next Lecture Topic - Natural Climate Variability Reading - Ahrens, pg Problems , 14.6, 14.7, 14.10