1. Photochemistry of Ozone
Stratospheric ozone formation and loss
Tropospheric ozone formation and loss
Some aspects of tropospheric ozone chemistry
Reactive NOx chemistry

2. Stratospheric ozone formation and loss
Oxygen Photolysis
The Chapman Cycle
Catalytic Ozone Destruction

3. Penetration of UV radiation

4. Oxygen Photolysis
Energy diagram and absorption cross section of molecular oxygen
Photodissociation of oxygen occurs only in the stratosphere and above. The produced oxygen atoms are partly in excited state O(1D).

5. Oxygen Photolysis (2)

6. The Chapman Cycle (1) O2 + h O + O (2) O + O2 + M  O3 + M
The formation rate of O3 is thus
theory
observed

7. Catalytic ozone destruction
(5) X + O3  XO + O2
(6) XO + O  X + O2
net O3 + O  O2 + O2
X can be H, OH, NO, Cl, or Br. (6) is usually the rate-limiting step.
Ozone hole: chlorine and bromine „activation“ on polar stratospheric clouds, then catalytic destruction as above.

8. Brief history of stratospheric ozone
1881 Hartley identifies ozone as main cause for cutoff of solar spectrum at 300 nm
1921 Fabry and Buisson obtain first reliable measurement of overhead column ozone
1918 Strutt measured tropospheric „column“ with „40 ppb or less“  bulk of ozone in stratosphere
1926 Dobson and Harrison measure latitudinal distribution of total ozone
1930 Chapman theory; Schumacher measured rate coefficients
Götz identified an ozone layer and located maximum near 22 km
1960 McGrath and Norris discover OH production and propose catalytic ozone destruction cycle
1971 Crutzen and Johnston discover NOx cycle
1974 Molina and Rowland recognize impact of man-made chlorofluoromethanes
1985 Farman discovers Antarctic ozone hole
1987 Montreal protocol
between 1934 and the 1970s, poeple were convinced that the Chapman theory is correct; the obvious disagreement of the latitudinal distribution and the seasonal cycle were thought to be uncertainties in transport

9. Tropospheric Ozone Chemistry
stratosphere
troposphere OH hn
H2O
HCHO
Acetone
PAN
Isoprene, Terpenes
CH3OOH
H2O2
Alcohols
Br, Cl, I
Sulphur
Ammonia H2 O3 NO
NO2
deposition
HNO3
HO2
RO2 CO
VOC
Tropospheric ozone chemistry is initiated by the photodissociation of ozone entering the troposphere from the stratosphere. This process leads to formation of hydroxyl radicals (OH), which are the main oxidising agent for many trace gases. In the oxidation process, peroxy radicals are formed, which interact with the fast photochemical cycle converting NO into NO2 and back. It is through this cycle, that ozone can be formed, but net ozone production only takes place in the presence of peroxy radicals.
While this simple picture already explains a large part of the tropospheric ozone abundance, there are many more species and reactions interfering with the system, and the thorough treatment of these processes requires advanced numerical models.

10. Stratospheric ozone intrusions
The tropospheric ozone „paradox“:
Ozone photochemistry in the troposphere can only proceed with help of ozone transported from the stratosphere.
DJF
Climatology of „deep“ STE events (Sprenger & Wernli, 2002)

11. Ozone photolysis O3+h  O(3P)+O2, l<800 nm
O(1D) quantum yield
O3+h  O(3P)+O2, l<800 nm
O3+h  O(1D)+O2, l<320 nm

12. The NOx-free atmosphere
1. OH formation (ozone  HOx conversion)
O3+h  O(1D)+O2 (majority yields O(3P), which does not react with H2O!)
O(1D)+H2O  2*OH (a large fraction is quenched by collision with N2 or O2: O(1D)+M  O(3P)+M)
2. HOx (and ozone) loss
OH+OH  H2O2 or H2O+O
OH+O3  HO2+O (peroxy radical formation - a minor channel)
HO2+O3  OH+2*O2
HO2+HO2  H2O2+O2
HO2+OH  H2O+O2

13. CO and hydrocarbon oxidation
3. CO oxidation
OH+CO+O2  HO2+CO2
4. Methane oxidation
OH+CH4+O2  CH3O2+H2O (the methyl peroxy radical is born)
CH3O2+HO2  CH3O2H+O2
CH3O2+CH3O2  (e.g. methanol: CH3OH)
5. HOx regeneration
H2O2+h  2*OH (also reaction with OH possible, i.e. HOx loss)
CH3O2H+h +O2  OH+HO2+HCHO (formaldehyde)

14. CO and hydrocarbon oxidation (2)
5. HOx regeneration (continued)
HCHO+h  H2+CO (ca. 60%)
HCHO +h+O2  2*HO2+CO (ca. 40%)
HCHO+OH+O2  HO2+CO+H2O
6. Simplified NMHC scheme
OH+RH+O2  RO2+H2O (R=C2H5, C3H7, ...)
RO2+HO2 or RO2+CH3O2 or RO2+RO2  peroxide
peroxide+h+O2  HOx+aldehyde
aldehyde+h+O2  HOx and RO2
aldehyde+OH  other stuff

15. The crucial role of NOx 7. The catalytic ozone formation cycle
NO+O3  NO2+O2
NO2+h+O2  NO+O3
NO+HO2  NO2+OH (this is the key reaction!)
NO+CH3O2 NO2+CH3O (CH3O immediately reacts with O2 to form HO2+HCHO)
NO+RO2  NO2+RO
8. The end of the story
OH+NO2  HNO3
Note: in the stratosphere catalytic ozone destruction, in the troposphere catalytic ozone formation!

16. The NOx cycle
from Chatfield, 1994

17. The NOx cycle Nighttime NOx losses
NO2+O3  NO3+O (nighttime reaction)
NO2+NO3  N2O (nighttime reaction)
NO3+h  NO2+O or NO+O (daytime reaction)
PAN (an important reservoir for NOx)
RCHO+OH  CH3COO (aldehyde oxidation  peroxy acetyl radical)
NO2+CH3COO2  CH3CONO2+O2 (PAN formation)
PAN  NO2+CH3COO (thermal decomposition)
PAN+h  products
Terminal loss of NOx occurs through deposition of HNO3, aldehydes, peroxides, …

18. Heterogeneous loss of nitrogen oxides
1000 hPa
Zonal and monthly mean ratio
of NOx without and with the heterogeneous reaction of N2O5 on (ammonium sulfate) aerosol
500 hPa
from Dentener and Crutzen, 1993

19. The NOx-HOx connection
from Logan, 1981

20. Summary: The ingredients
In order to form ozone in the troposphere, we need:
ozone itself (no ozone  no OH radical) source: stratosphere
CO and hydrocarbons source: anthropogenic and natural emissions
NOx source: anthropogenic and natural emissions, lightning

21. Bibliography A few key papers (incomplete listing):
Haagen-Smit, A.J., Chemistry and physiology of Los Angeles Smog, Industrial and Engineering Chemistry, 44(6), 1952.
Chameides, W., and Walker, J.C.G., A photochemical theory of tropospheric ozone, J. Geophys. Res., 78(36), 1973.
Chatfield, R.B., Anomalous HNO3/NOx ratio of rmote tropospheric air, Geophys. Res. Lett., 21(24), 1994.
Crutzen, P.J., The role of NO and NO2 in the chemistry of the troposphere and stratosphere, Ann. Rev. Earth Planet. Sci., 1979.
Dentener, F.J., and Crutzen, P.J., Reaction of N2O5 on tropospheric aerosols, J. Geophys. Res., 98(D4), 1993.
Fishman, J., and Crutzen, P.J., The origin of ozone in the troposphere, Nature, 274(31), 1978.
Lin, X., Trainer, M., and Liu, S.C., On the nonlinearity of the tropospheric ozone production, J. Geophys. Res., 93(D12), 1988.

22. Bibliography (2)
Logan, J.A., Prather, M.J., Wofsy, S.C., and McElroy, M.B., Tropospheric chemistry: A global perspective, J. Geophys. Res., 86(C8), 1981.
Logan, J.A., Nitrogen oxides in the troposphere: Global and regional budgets, J. Geophys. Res., 88(C15), 1983.
Prather, M.J., and Jacob, D.J., A persistent imbalance in HOx and NOx photochemistry of the upper troposphere driven by deep tropical convection, Geophys. Res. Lett., 24(24), 1997.
See also the books from Seinfeld&Pandis, Warneck, and Finnalyson- Pitts&Pitts as well as:
Brasseur, G.P:, Orlando, J.J., and Tyndall, G.S. (eds.), Atmospheric Chemistry and Global Change, Oxford University Press, New York, Oxford, 1999.