ozone stratospheric tropospheric summary basics issues ©2003, perry samson, university of michigan...

OZONE

Stratospheric

Tropospheric

Summary

Basics

Issues

©2003, Perry Samson, University of Michigan

The good,the bad &

the ugly

OZONE

Stratospheric

Tropospheric

Summary

Basics

Issues

©2003, Perry Samson, University of Michigan

Driving Questions

1. Why is ozone important?

2. How is it formed?

3. How is it destroyed?

4. How is it different in the stratosphere vs. the troposphere?

5. If stratospheric ozone is decreasing and tropospheric ozone is increasing, why not just build a REALLY big chimney?

Issues to be Considered:

OZONE

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Summary

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©2003, Perry Samson, University of Michigan

Greenhouse Gases

1. Carbon Dioxide (CO2)

2. Nitrous Oxide (N2O)

3. Methane (CH4)

4. Ozone (O3)

5. Chlorofluorocarbons (CCFs)

and

Water VaporClick me

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Tropospheric

Summary

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©2003, Perry Samson, University of Michigan

Ultraviolet Radiation

0

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1

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0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2Wavelength (µm)

Radiation

uv-b

.280 µ

m

uv-a

.315 µ

m.4

00 µ

m

uv-c

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©2003, Perry Samson, University of Michigan

Ozone Formation

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©2003, Perry Samson, University of Michigan

Ozone Maxima

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©2003, Perry Samson, University of Michigan

Why the Stratosphere?

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©2003, Perry Samson, University of Michigan

Ozone Formation

• When an oxygen molecule absorbs a photon of light with a wavelength shorter than 0.2 µm the energy splits the molecule into two oxygen atoms.

O2 + h —> O + O• One of these atoms can react with another oxygen

molecule to form an ozone molecule.

O + O2 + M —> O3 + MUp to 98% of the sun's high-energy ultraviolet light(UV-B and UV-C) are absorbed by the destruction and formation of atmospheric ozone.

OZONE

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©2003, Perry Samson, University of Michigan

Ozone Formation

O2 + h —> O + O

2 (O + O2 + M —> O3 + M)

3O2 —> 2O3

Net production

OZONE

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©2003, Perry Samson, University of Michigan

Ozone Destruction

• Ozone, on the other hand, absorbs photons of light with a wavelength between 0.2-0.3 µm that splits the molecule into an oxygen atom and an oxygen molecule.

O3 + h —> O2 + O• The oxygen atom react with another ozone molecule to

form two oxygen molecules.

O + O3 —> O2 + O2

2 O3 —> 3O2

OZONE

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Tropospheric

Summary

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©2003, Perry Samson, University of Michigan

Catalytic Reactions

• The term catalytic means that X can react with and remove ozone while X itself is regenerated in the process

O3 + X —> XO + O2

XO + O3 —> X + 2O2

2 O3 —> 3 O2

O3 + X —> XO + O2

XO + O —> X + O2

O + O3 —> 3 O2

and

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Summary

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©2003, Perry Samson, University of Michigan

Ozone Formation

• Ozone has a natural photochemical balance between formation and destruction as other gases participate. Example: Nitrogen oxides

NO2 —> NO + O

O + O2 —> O3

NO + O —> NO2

O + O2 —> 2O2

OZONE

Stratospheric

Tropospheric

Summary

Basics

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©2003, Perry Samson, University of Michigan

Ozone Formation

• But chlorofluorocarbons shift this balance:

Cl2CF2 + UV light —> ClCF2 + Cl

• Then there is a catalytic destruction of ozone:

Cl + O3 —> ClO + O2

ClO + O3 —> Cl + O2

• The net effect of this pair of fast reactions is to turn two ozone molecules into three normal molecules of oxygen. The (catalyst) atomic chlorine is recovered in the second reaction, making it available to start over. In fact, each chlorine atom can destroy hundreds of thousands of ozone molecules!

OZONE

Stratospheric

Tropospheric

Summary

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©2003, Perry Samson, University of Michigan

Lifetime of CFC1. Emitted into atmosphere as gas.2. Disperses rapidly throughout Northern hemisphere in ~1

year3. After a few years is uniformly mixes throughout whole

troposphere.4. Movement into stratosphere is slow. Some leaks upwards to

region where sufficient UV light is available to break bonds and release Cl.

5. Each atom of Cl participates in thousands of catalytic reactions, destroying hundreds of thousands of ozone molecules.

6. Ultimately each Cl atom reacts with methane to produce HCl.

7. Some of the HCl is transported back into the troposphere and gets rained out, the rest gets broken apart and the Cl cycle begins again.

OZONE

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Tropospheric

Summary

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©2003, Perry Samson, University of Michigan

Ozone Depletion Potential

•The Ozone Depletion Potential (ODP) is the ratio of the impact on ozone of a chemical compared to the impact of a similar mass of CFC-11.

o Thus, the ODP of CFC-11 is defined to be 1.0.

o Other CFCs and Hydrochlorofluorocarbons (HCFC’s) have ODPs that range from 0.01 to 1.0.

o Halons have ODPs ranging up to 10.

o Carbon tetrachloride (CCl4) has an ODP of 1.2, and methyl chloroform's ODP is 0.11.

o Hydrofluorocarbons (HFCs) have zero ODP because they do not contain chlorine.

OZONE

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©2003, Perry Samson, University of Michigan

Ozone Column Measure

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©2003, Perry Samson, University of Michigan

Problems with Ozone Depletion• More than 90% of

non-melanoma skin cancers are related to UV-B exposure.

• 1% ozone decrease produces a ~2% increase in UV-B.

• Each 2% increase in UV-B is epidemiological linked with a 2%-5% increase in basal-cell cancer cases and a 4-10% increase in squamous cell cancer.

http://www.skincancer.org/

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©2003, Perry Samson, University of Michigan

Problems with Ozone Depletion

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

OZONE

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©2003, Perry Samson, University of Michigan

Problems with Ozone Depletion

A national UV Index is issued daily to advise the public on the strength of the sun's UV rays.

The chart to the right shows the levels of the UV Index, your time to burn in the sun without sunscreen, and what you can do to protect yourself.

OZONE

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©2003, Perry Samson, University of Michigan

Problems with Ozone Depletion

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©2003, Perry Samson, University of Michigan

Problems with Ozone Depletion

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©2003, Perry Samson, University of Michigan

OZONE

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©2003, Perry Samson, University of Michigan

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©2003, Perry Samson, University of Michigan

Ozone Hole Discovery

In 1985 Farmen et al. published a paper containing this figure. It showed a dramatic decrease in total ozone abundance during October over the Antarctic station at Halley Bay.

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©2003, Perry Samson, University of Michigan

Ozone Hole Discovery

Moreover the amount of ozone column has decreased steadily.

0

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100

150

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1979 1981 1983 1985 1987 1989 1991 1993 1995 1997 1999 2001

Ozone Column (Dobson Units)

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©2003, Perry Samson, University of Michigan

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©2003, Perry Samson, University of Michigan

Changing Ozone Concentrations

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©2003, Perry Samson, University of Michigan

Changing Ozone Concentrations

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©2003, Perry Samson, University of Michigan

Changing Ozone Concentrations

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1979 1981 1983 1985 1987 1989 1991 1993 1995 1997 1999 2001

Ozone Hole Area (10^6 km^2)

The ozone hole is defined geographically as the area wherein the total ozone amount is less than 220 Dobson Units.

OZONE

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©2003, Perry Samson, University of MichiganLast Updated: April 18, 2023

QuickTime™ and aYUV420 codec decompressor

are needed to see this picture.

OZONE

Stratospheric

Tropospheric

Summary

Basics

Issues

©2003, Perry Samson, University of Michigan

QuickTime™ and aYUV420 codec decompressor

are needed to see this picture.

OZONE

Stratospheric

Tropospheric

Summary

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©2003, Perry Samson, University of MichiganLast Updated: April 18, 2023

OZONE

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©2003, Perry Samson, University of Michigan

So Why an Ozone “Hole”?

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©2003, Perry Samson, University of Michigan

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©2003, Perry Samson, University of Michigan

Role of Polar Stratospheric Clouds

Source: Turco, Earth Under Siege

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©2003, Perry Samson, University of MichiganLast Updated: April 18, 2023

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0 5 10 15Ozone (partial pressure)

HEIGHT (km)

8/4/01

9/28/01

10/2/01

10/14/01

Ozone depletion depends on altitude

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©2003, Perry Samson, University of MichiganLast Updated: April 18, 2023

Ozone depletion depends on altitude and is highly influenced by temperature.

OZONE

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Tropospheric

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©2003, Perry Samson, University of Michigan

Changing Ozone Concentrations

Last Updated: April 18, 2023

QuickTime™ and aGIF decompressor

are needed to see this picture.

OZONE

Stratospheric

Tropospheric

Summary

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©2003, Perry Samson, University of MichiganLast Updated: April 18, 2023

OZONE

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Tropospheric

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©2003, Perry Samson, University of MichiganLast Updated: April 18, 2023

• UV radiation is weaker and the photolysis of O2 is less important.

• Near the surface the production of ozone is still initiated by sunlight, but the reactors are volatile organic compounds (VOCs) and nitrogen oxides (NOx = NO + NO2)

Meanwhile, down near the surface

OZONE

Stratospheric

Tropospheric

Summary

Basics

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©2003, Perry Samson, University of MichiganLast Updated: April 18, 2023

NO2 —> NO + O

O + O2 + M —> O3 + M

NO + O3 —> NO2 + O2

Meanwhile, down near the surface

Net result, no ozone buildup.

OZONE

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Tropospheric

Summary

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©2003, Perry Samson, University of MichiganLast Updated: April 18, 2023

• VOCs produce “radical organics” (RO2) that interfere with the NO chemistry, basically producing extra atomic oxygen

RO2 + NO —> NO2 + RO

NO2 —> NO + O

O + O2 + M —> O3 + M

Meanwhile, down near the surface

Net result, ozone buildup.

OZONE

Stratospheric

Tropospheric

Summary

Basics

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©2003, Perry Samson, University of MichiganLast Updated: April 18, 2023

Meanwhile, down near the surface

QuickTime™ and aGIF decompressor

are needed to see this picture.

OZONE

Stratospheric

Tropospheric

Summary

Basics

Issues

©2003, Perry Samson, University of MichiganLast Updated: April 18, 2023

Meanwhile, down near the surface

QuickTime™ and aGIF decompressor

are needed to see this picture.

OZONE

Stratospheric

Tropospheric

Summary

Basics

Issues

©2003, Perry Samson, University of MichiganLast Updated: April 18, 2023

Ozone Control