NATS 101

Lecture 32Ozone Depletion

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)

Review: Ultraviolet (UV)

Absorption 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

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 mO2 + UV O + O

O3 forms when O2 and O molecules collideO2 + O O3

Disassociation of O3 absorbs 0.2-0.3 m UVO3 + 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

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 ended today due to leakage

of CFC’s from old appliances, etc.

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

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?

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 O3 over Antarctica during the period 1979-92.

Setting the StageConditions 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.

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 -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.

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

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

Moran and Morgan, Fig 2.19

CFC-11

Annual Cycle of Ozone over SP

http://www.cmdl.noaa.gov/ozwv/ozsondes/spo/index.htmlhttp://www.cmdl.noaa.gov/ozwv/ozsondes/images/ozone_anim2001.avi

Mean Monthly Total Ozone

Decrease in O3 over N.H. during the period 1979 to 1993.

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

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

Assignment for Next Lecture

• Topic - Natural Climate Variability

• Reading - Ahrens, pg 373-390

• Problems - 14.5, 14.6, 14.7, 14.10