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

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• 作者：G. Krysztofiak ; V. Catoire ; G. Poulet ; V. Mar¨¦cal ; M. Pirre ; F. Louis ; S. Canneaux ; B. Josse
• 关键词：Modeling ; ab initio Calculation ; Bromine ; Henry&rsquo ; s law constants ; Chemical degradation mechanism ; Very short-lived species ; CHBr3 ; CH2Br2 ; Stratosphere ; Tropical tropopause layer
• 刊名：Atmospheric Environment
• 出版年：2012
• 出版时间：November, 2012
• 年：2012
• 卷：59
• 期：Complete
• 页码：514-532
• 全文大小：1452 K

文摘

Detailed chemical reaction schemes for the atmospheric degradations of the very short-lived species (VSLS) bromoform (CHBr₃) and dibromomethane (CH₂Br₂) 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 CH₄, CO, HO_x, NO_x, NO_y and O_x. They include the main possible reactions of the intermediate brominated peroxy radicals RO₂ (with R?=?CH₂Br, CHBr₂ and CBr₃) for which the most likely reaction pathways with HO₂ 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; ) or the Molecular Connectivity Index (MCI; ). Using these constants, the least soluble species formed from the VSLS degradation are found to be CBr₂O, CHBrO, CBr₃O₂NO₂, CHBr₂O₂NO₂, BrO, BrONO₂ 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 CHBr₃ and from 8.8 % to 20.2 % for CH₂Br₂). The influence of the reactions of the RO₂ radicals with HO₂, CH₃O₂ and NO₂ on the nature and abundance of the stable intermediate and end-products has been tested for CHBr₃ degradation. As a result, the reactions of the RO₂ radicals with NO₂ have no impact. Taking into account the reaction between RO₂ and CH₃O₂ and modifying the branching ratios of the reaction between RO₂ and HO₂ 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 CHBr₃, CH₂Br₂ 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 CHBr₃ and CH₂Br₂ are proposed.