COBRA: Coupled bromine cheimstry affecting stratospheric ozone amounts

• Unité de recherche: PCRD EU
• Numéro de projet: 97.0008
• Titre du projet anglais: COBRA: Coupled bromine cheimstry affecting stratospheric ozone amounts

Mots-clé (Anglais)

Atmospheric chemistry; heterogeneous kinetics; Cirrus clouds; ice chemistry

Autre Numéro de projet (Anglais)

EU project number: ENV4-CT97-0576

Programme de recherche (Anglais)

EU-programme: 4. Frame Research Programme - 3.1 Environment

Description succincte (Anglais)

See abstract

Autres indications (Anglais)

Full name of research-institution/enterprise:
EPF Lausanne
Département de Génie Rural (DRG)
Laboratoire de pollution atmosphérique et sol (LPAS)

Partenaires et organisations internationales (Anglais)

University of Cambridge (UK) Laboratoire de Combustion et Systèmes Réactifs, Orléans (F), University of Oxford (UK), Max-Planck Institut für Chemie, Mainz (D)

Résumé des résultats (Abstract) (Anglais)

The subject of this work is the study of the reactivity of trace gases on ice as a proxy for ice particles formed in aviation contrails as well as for naturally occurring Cirrus clouds encountered in the upper troposphere. In addition, basic information on the nature of the ice interface in relation to effective bimolecular reactions taking place on the ice has also been sought in order to obtain basic understanding of the complex interplay between interfacial reactions and diffusion processes. The main conclusions resulting from this work follow:
o The uptake coefficients (g) of HCl and HBr on pure H2O-ice are independent of the type of ice used and have a negative temperature dependence given by Ea=-1.7 and -3.9 kcal/Mol in the range 190-210K and 210-225K, respectively. The HCl-Ice interaction can be described using a chemical kinetic model using two precursors. The uptake reaction of HCl never saturates because gas phase HCl may adsorb onto already adsorbed HCl. The thickness of the HCl/ice interface is on the order of 100, 200 and 1000 nm for single crystal (SC), condensed (C) and bulk (B) ice, respectively. This has consequences for small ice particles as far as the availability of HCl for heterogeneous reaction is concerned. The HCl diffusion coefficient D at 200K for diffusion from the interface region into the bulk is (1.2±0.5)x10-14, (5.6±1.0)x10-13 and (2.1±1.0)x10-12 cm2s-1 for SC-, C- and B-type ice, respectively.
o g for HOBr on water-ice is independent of the type of ice used as well as of previous HNO3 contamination. Ea = -9.7 kcal/mol in the range 175-205K. Ice supports a HOBr vapor pressure whose enthalpy of vaporization is 9.4±1.0 kcal/mol.
g for reaction of HOBr with HCl and HBr is constant at g = 0.35 in the range 180 to 195K and decreases with T at T>195K. BrCl and Br2 are the reaction products.
o The uptake coefficient of HNO3 on ice is independent of the type of ice and has a negative T-dependence: Ea = -6.8±1.0 kcal/Mol in the range 195-211K. g for HNO3 uptake on solid frozen H2SO4/H2O is constant for 10-60 wt% and decreases for higher concentrations, both at 180 and 200K. g for HNO3 uptake on saturated ternary solutions of H2SO4/HNO3/H2O is 0.10±0.03 in the range 185-195K.
o g for BrONO2 uptake on ice is fast in the range 180-200K and slightly depends on the type of ice in the range 170-210K. g for BrONO2 uptake on HNO3-contaminated ice (B-type) is similar to C-type pure ice in the range 180 to 210K. g for BrONO2 uptake on HBr-doped ice is fast. Br2 and HOBr have been observed as products. g for BrONO2 uptake on solid H2SO4/H2O mixtures is independent in the concentration range 50-90wt% and decreases with increasing temperature: 0.20, 0.12 and 0.07 for 180, 200 and 210K. HOBr is the major reaction product.
o g for Br2O uptake on ice decreases from 0.33±0.09 at 180K to 0.11±0.08 at 210K. HOBr has been observed as the sole product in the range 180 to 210K.

Références bases de données (Anglais)

Swiss Database: Euro-DB of the
State Secretariat for Education and Research
Hallwylstrasse 4
CH-3003 Berne, Switzerland
Tel. [phone]
Swiss Project-Number: 97.0008