Stratospheric Ozone Depletion

1. 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?

2. Anthropogenic Enhancement of the Stratospheric HOx Cycle From: http://www.ipcc.ch/present/graphics.htm

3. From: IPCC report, 2007

4. Anthropogenic Enhancement of the Stratospheric NOx Cycle From: http://www.ipcc.ch/present/graphics.htm

5. From: IPCC report, 2007

6. Chlorine Chemistry in the Stratosphere (ClOx family) • CFCs photodissociate in the stratosphere:

7. Reservoirs of ClOx • Reservoirs are transported to the troposphere for removal, or: ClNO3

8. 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.

9. The ClOx Cycle http://www-as.harvard.edu/people/faculty/djj/book/powerpoints/

10. 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

11. 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

12. 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).

13. Why did the use of CFCs Expand? • 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.

14. The Rise and Fall of CFCs WMO/UNEPScientific Assessment of Ozone Depletion: 2006

15. The Rise and Fall of CFCs WMO/UNEPScientific Assessment of Ozone Depletion: 2002

16. CFCs in the Statosphere http://www-as.harvard.edu/people/faculty/djj/book/powerpoints/

17. Mixed Cycles • We have seen the coupling of NOx and HOx: • NOx is coupled with ClOx: • HOx is coupled with ClOx:

18. 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.

19. Mixed Cycles HOx/NOx HOx/ClOx Null cycle Ozone destruction

20. Mixed Cycles NOx/ClOx NOx/ClOx Null cycle Ozone destruction

21. BrOx in the Stratosphere 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.

22. Sources of BrOx • Natural: • CH3Br is naturally emitted from the ocean. • Anthropogenic: • CH3Br is used as an agricultural fumigant.

23. Methylbromide WMO/UNEP Scientific Assessment of Ozone Depletion: 2006

24. Non-Polar StratosphericOzone Depletion WMO/UNEP Scientific Assessment of Ozone Depletion: 2002

25. 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.

26. 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.

27. 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.

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

29. Heterogeneous Chemistry • 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:

30. Mixed Cycle • If the amount of active NOx is decreased: • the amount of active ClOx is increased

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

32. Increased UV Radiation WMO/UNEPScientific Assessment of Ozone Depletion: 2006

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

34. Ozone Depleting Chemicals WMO/UNEPScientific Assessment of Ozone Depletion: 2004

35. The Ozone Hole http://www-as.harvard.edu/people/faculty/djj/book/powerpoints/

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

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

38. Seasonality of the Ozone Hole http://www-as.harvard.edu/people/faculty/djj/book/powerpoints/

39. The Ozone Hole Area From: NASA

40. The Ozone Hole From: NASA

41. Vertical Structure of the Ozone Hole Antarctic Ozone Bulletin No 3/2008 World Meteorological Organization

42. 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)

43. A New Catalytic Cycle • 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:

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

45. PSCs • Type I: • mixture of water and nitric acid • form at temperatures under 197K • Type II: • pure water ice • form at temperatures under 188K

46. Formation of PSCs WMO/UNEPScientific Assessment of Ozone Depletion: 2002

47. PSCs • PSC help to activate ClOx • At the same time removing NOx • disabling the ClONO2 reservoir for ClOx

48. Heterogeneous Chemistryin the Dark • 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.

49. When the Sun Comes Up • 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.

50. 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