LEXIS: Laboratory experiments of iodine chemistry in the stratosphere

• Research unit: EU RFP
• Project number: 95.0552

Key words (English)

Hheterogeneous reactions; stratospheric chemistry; Iodine-containing molecules; hypoiodous acid (HOI); IOo free radical

Alternative project number (English)

EU project number: ENV4-CT95-0013

Research programs (English)

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

Short description (English)

See abstract

Further information (English)

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

Partners and International Organizations (English)

LCSR (Laboratoire de Chimie des Systèmes Réactifs)/CNRS, Orléans (F); Max-Planck Institut für Chemie, Mainz(D), University of Cambridge (UK), Universität Wuppertal (D)

Abstract (English)

It was recently suggested that homogeneous and heterogeneous chemical reactions involving iodine-containing species could very well explain midlatitude and tropical ozone depletions at low altitudes between 12 and 20 km. In this altitude range chlorine- and bromine-based chemistry alone is not able to obtain agreement between modeling results and observations. Very little is known about uptake coefficients of iodine-containing molecules and free radicals on atmospherically relevant surfaces under conditions approaching those of the lower stratosphere.
The major conclusions resulting from our work are as follows:
oThe uptake coefficient g of HI on ice shows a negative temperature dependence over the range T=180 to 200K and is identical in value to the one for HCl and HBr. The rate of uptake is first order in HI under our experimental conditions. The HI vapor pressure over ice is unmeasurably small over the same temperature range.
oThere is a strong interaction of HI adsorbed on ice with reservoir compounds containing Cl and Br such as ClONO2 and BrONO2 resulting in g=0.3±0.02 and 0.4±0.02, respectively. These reactions are complicated by the interference of fast secondary reactions of the primary interhalogen compounds ICl and IBr which react with excess HI to Cl2 and Br2. However, they are observable on a transient basis at low surface concentrations of HI on the ice. I2 seems to be sequestered in the condensed phase at low temperatures in the presence of hydrohalic acids HX. We note a propensity to form homonuclear halogens in interfacial reactions at the expense of the formation of interhalogen compounds.
oThe reactivity of the halogen and interhalogen compounds with HX adsorbed on ice is controlled by the formation of the hydrohalic acid whose anion has the highest electronegativity. Specifically, the reaction of ICl + HBr and ICl + HI lead to HCl, the reaction of IBr + HI to HBr. All uptake coefficients are first order and large with the exception of IBr and I2 on ice and I2 on HCl-doped ice at 200K. The large uptake coefficients may be understood in terms of the formation of trihalide complexes in agreement with the observed negative temperature dependence.
oHOI rapidly decomposes on surfaces including Teflon at ambient temperature and on ice at 200K to result in I2. On ice surfaces the uptake coefficient leading to decomposition has a slight negative temperature dependence in the range 180 to 200K and varies from 7.5x10-2 to 4.8x10-2. The uptake is not significantly affected by saturation. The uptake coefficient does not significantly change upon doping the ice surface with HCl and HBr; no reaction products except I2 have been observed and is therefore believed to be also controlled by decomposition.
oIO free radical does not interact with ice at T=200K under our conditions. No products have been observed.

References in databases (English)

Swiss Database: Euro-DB of the
State Secretariat for Education and Research
Hallwylstrasse 4
CH-3003 Berne, Switzerland
Tel. [phone]
Swiss Project-Number: 95.0552