CUT-ICE: Chemistry of the upper troposphere : laboratory studies of heterogeneous processes on ice

• Unité de recherche: PCRD EU
• Numéro de projet: 99.0491-1
• Titre du projet anglais: CUT-ICE: Chemistry of the upper troposphere : laboratory studies of heterogeneous processes on ice

Mots-clé (Anglais)

Heterogeneous chemistry; ice; HNO3; HCl; HBr; Br2O; BrONO2; diffusion coefficients; interfacial processes

Autre Numéro de projet (Anglais)

EU project number: EVK2-1999-00005

Programme de recherche (Anglais)

EU-programme: 5. Frame Research Programme - 1.4a.2 Global change, climate and biodiversity

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)

Coordinator: Max Planck Institut für Chemie (D)

Résumé des résultats (Abstract) (Anglais)

We have generated ice in the laboratory using different techniques at temperatures of 180 to 210K by freezing liquid samples ('Bulk' (B) ices), by vapor phase condensation under molecular flow conditions (condensed (C) ices) and single crystal (SC) ice. The goal of this project is to study both Ice physicochemical parameters and its reactivity in the presence of atmospherically relevant molecules under conditions approaching those of the upper troposphere. The following main results have been obtained in the contracting period:
- Measurement of the diffusion coefficient DHCl and DHBr of HCl and HBr in different types of ice (B-, C- and SC-type ice) in the range 190 to 205K. The measurements have been performed in a low pressure flow reactor using the 'Dope and Probe' technique wherein a known dose of HCl or HBr has been applied to the ice ('dope' pulse) and probed at different time delays using the titration reaction ClONO2 + HCl => Cl2 + HNO3. The resulting diffusion coefficient DHCl is a strong function of the type of ice and lies in the range 5x10-13 cm2s-1 for B- and C-ice and 5x10-14 cm2s-1 for SC ice. DHBr is lower than DHCl, as expected, and ranges from 5x10-15 to 6x10-14 cm2s-1 for C-ice over the temperature range indicated with similar values for B-ice. An interface region of several tens of nm thickness d has been measured for HCl and HBr adsorbed on ice whose values also depend on the type of ice. This interface region represents an open structure of the ice-atmosphere interface in which the mobility of HX is immeasurably fast using our technique and which corresponds to that volume of the ice which hosts HX that instantaneously reacts with the gas phase titrant (ClONO2). Typical values for HCl are d=100nm for C-ice, d=75nm for SC-ice and 100 to 200 nm for B-ice. Smaller values for HBr/ice have been measured.
- FTIR transmission spectra of thin films of ice dosed with known amounts of HCl and HBr have been recorded in transmission in order to characterize the condensed phase under identical conditions under which many of the kinetic results have been obtained in the flow reactor. Both well-defined HCl hydrates as well as amorphous HCl/ice mixtures have been identified in the OH-stretch region of the ice matrix whereas the spectral signature of HBr/ice mixtures is marginal in the IR. The evaporation rate of H2O decreases by a factor of 4-6 with respect to pure ice when it is dosed with a monolayer or so of HCl whereas it decreases by a factor of 20 in the HBr case in the temperature range 200 to 240K. This result may be of direct consequence to the lifetime of atmospheric ice particles.
- Chemical kinetic modelling of the interaction of H2O/ice, HCl/ice, HBr/ice and HOBr/ice in the range 180 to 210K points to the existence of a common precursor-mediated adsorption mechanism which explains the negative temperature dependence of adsorption observed in all cases.

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: 99.0491-1