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 (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

The Stratosphere

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

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

Ozone profiles within the three regimes

Observed Ozone Abundance

Calculated Ozone Abundance

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  odd-oxygen O 2 + h  → O + O +21 O + O 2 + M → O 3 + M 02 O 3 + h → O + O 2 03 O + O 3 → O 2 + O [O + O + M → O 2 + M -25] 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.

Ozone-only chemistry Reactions 2 and 3 interconvert O and O 3 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+O 3 ] Reactions 2 and 3 ‘do nothing’ as far as odd oxygen is concerned. Odd oxygen is produces in reaction1, and destroyed in reaction 4.

Ozone-only chemistry Altitude, km

Chapman Layers

Ozone-only chemistry Let the rate of production of odd-oxygen for reaction 1 be P 1, and that for reaction 3 be P 3. 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

Ozone-only chemistry

Comparison of experiment and theory Given the oxygen only chemistry, the concentrations and production rates for ozone ought to be proportional to P 1 1/2 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.

Ozone-only chemistry Zonally averaged ozone concentration vs altitude Zonally averaged rate of ozone formation from O 2 photolysis