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NATS 101

Ozone Depletion

Last air pollution topic we’ll cover is the ozone hole.

Is stratospheric ozone good or bad? Why?

inversion

isothermal

6.5oC/km

Recall that the warming in the stratosphere occurs due to the photodissociation of O3 and O2.

Photodissociation occurs because of absorption of the sun’s UV rays.

Ultraviolet (UV) Absorption

A reduction in O3 would increase the UV that penetrates to the surface

IR

Ahrens, p 36

UV Visible

Possible Harm from O3 Loss

• UV damages DNA (genetic material)

Damage to plants (including crops)

Reduction in ocean phytoplankton-base of food chain and major sink of CO2

Human impacts include…

Skin cancers

Eye cataracts

Immunity suppression

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 Chlorofluorocarbons• 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 years for CFC levels to start falling if all production ceased today owing 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 ProtocolMontreal 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 ProtocolMontreal 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 ProtocolMontreal Protocol calling for an accelerated phase out by ozone depleting chemicals

2100 Time needed until ozone layer heals completely?

How O3 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 There are only a few millimeters (few hundred Dobsons) of total ozone in a column of airDobsons) of total ozone in a column of air

Mean October Total Ozone

O3 evolution over Antarctica during the period 1979-2008.

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Ozone < 220 DU Considered Hole

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 -85oC in stratosphere.

Chemical reactions unique to extreme cold occur in air isolated inside vortex.

Williams, The Weather Book

Ingredient #2Polar-Stratospheric Clouds

June: Winter begins

Polar vortex strengthens and temperatures begin to fall.

July-August: Temperatures fall to below -85oC

Ice clouds form from water vapor and nitric acid.

Chemical reactions that can occur on HNO3 ice crystals, but not in air, free chlorine atoms from the CFC. nacreous cloud

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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 + ClOnce a chlorine atom is freed,

it can destroy thousands of ozone molecules before its sequestering from the air

Cl + O3 O2 + ClO

ClO + O O2 + Cl

Chlorine acts as a catalyst Moran and Morgan, Fig 2.19

CFC-11

Annual Cycle of Ozone over SP

Ozone time-height cross-sectionhttp://www.esrl.noaa.gov/gmd/dv/spo_oz/

Arctic O3 “Mini-Hole”?

Trend of about 10% per decade?

Northern Hemisphere Ozone Loss Partial Skin Damage Risk

About a 5% decrease in mid-latitudesErythema: reddening of skin from ionizing radiation

http://www.esrl.noaa.gov/gmd/about/ozone.html

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Key Points: Ozone Hole

• Chlorofluorocarbons (CFCs) disrupt the natural balance of O3 in S.H. stratosphere

CFCs responsible for the ozone hole over SP!CFCs responsible for the ozone hole over SP!

Responsible for lesser reductions worldwide.Responsible for lesser reductions worldwide.

• Special conditions exist in stratosphere over Antarctica that promote ozone destruction:

Air cools to -85Air cools to -85ooC inside circumpolar vortex C inside circumpolar vortex

Allows formation of HNO3 nacreous cloudsAllows formation of HNO3 nacreous clouds

Key Points: Ozone Hole

• CFCs stay in atmosphere for ~100 years

One freed chlorine atom destroys thousands One freed chlorine atom destroys thousands of Oof O33 molecules before leaving stratosphere 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 decrease for another 10 yearsWould not decrease for another 10 years

Would not completely recover for ~100 yearsWould not completely recover for ~100 years

Anthropogenic Damage And a Greater World Promise

• The Ozone HoleOzone Hole showed unequivocally that humans and industrialization can damage the environment on a large scale.

• The Montreal ProtocolMontreal Protocol showed that the world community can come together to solve major global issues.

A SUCCESS STORYA SUCCESS STORY

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A UNEQUIVOCAL SUCCESS STORYA UNEQUIVOCAL SUCCESS STORY

Montreal Protocol is

But… Corrective Measures Were Relatively Painless

Assignment for Next Lecture Climate Variability, Climate Change

• Reading - Ahrens4th: 324-327, 373-399

5th: 334-337, 383-409

• Homework11 - D2L Due Friday May 74th-Pg. 399: 14.5, 6, 7, 10

5th-Pg. 412: 14.5, 6, 7, 10

Do Not Submit Homework on D2L