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Content archived on 2024-04-19

Chlorine and bromine reservoirs in the stratosphere: laboratory studies of homogeneous and heterogeneous reactions


The double aim of CABRIS is to obtain detailed kinetic and mechanistic information, from laboratory studies, on the formation and on the transformation of chlorine and bromine reservoirs in the stratosphere. The reactions involving these reservoirs will be studied in the gas phase and/or on PSC type surfaces at low temperatures relevant to the polar stratosphere. The data obtained will aid both in the interpretation of the observations of chlorine and bromine species (ClO, BrO, OClO...) made, for example, during the SESAME campaign, and in the improvement of the predictive capability of models of stratospheric ozone depletion.

Three different and complementary topics will be investigated with CABRIS using different experimenetal approaches. Kinetic, mechanistic and spectroscopic studies will concern these three topics:

1. Formation of halogen reservoirs from the reactions of ClO and BrO with HO2 and CH3O2

The interactions of halogen oxides radicals (ClO, BrO) and peroxyradicals (HO2 and RO2) lead to the formation of halogen species which can be efficient reservoirs and/or which can react on PSC-type surfaces. Kinetic and mechanistic studies of the BrO + HO2 reaction at low temperature will be performed. The adduct, if it exists, will be identified. A similar study of the ClO + HO2 will be also undertaken. More generally, adducts similar to the ClO dimer, which may be formed in the reactions of BrO, ClO and HO2 under stratospheric conditions will be searched for and their spectroscopic properties will be determined over a wide temperature range.

2. Homogeneous and heterogeneous formation of higher halogen oxides

Although the BrO+ClO reaction is considered to be the unique source of OClO in the polar stratosphere, recent laboratory studies have suggested tha OClO could also be produced in a bromine-free system in the presence of PSC type surfaces. A quantitative characterization of this possible heterogeneous OClO formation will be carried out. The kinetics of Cl2O3 formation will also be studied in detail. ClO3, which is considered to be a key species in the chemical formation of these higher chlorine oxides, will be searched for. Concerning the higher oxides of bromine, special attention will be directed towards the possible formation of the BrO dimer.

3. Heterogeneous reactions of halogen reservoirs

If the heterogeneous processes involving chlorine reservoirs have been the object of many recent laboratory studies, it is not the case for bromine. Uptake coefficients and reactions of BrONO2, HOBr and HBr on PSC type surfaces will be investigated at low temperature. The possible heterogeneous formation of CH3OCl and CH3 OBr will be elucidated at low temperature. The study of CH3OCI uptake and of its heterogeneous reaction with HCl is also planned, since this molecule may behave similarly to HOCl on cold surfaces.


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