SALT: Sea-salt aerosols : Laboratory investigations of heterogeneous halogen activation in the troposphere

• Research unit: EU RFP
• Project number: 95.0551

Key words (English)

Tropospheric chemistry; heterogeneous reactions; marine aerosols; oxidizing capacity of the troposphere

Alternative project number (English)

EU project number: ENV4-CT95-0037

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)

Université de Lille (F), Université Louis Pasteur, Strasbourg (F), Max-Planck Institut für Chemie, Mainz (D), Russian Academy of Science, Moscow (R)

Abstract (English)

This project is aimed at investigating the heterogeneous atmospheric reactions which are suspected to lead to high concentrations of halogenated reservoir species and free radicals, especially bromine-containing radicals, recently observed in the marine boundary layer. The presence of active halogen-containing species under photolyzing conditions is expected to affect the oxidizing capacity of the troposphere. The work is therefore focussed on the heterogeneous interaction between active nitrogen compounds (NOy) that are occurring in polluted air masses and salt of marine origin. The reactions are occurring on the surface of sea-salt aerosols which subsequently may release photo-labile precursors to halogenated free radicals.
We have obtained the following results on sea-salt aerosol substitutes:
oBoth ClNO2 and BrNO2 are the result of NOy interaction with solid alkali halide salts. ClNO2 is unreactive from a heterogeneous chemistry point of view such that photolysis should be the major loss process in the troposphere. BrNO2 is very reactive in heterogeneous reactions involving bromides and easily hydrolyzes to HONO and HOBr. It is, however, unreactive towards chlorides. Therefore, NOy undergoes oxidation-reduction reactions only with bromides, directly resulting in molecular bromine and HONO according to BrNO2 + KBr -> Br2 + KNO2. On the other hand only ClNO2 is formed when NOy interacts with chlorides leaving the oxidation state of nitrogen and chlorine unchanged as oppposed to BrNO2 which is the anhydride of HOBr and HONO.
oNitrate free radical has been found to undergo exothermic reactions on salt resulting in free Br and I but not Cl. In the latter case gas phase HCl has been found as the major product, whereas in the former case a mixture of both free atomic Br and I as well as Br2 and I2 have been found which result from the heterogeneous recombination of the halogen atoms on the Teflon-coated internal walls of the flow reactor.
oBromine nitrate efficiently reacts both with chlorides and bromides to result in free halogen and interhalogen compounds. However, the uptake kinetics does not follow a first order rate law. In addition, BrONO2 competes for both solid alkali halide as well as for H2O adsorbed on salt to result in Br2 even on non-reactive salt substrates. In addition, there is a clear tendency of BrONO2 to decompose to Br2 even on non-reactive salts such as NaNO3.
oHOI is reactive with solid alkali bromides resulting in IBr, but not on solid alkali chlorides. HOI undergoes fast heterogeneous decomposition on salt surfaces, both potentially reactive as well as non-reactive, to result in I2. The tendency for heterogeneous decomposition of HOX (X=Cl, Br, I) to X2 on solid alkali salt surfaces increases in the series Cl

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: 95.0551