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 HO x Cycle From:

From: IPCC report, 2007

Anthropogenic Enhancement of the Stratospheric NO x Cycle From:

From: IPCC report, 2007

Chlorine Chemistry in the Stratosphere (ClO x family) CFCs photodissociate in the stratosphere:

Reservoirs of ClO x Reservoirs are transported to the troposphere for removal, or: ClNO 3

Reservoirs of ClO x (Cl y ) ClO x includes: –Cl + ClO Cl y includes: –Cl + ClO + ClONO 2 + HCl + Cl 2 + ClO 2 + Cl 2 O 2 + HOCl + BrCl About 99% of the chlorine in the stratosphere is in the form of Cl y. –This chlorine is not active towards O 3.

The ClO x Cycle

Sources of Stratospheric Chlorine Natural sources: –HCl: dechlorination of sea-salt, volcanic emissions, biomass burning Very short lifetime in the troposphere –CH 3 Cl: 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 = 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 CCl 3 F

Why were CFCs Invented? Early refrigerators used toxic chemicals as refrigerants: –NH 3, CH 3 Cl, SO 2. 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/UNEP Scientific Assessment of Ozone Depletion: 2006 The Rise and Fall of CFCs

WMO/UNEP Scientific Assessment of Ozone Depletion: 2002

CFCs in the Statosphere

Mixed Cycles We have seen the coupling of NO x and HO x : NO x is coupled with ClO x : HO x is coupled with ClO x :

Mixed Cycles As HO x increases, active NO x decreases, but active ClO x increases. As NO x increases, active ClO x and HO x decreases. As ClO x increases, active NO x and HO x decreases.

Mixed Cycles HO x /ClO x HO x /NO x Ozone destruction Null cycle

Mixed Cycles NO x /ClO x Ozone destruction Null cycle

BrO x in the Stratosphere Stratospheric bromine chemistry is analogous to chlorine chemistry. BrO x /ClO x mixed cycle BrO x is a more effective catalyst for O 3 destruction because its reservoir species are more difficult to form.

Sources of BrO x Natural: –CH 3 Br is naturally emitted from the ocean. Anthropogenic: –CH 3 Br is used as an agricultural fumigant.

Methylbromide WMO/UNEP Scientific Assessment of Ozone Depletion: 2006

Non-Polar Stratospheric Ozone 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 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 H 2 SO 4 aerosols directly into the stratosphere. –normal stratospheric aerosol surface area: 0.5 – 1.0µm 2 cm -3 –surface area after Mt. Pinatubo erupted: 20µm 2 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 HNO 3 is days –the lifetime of N 2 O 5 is hours The conversion of N 2 O 5 to HNO 3 represents a decrease in: Heterogeneous Chemistry

If the amount of active NO x is decreased: –the amount of active ClO x is increased Mixed Cycle

Stratospheric Aerosol WMO/UNEP Scientific Assessment of Ozone Depletion: 2002

Increased UV Radiation WMO/UNEP Scientific Assessment of Ozone Depletion: 2006

CFC Replacements

Ozone Depleting Chemicals WMO/UNEP Scientific Assessment of Ozone Depletion: 2004

The Ozone Hole

Antarctic Ozone Bulletin No 8/2005 Winter/spring summary January 2006 Global Atmosphere Watch ‘Density’ of Ozone Hole

Area of Ozone Hole Antarctic Ozone Bulletin No 8/2005 Winter/spring summary January 2006 Global Atmosphere Watch

Seasonality of the Ozone Hole

The Ozone Hole Area From: NASA

The Ozone Hole From: NASA

Vertical Structure of the Ozone Hole Antarctic Ozone Bulletin No 3/2008 World 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 This cycle does not depend on atomic O Cl 2 O 2 is easily photolyzed by hν, O 2 is not. How could so much ClO be produced to account for the observed O 3 loss? Since:

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/UNEP Scientific Assessment of Ozone Depletion: 2002

PSCs PSC help to activate ClO x At the same time removing NO x –disabling the ClONO 2 reservoir for ClO x

Heterogeneous Chemistry in the Dark During the winter, all of the chlorine reservoirs are converted to Cl 2 on the surface of PSCs. All of the NO y is removed from the Antarctic stratosphere. Much of the NO y settles out of the atmosphere altogether.

When the Sun Comes Up There is no NO x to sequester the ClO x, so O 3 destruction goes on unabated. Remember, in the mid-latitude stratosphere, about 99% of Cl is in the form of Cl y. During Antarctic spring, almost all Cl is in the form of ClO x.

In the Summer The vortex breaks up –This allows mid-latitude NO x and O 3 rich air to mix over Antarctica. –However, O 3 depleted air mixes over populated areas of the southern hemisphere. The stratosphere warms and the PSCs evaporate

Ozone Hole Chemistry

Ozone Hole Calendar

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