Detailed modeling of the atmospheric degradation mechanism of very-short lived brominated species

Abstract

Detailed chemical reaction schemes for the atmospheric degradations of the very short-lived species (VSLS) bromoform (CHBr 3) and dibromomethane (CH 2Br 2) have been established. These degradation schemes have been implemented in the meteorological/tracer transport model CATT-BRAMS used in the present case as pseudo one-dimensional model with chemistry of CH 4, CO, HO x, NO x, NO y and O x. They include the main possible reactions of the intermediate brominated peroxy radicals RO 2 (with R = CH 2Br, CHBr 2 and CBr 3) for which the most likely reaction pathways with HO 2 have been found using ab initio computational calculations. The full degradation schemes have been run for two well-defined realistic scenarios, " clean" atmosphere and " moderately" NO y-polluted atmosphere, as representative of a tropical coastal region where these VSLS natural emissions are expected to be important. The Henry's law constants of the brominated organics products have been estimated by using the Bond Contribution Method (BCM; Meylan and Howard, 1991) or the Molecular Connectivity Index (MCI; Nirmalakhandan and Speece, 1988). Using these constants, the least soluble species formed from the VSLS degradation are found to be CBr 2O, CHBrO, CBr 3O 2NO 2, CHBr 2O 2NO 2, BrO, BrONO 2 and HOBr, which leads those to be potentially transported into the tropical tropopause layer (TTL) in case of deep convection and contribute to stratospheric bromine additionally to the original substances. For bromoform and dibromomethane degradation, the moderate NO y pollution increases the production of the least soluble species and thus approximately doubles the bromine quantity potentially able to reach the TTL (from 22.5% to 43% for CHBr 3 and from 8.8% to 20.2% for CH 2Br 2). The influence of the reactions of the RO 2 radicals with HO 2, CH 3O 2 and NO 2 on the nature and abundance of the stable intermediate and end-products has been tested for CHBr 3 degradation. As a result, the reactions of the RO 2 radicals with NO 2 have no impact. Taking into account the reaction between RO 2 and CH 3O 2 and modifying the branching ratios of the reaction between RO 2 and HO 2 lead to a small impact on the bromoform degradation by slightly decreasing (by 10%) the bromine quantity potentially able to reach the TTL. As a final point, in contrast to CHBr 3, CH 2Br 2 degradation produces negligible quantities of organics species and the effects of pollution increase only the inorganic species production. By taking into account the results of these tests, new simplified degradation schemes for CHBr 3 and CH 2Br 2 are proposed. © 2012 Elsevier Ltd.