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Stratospheric Ozone Depletion

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.

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Stratospheric Ozone Depletion

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  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

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