HAMLET: Halogens in the marine environment - Laboratory investigations of heterogeneous chemistry

• Unité de recherche: PCRD EU
• Numéro de projet: 97.0009
• Titre du projet anglais: HAMLET: Halogens in the marine environment - Laboratory investigations of heterogeneous chemistry

Mots-clé (Anglais)

Atmospheric chemistry; heterogeneous kinetics and mechanism; sea salt; aerosols

Autre Numéro de projet (Anglais)

EU project number: ENV4-CT97-0394

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)

Max Planck Institut, Mainz (D); Fraunhofer Institut, Hannover (D); Université Louis Pasteur, Strausbourg (F); Université des Sciences et Technologies, Lille (F)

Résumé des résultats (Abstract) (Anglais)

Heterogeneous chemical reactions occurring on surfaces of atmospheric particulates have been shown to be of prime importance in a number of cases. The field of homogeneous gas phase reactions has benefitted from uninterrupted research interest since its beginnings, whereas its corresponding heterogeneous counterpart has been recognized to be of importance only recently. Therefore, there is a serious shortage of information regarding the nature of interfacial chemical reactions of atmospheric interest. We have studied several heterogeneous reactions of atmospheric interest in the laboratory using a low pressure flow reactor in order to avoid secondary reactions to the greatest extent possible. The subject of the present project is the laboratory study of several chemical reactions of atmospheric gases thought to occur on marine aerosols. The work has allowed an improved understanding in terms of reaction rates or uptake coefficients of atmospheric trace gases, reaction products as well as branching ratios and the influence of the substrate on the reactivity of the trace gases. Some reactions have been studied for the first time using alkali halide salts as a substrate.
BrONO2 has a high uptake coefficient on solid NaCl and KBr on the order of 0.3±0.03. The quantity of adsorbed water plays a crucial role in the rate of appearance of the products, but does not affect the initial uptake coefficient. There is competition between the ionic exchange reaction, self reaction, decomposition and hydrolysis. Even though the BrONO2 uptake is complex it follows an apparent first order rate law. Chemical kinetic modeling using the above fundamental reactions results in good agreement with experiment.
The heterogeneous reactions of Cl2O and Br2O on solid NaCl are observable in the Knudsen flow reactor in contrast to the HOCl interaction according to the series Br2O>Cl2O. Pure HOCl does not react on solid KBr as well as on NaCl. Cl2O seems to be a reactive intermediate in the HOCl/KBr reaction which kicks off a halogen activation reaction for bromide based on the reactivity of BrCl with solid KBr. The reaction mechanism includes the autocatalytic behavior of HOCl uptake on KBr at high partial pressure whereas it shows normal saturation behavior at low partial pressures of HOCl. Adsorbed water plays a critical role in the reaction mechanism as well as in the regeneration of the solid substrates. A striking example of the surface reaction of adsorbed H2O with Br2 resulting in adsorbed HBr and HOBr, which is a critical reaction step in the HOCl/KBr system is the recrystallization of a thin amorphous film of KBr to polycrystalline KBr. Although there is no net reaction the change in morphology of the KBr substrate is quite obvious. The new reaction product BrOCl has been observed using MS in the reaction of Cl2O + KBr which is the initiation reaction in the HOCl/KBr system. These complex reactions in general show the increased reactivity of bromides in relation to chlorides.

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: 97.0009