Modelling of Ozone Chemistry in the Arctic Stratosphere
Background
Ozone depletion in the polar spring (to a large extent over Antarctica, but also over the Arctic) is initiated by heterogeneous reactions on Polar Stratospheric Clouds (PSCs) that form at low temperatures. These reactions convert chlorine-containing reservoir gases (HCl, ClONO2) into Cl2. In sunlight Cl2 is decomposed into 2 Cl atoms, which start a catalytic ozone destruction cycle:
Cl + O3 → ClO + O2 (2x)
ClO + ClO + M → Cl2O2 + M
Cl2O2 + hν → ClOO + Cl
ClOO + M → Cl + O2 + M
net: 2 O3 → 3 O2
The processes terminating this cycle also depend on sunlight (but proceed slowly): e.g. HNO3 releases NO2, which removes ClO from the cycle above and re-converts it into a reservoir gas:
ClO + NO2 + M → ClONO2 + M
Starting Point
Measurements of
- Arctic stratospheric ozone destruction (Match)
- related trace gases (FTIR) Questions
How can we explain the observations?
Which chemical reactions are important?
Tool
Chemical box model and one-dimensional chemical model ("column of boxes"):
- approximately 150 chemical reactions between 50 species
- based on a module by G. Brasseur (NCAR, USA)
Selected Results
- In the Match data of 1995/96 a decline in the ozone destruction rate is seen in February, although PSCs are still existent. An explanation for that (found with the help of the chemical model) is: when chlorine "leaves" the catalytic ozone destruction cycle, most of it is first converted into ClONO2 (cf. above), only small quantities react directly to HCl. That is why the chlorine gets "trapped" in ClONO2, and the reaction partner HCl for the activation reaction
HCl + ClONO → Cl2 + HNO3
is "missing".
- An increase of ClONO2 in the Arctic spring can also be seen in the column densities measured by the FTIR and the corresponding model results.