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

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

Penetration of UV radiation

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).

Oxygen Photolysis (2)

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

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.

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 1931-34 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

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.

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)

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

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+O2 (peroxy radical formation - a minor channel) HO2+O3  OH+2*O2 HO2+HO2  H2O2+O2 HO2+OH  H2O+O2

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)

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

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!

The NOx cycle from Chatfield, 1994

The NOx cycle Nighttime NOx losses NO2+O3  NO3+O2 (nighttime reaction) NO2+NO3  N2O5 (nighttime reaction) NO3+h  NO2+O or NO+O2 (daytime reaction) PAN (an important reservoir for NOx) RCHO+OH  CH3COO2+... (aldehyde oxidation  peroxy acetyl radical) NO2+CH3COO2  CH3CONO2+O2 (PAN formation) PAN  NO2+CH3COO2 (thermal decomposition) PAN+h  products Terminal loss of NOx occurs through deposition of HNO3, aldehydes, peroxides, …

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

The NOx-HOx connection from Logan, 1981

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

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.

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.