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This Week—Polar Stratospheric Chemistry

This Week—Polar Stratospheric Chemistry. READING: Chapter 10 of text SPADE Data Analysis Pt 1: Due Tuesday Nov 13. Today—Ozone Hole Part 1. Characterization of Ozone Hole Finding the “Smoking Gun” With Human Fingerprints. Polar Ozone Loss.

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This Week—Polar Stratospheric Chemistry

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  1. This Week—Polar Stratospheric Chemistry READING: Chapter 10 of text SPADE Data Analysis Pt 1: Due Tuesday Nov 13

  2. Today—Ozone Hole Part 1 • Characterization of Ozone Hole • Finding the “Smoking Gun” With Human Fingerprints

  3. Polar Ozone Loss Severe depletion of stratospheric O3 occurs every spring (since 1980’s) over the poles (especially South Pole). • Example of: • far reach of human activities • environmental catastrophe and political blame game • scientific process and ability of humans to correct

  4. Discovery of Antarctic Ozone Hole

  5. Antarctic O3 Hole A Springtime Event Ozone Watch Web Page 2006 Ozone Hole Movie From Satellite Observations

  6. Vertical Extent of Ozone Hole Chlorine from CFCs predicted to be most effective ~ 40 km. So why is depletion at 15 – 20 km?

  7. Discovery of Antarctic Ozone Hole The Conundrum: • Known catalytic reactions with chlorine not fast enough to explain near complete depletion in couple months. • Why only in spring? • Why only between 15 – 25 km? • Why only in polar region?

  8. Debate Over Causes of Ozone Hole There was also a real scientific debate over the relative roles of chemistry and meteorology. Turns out to be both (of course!)

  9. Chlorine: The Smoking Gun? ClO and O3 certainly anti-correlated. But how is there so much ClO? What is the mechanism for ClO to destroy ozone so fast? Can’t be ClO + O…

  10. QUESTIONS 1. Where in the stratosphere (altitude) would the ClO dimer mechanism be most important? 2. If CFC concentrations were to double, how much faster would ozoneloss by the ClO dimer mechanism occur?

  11. Polar Stratospheric Clouds (PSC’s)

  12. PSC Formation Starts on S.A.L Stratosphere Stratospheric Aerosol Layer OCS SO2 H2SO4 15-30 km volcanoes Troposphere oceans • Extends vertically from 15 to 35 km • Aerosol composition primarily H2SO4-H2O • Typical particle radius ~ 700 nm • Typical particle number density ~ 10 cm-3

  13. Sunrise over Texas From Space Shuttle Stratospheric Aerosol Layer Tropospheric Clouds

  14. PSC Formation and Types From stratospheric aerosol layer H2SO4-H2O HNO3 Crystalline HNO3-3H2O (NAT) containing particles HNO3/H2O/ H2SO4 Type Ia PSC 197 K T Supercooled Ternary Solutions (STS) H2SO4/HNO3/H2O Type Ib PSC H2O 185 K Mainly H2O- ice particles Type II PSC

  15. PSC Formation Requires Very Low T Arctic T Color shows westerly wind speed Antarctic T Polar Vortex cuts off warm, ozone/NOx rich mid-latitude air

  16. Heterogeneous Chemistry on PSCs PSCs convert inactive Cly to active ClOx Turco, 1987

  17. Chlorine Activation in Polar Vortex

  18. Antarctic O3 Hole A Springtime Event Ozone Watch Web Page 2006 Ozone Hole Movie From Satellite Observations

  19. Antarctic Ozone Hole: Key Points • Why only in spring? • Why only between 15 – 25 km? • Why only in (south) polar region? Wintertime processing on PSCs needed for active chlorine production and denitrification. Sunlight required to generate Cl atoms from Cl2 and ClOOCl. Where PSCs form (on aerosol layer), and where the ClOOCl mechanism is fastest. PSCs are required and only form under cold conditions achieved during polar winter (and mainly only Antarctic winter).

  20. The Solution: Montreal Protocol The persistent observations of the ozone hole from 1984 – 1987 led to legally binding international agreements. The Montreal Protocol and its amendments eventually called for a near complete ban on the production and use of CFCs. A suitable and easy replacement for CFCs, known as HCFCs, made these acts easier to swallow.

  21. Ozone Recovery • Computer model predictions: • Antarctic ozone will recover to pre-1980 values by ~2040. • Extra-polar ozone should recover by 2020-2040. • Predictions assume strict adherence to Montreal Protocol.

  22. Questions • Large volcanic eruptions can inject sulfur and particles into the stratosphere. How would that affect stratospheric ozone at present? • How might eruptions in the distant past affect stratospheric ozone? • 3. Increasing greenhouse gases in the troposphere lead to a cooling of the stratosphere. How might global warming affect stratospheric ozone levels?

  23. Effect of N2O5 + Aerosols 2HNO3 Only gas-phase chemistry Including Heterogeneous Chemistry HOx becomes most important catalyst in aerosol layer!

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