stratospheric ozone depletion last time, we learned about natural ozone chemistry in the...

Stratospheric Ozone Depletion

• Last time, we learned about natural ozone chemistry in the stratosphere.

• How do anthropogenic emissions of certain chemicals affect the ozone layer?

• Why does the ozone ‘hole’ form over Antarctica?

Anthropogenic Enhancement of the Stratospheric HOx Cycle

From: http://www.ipcc.ch/present/graphics.htm

From: IPCC report, 2007

Anthropogenic Enhancement of the Stratospheric NOx Cycle

From: http://www.ipcc.ch/present/graphics.htm

From: IPCC report, 2007

Chlorine Chemistry in the Stratosphere (ClOx family)

223

2

23

222

2210nm-185

3

OOOO :net

OClOClO

OClOO Cl

ClClCFhClCF

ClCFClhCFCl

• CFCs photodissociate in the stratosphere:

Reservoirs of ClOx

• Reservoirs are transported to the troposphere for removal, or:

MClONOMNOClO

HClCHCH Cl

22

34

ClNO3

3

22

2

NOCl

NOClOhClONO

OH ClOHHCl

Reservoirs of ClOx (Cly)

• ClOx includes:

– Cl + ClO

• Cly includes:

– Cl + ClO + ClONO2 + HCl + Cl2 + ClO2 + Cl2O2 + HOCl + BrCl

• About 99% of the chlorine in the stratosphere is in the form of Cly.

– This chlorine is not active towards O3.

The ClOx Cycle

http://www-as.harvard.edu/people/faculty/djj/book/powerpoints/

Sources of Stratospheric Chlorine• Natural sources:

– HCl: dechlorination of sea-salt, volcanic emissions, biomass burning• Very short lifetime in the troposphere

– CH3Cl: ocean and biomass burning emissions• Lifetime of about 1.5 years

• Anthropogenic sources:– Emissions of CFCs

• No tropospheric sink• Lifetimes on the order of 100 years

What is a CFC?

• Chlorofluorocarbon – contains only carbon, chlorine, and fluorine.

• ‘Rule of 90’ – take CFC-11 for example:– add 11+90 = 101

• First digit is the # of carbon = 1• Second digit is the # of hydrogen = 0• Third digit is the # of fluorine = 1• The remainder is chlorine

CCl3F

Why were CFCs Invented?

• Early refrigerators used toxic chemicals as refrigerants:

– NH3, CH3Cl, SO2.

• CFCs were invented by Thomas Midgley in 1928

– By 1935 over 8 million refrigerators had been sold containing Freon (the trade name for CFCs).

• CFCs were considered incredibly non-toxic:

– Expanded the use of refrigerators and air-conditioning in non industrial settings.

– Many additional uses were discovered:• aerosol propellant• blowing Styrofoam• fire retardant• electronics cleaning• etc.

Why did the use of CFCs Expand?

WMO/UNEPScientific Assessment of Ozone Depletion: 2006

The Rise and Fall of CFCs

The Rise and Fall of CFCs

WMO/UNEPScientific Assessment of Ozone Depletion: 2002

http://www-as.harvard.edu/people/faculty/djj/book/powerpoints/

CFCs in the Statosphere

Mixed Cycles• We have seen the coupling of NOx and HOx:

• NOx is coupled with ClOx:

• HOx is coupled with ClOx:

MHNOMNOOH 32

MClONOMNOClO 22

22

2

OHOClClOHO

ClOHHCl OH

Mixed Cycles• As HOx increases, active NOx decreases,

but active ClOx increases.

• As NOx increases, active ClOx and HOx decreases.

• As ClOx increases, active NOx and HOx decreases.

Mixed Cycles

22233

22

223

23

OOOOO

ClOHhHOCl

OHOClHOClO

OHOOOH

OClOOCl

HOx/ClOx HOx/NOx

OOhO

ONOhNO

OHNONOHO

OHOOOH

23

2

22

223

Ozone destruction Null cycle

Mixed Cycles

22233

23

32

2M

2

223

23

OOOOO

ONOhNO

NOClhClONO

ClONONOClO

ONOONO

OClOOCl

NOx/ClOx NOx/ClOx

OOhO

ONOhNO

NOClNOClO

OClOOCl

23

2

2

23

Ozone destruction Null cycle

BrOx in the Stratosphere

22233

23

23

2

2

OOOOO

OClOOCl

ClBrhBrCl

OBrOOBr

OBrCl

OClOBr

OBrClBrOClO

223

2

23

OOOO :net

OBrOBrO

OBrOOBr

Stratospheric bromine chemistry is analogous to chlorine chemistry.

BrOx/ClOx mixed cycle

BrOx is a more effective catalyst for O3 destruction because its reservoir species are more difficult to form.

y

x

y

x

Br

BrO

Cl

ClO

Sources of BrOx

• Natural:– CH3Br is naturally emitted from the ocean.

• Anthropogenic:– CH3Br is used as an agricultural fumigant.

Methylbromide

WMO/UNEP Scientific Assessment of Ozone Depletion: 2006

Non-Polar StratosphericOzone Depletion

WMO/UNEP Scientific Assessment of Ozone Depletion: 2002

Stratospheric Aerosol Layer• There is a persistent layer of sulfate

aerosols in the lower stratosphere.– The background source is carbonyl sulfide

(COS).– COS is naturally emitted from the ocean’s

surface

• tropospheric concentrations are about 500pptv.

COS in the Stratosphere

4(l)2223

2

SOHOHSOHO,OOH,HSSO,S,

HSCOOH COS

(22%) SOCOO COS

(71%) S COh COS

Since there is no precipitation in the stratosphere,the lifetime of the aerosol is 1-2 years.

The aerosols absorb water to their surface.

Volcanic Aerosols

• Major volcanic eruptions can inject H2SO4 aerosols directly into the stratosphere.

– normal stratospheric aerosol surface area:

0.5 – 1.0µm2 cm-3

– surface area after Mt. Pinatubo erupted:

20µm2 cm-3

• Since there is usually a major volcanic eruption every few years, it is uncommon for the stratospheric aerosol layer to not be influenced by volcanic activity.

Stratospheric Aerosol

WMO/UNEP Scientific Assessment of Ozone Depletion: 2002

• Remember the reaction:

– the lifetime of HNO3 is days

– the lifetime of N2O5 is hours

• The conversion of N2O5 to HNO3 represents a decrease in:

Heterogeneous Chemistry

3(s)252 2HNOOHON

y

x

NO

NO

• If the amount of active NOx is decreased:

– the amount of active ClOx is increased

Mixed Cycle

22 ClONONOClO

Stratospheric Aerosol

WMO/UNEP Scientific Assessment of Ozone Depletion: 2002

Increased UV Radiation

WMO/UNEPScientific Assessment of Ozone Depletion: 2006

CFC Replacements

http://www-as.harvard.edu/people/faculty/djj/book/powerpoints/

Ozone Depleting Chemicals

WMO/UNEPScientific Assessment of Ozone Depletion: 2004

The Ozone Hole

http://www-as.harvard.edu/people/faculty/djj/book/powerpoints/

Antarctic Ozone BulletinNo 8/2005Winter/spring summaryJanuary 2006Global Atmosphere Watch

‘Density’ of Ozone Hole

Area of Ozone Hole

Antarctic Ozone BulletinNo 8/2005Winter/spring summaryJanuary 2006Global Atmosphere Watch

Seasonality of the Ozone Hole

http://www-as.harvard.edu/people/faculty/djj/book/powerpoints/

The Ozone Hole Area

From: NASA

The Ozone Hole

From: NASA

Vertical Structure of the Ozone Hole

Antarctic Ozone BulletinNo 3/2008World Meteorological Organization

The Antarctic Stratosphere

• What is special about the Antarctic stratosphere?

– Polar vortex• Extremely stable westerly circulation that

essentially cuts the Antarctic stratosphere off from the midlatitudes.

• The vortex persists until November (late spring)

– Extremely cold temperatures• As low as 180K

– Its totally dark during the winter (until September)

A New Catalytic Cycle

23

23

222

22

3OhO2

OClOOCl2

OClClhOCl

MOClMClOClO

This cycle does not depend on atomic O•Cl2O2 is easily photolyzed by hν, O2 is not.

•How could so much ClO be produced to account for the observed O3 loss? Since:

22 ClONONOClO

http://www.atm.damtp.cam.ac.uk/people/efs20/media/kiruna/PSCbest.jpg

Polar Stratospheric Clouds

PSCs

• Type I:– mixture of water and nitric acid– form at temperatures under 197K

• Type II:– pure water ice– form at temperatures under 188K

Formation of PSCs

WMO/UNEPScientific Assessment of Ozone Depletion: 2002

PSCs

• PSC help to activate ClOx

• At the same time removing NOx

– disabling the ClONO2 reservoir for ClOx

Heterogeneous Chemistryin the Dark

OHClHClHOCl

HNOHOClOHClONO

HNOClHClClONO

22PSC

3PSC

22

32PSC

2

•During the winter, all of the chlorine reservoirs are converted to Cl2 on the surface of PSCs.

•All of the NOy is removed from the Antarctic stratosphere. Much of the NOy settles out of the atmosphere altogether.

When the Sun Comes Up

23

23

222

22

3OhO2

OClOOCl2

OClClhOCl

MOClMClOClO

Cl2hCl2

•There is no NOx to sequester the ClOx, so O3 destruction goes on unabated.

•Remember, in the mid-latitude stratosphere, about 99% of Cl is in the form of Cly.•During Antarctic spring, almost all Cl is in the form of ClOx.

In the Summer

• The vortex breaks up– This allows mid-latitude NOx and O3 rich air

to mix over Antarctica.

– However, O3 depleted air mixes over populated areas of the southern hemisphere.

• The stratosphere warms and the PSCs evaporate

Ozone Hole Chemistry

Ozone Hole Calendar

http://www-as.harvard.edu/people/faculty/djj/book/powerpoints/

The Smoking Gun

Next Week’s Exam• The exam is on Wednesday, October 7

during the first 50 minutes of regular class time.

• You are allowed to use a calculator (no PDAs or cell phone calculators).– You may not use your calculator to look at

pre-recorded notes – this is considered cheating.

– Don’t forget your calculator, there will be no extras to hand out.

Next Week’s Exam• No aids in addition to a calculator are

allowed.• Do everything possible to avoid even the

suggestion of cheating:– no hats, sunglasses, wandering eyes, etc.

• Partial credit can be given only if your work is well organized and easily readable.

• All calculations need to be worked out on the exam to receive credit, even if you have the correct answer.

Next Week’s Exam

• The form of the exam:– Multiple choice:

• 20-30%

– Short answer• 30-40%

– Worked out calculations• 30-40%

• This exam is worth 22% of your grade.

Next Week’s Exam

• Priority of material on the exam:– Topics covered in lecture– Topics covered by the assigned reading

• You are responsible for material in the readings, even if it was not explicitly covered in lecture.

• Preparing for the exam:– Do the suggested problems– Study the lecture notes– Study text