Week 12: Stratospheric Chemistry I

 

Friday, 14th and Monday, 17th November, 2006, lecture I (pdf), lecture II (pdf)

Overhead sources are
Chemistry of the Natural Atmospheres, Peter Warneck, 2nd Ed., Academic Press, 1999

Atmospheric Chemistry and Global Change, Brasseur, Orlando, Tyndall, Eds., Oxford University Press, 1999

  Points/Topics to remember from this week’s classes:

 

  • More than 90 percent of ozone in our atmosphere is concentrated in the stratosphere. The total column ozone is measured in Dobson Units (1 DU = 10-5 m ozone layer thickness brought to STP; 100 DU = 1 mm). Ozone in the stratosphere is formed as a result of oxygen photolysis at wavelengths smaller than 240 nm. The Chapman mechanism
    1. O2 + hν  →  O + O
    2. O + O2 + M  →  O3 + M
    3. O + O + M  →  O2 + M         (very small contribution)
    4. O3 + hν  →  O(1D) + O2
    5. O3 + O  →  2 O2

… was the first to explain the ozone layer in the stratosphere consistently. Ozone is produced in reactions 1&2, and lost in reaction 5. It is also lost as a result of reaction 4 when the oxygen atom does not recombine with molecular oxygen as in #2. Model calculations incorporating only reactions 1-5 correctly predict a layer of maximum ozone between 20 and 30 km elevation, and the Chapman mechanism was deemed sufficient to explain the observed ozone layer until the early 1960s. However, after more precise measurements of overhead ozone were available, by the end of the 60s it was clear that this mechanism alone results in too much ozone in the stratosphere. Other loss mechanisms had to be invoked.

  • Because the amount of radiation plays such an important factor in stratospheric ozone formation, the highest concentration (density) of ozone is found in the tropics. Due to atmospheric dynamics and the resulting atmospheric structure with a deeper troposphere in the tropics and lower tropopause levels at higher latitudes, a higher column density (thickness) of the ozone layer is found in mid- and high latitudes. Higher mixing ratios in the tropical stratosphere result in a meridional gradient and transport of ozone towards the poles from its major production areas in the tropics. Hence, the amount of UV-B radiation reaching the surface is higher in the tropics and you are more likely to get a sunburn in the tropics than at mid-latitudes, even for the same zenith angles!
  • The sum of ozone and atomic oxygen is called odd oxygen, Ox (≡ O3 + O). While ozone is often in steady state in the stratosphere compared to mixing/transport, odd oxygen is not, especially towards the lower stratosphere. This is partially due to reactions of atomic oxygen other than reaction 5, namely conversion to HOx via H-atom abstraction from several stratospheric molecules, such as CH4, H2, HCHO, or H2O. Further H-atom abstraction by the OH radicals formed, represents a significant, additional chemical source of water to the stratosphere as compared to transport from the troposphere.
  • It was Paul J. Crutzen, then working as a graduate student for Bert Bolin in Sweden, who first realized in the late 60s that ozone in the stratosphere is consumed not just by reaction 5 but also by NOx, a work for which he was later to receive the Nobel Price for Chemistry in 1995. Crutzen showed that NOx, formed from N2O reaction with O(1D) in the lower stratosphere consumes ozone as follows:

6.    N2O + O(1D)  →  2 NO  (N2 + O2) (note that the majority of N2O is lost via N2O + hν  →  N2 + O)

7.    NO + O3  →  NO2 + O2

8.    NO2 + O  →  NO + O2

Series 7+8 creates the net reaction O3 + O → 2 O2 with NOx as the catalyst. It turns out that NOx catalyzed ozone loss in the stratosphere is responsible for the majority of all loss reactions, in particular at maximum ozone levels between 25 and 35 km.

·       At the onset of night, first all NO is converted to NO2 by reaction 7, then slowly further to NO3 by further reactions with ozone. Once a significant amount of NO3 has been formed, it combines with NO2 to N2O5. If N2O5 further reacts with water on the surface of particles, NOx is “lost” from the stratosphere in the form of HNO3. The reaction of N2O5 with H2O is very slow in the gas phase and little water is available in the stratosphere. As a result NOx is only slowly oxidized to HNO3 besides the equilibrium that exists via reaction with OH to form HNO3, and HNO3 photolysis. Both gas phase N2O5 and HNO3 photolyze in the stratosphere under UV-C wavelengths during the day (write out all these reactions for practice!), reforming NOx. HNO3 shows a poleward increase due to its own decreasing photolysis rate, NO2 does so too in summer but not in winter due to its inefficient recycling from HNO3 at higher latitudes. - The distribution and cycling of NOy in the stratosphere is an active field of research.

·       Other important ozone loss reactions, many of which also pioneered/pointed out by Paul Crutzen, include reactions with HOx radicals

9.            ·OH + O3  →  HO2· + O2

10.                       HO2· + O3  →  ·OH + 2 O2

…, the net reaction of which is 2 O3 → 3 O2. Series 9&10 occurs dominantly at the bottom and at the top of the ozone layer higher in the stratosphere (~35 km).

HOx amount and distribution throughout the stratosphere is likewise an active field of atmospheric chemistry research.

  • Halogen species (TBC)