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METO 637

METO 637. Lesson 6. The Stratosphere. We will now consider the chemistry of the troposphere and stratosphere. There are two reasons why we can separate these regions

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METO 637

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  1. METO 637 Lesson 6

  2. The Stratosphere • We will now consider the chemistry of the troposphere and stratosphere. There are two reasons why we can separate these regions • (1) The stratosphere absorbs most of the shortwave radiation from the sun, hence the stratosphere has high energy photons to induce photochemistry. The troposphere must make do with lower energy photons. • (2) The temperature decreases with altitude in the troposphere, implying a basically unstable atmosphere with considerable vertical mixing. In the stratosphere the temperature increases with altitude. This implies a stable atmosphere with little vertical mixing. • Substances injected into the stratosphere take a long time to reach the troposphere, and can build up to significant levels

  3. The Stratosphere

  4. Ozone Observations • Ozone is found in trace amounts in the atmosphere with the largest concentration in a well defined layer between 15 and 30 km. • First discovered by Fafry and by Dobson, although they both misplaced the altitude of the peak. • Ozone layer is highly variable, and average column densities vary with latitude and season. • The unit of column ozone (also known as total ozone) is the Dobson Unit (DU). The units of DU are milli-atmospheres.cm

  5. Total Ozone Field March 11, 1990 Nimbus 7 TOMS (Hudson et al., 2003)

  6. Ozone profiles within the three regimes

  7. Observed Ozone Abundance

  8. Calculated Ozone Abundance

  9. Ozone-only chemistry • First approach to the theoretical explanation of the ozone layer was by Chapman, 1930, who proposed a static pure oxygen photochemical stratosphere. • The reactions were Dodd-oxygen O2 + hn → O + O +2 1 O + O2 + M → O3 + M 0 2 O3 + hn→ O + O2 0 3 O + O3 → O2 + O2 -2 4 [O + O + M → O2 + M -2 5] • Reaction 5 can be ignored in the stratosphere. Reactions 1 and 3 give excited atoms, but these are quickly quenched to the ground state. No excited state chemistry is assumed.

  10. Ozone-only chemistry • Reactions 2 and 3 interconvert O and O3 fairly rapidly. • Even at the top of the atmosphere reaction 2 has a lifetime of 100 seconds. Reaction 3 has a similar photochemical lifetime. • Hence we talk about ‘odd oxygen’ [O+O3] • Reactions 2 and 3 ‘do nothing’ as far as odd oxygen is concerned. • Odd oxygen is produces in reaction1, and destroyed in reaction 4.

  11. Ozone-only chemistry Altitude, km

  12. Chapman Layers

  13. Chapman Layers

  14. Chapman Layers

  15. Ozone-only chemistry • Let the rate of production of odd-oxygen for reaction 1 be P1, and that for reaction 3 be P3. • In steady state the amount of odd-oxygen produced in reaction 1 must equal the number destroyed in reaction 4. • Now consider equations 2 and 3

  16. Ozone-only chemistry

  17. Comparison of experiment and theory • Given the oxygen only chemistry, the concentrations and production rates for ozone ought to be proportional to P11/2 • The data do not show this. • Ozone is being produced 5 times faster then predicted, which means that the odd oxygen destruction must be five times faster also.

  18. Ozone-only chemistry Zonally averaged ozone concentration vs altitude Zonally averaged rate of ozone formation from O2 photolysis

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