A Langmuir-Hinshelwood Fit of Atmospheric Reactions of OH Radicals with Semivolatile, Aerosol-Borne Compounds in Chamber Experiments
2 Institute of Sustainable and Environmental Chemistry, Univ. Lüneburg, Germany
3 Forschungsstelle für Atmosphärische Chemie, Univ. Bayreuth, Germany/Fraunhofer-Institut für Toxikologie und Experimentelle Medizin, Hannover, Germany
P 3.13 in Ökosysteme: Funktion und Leistungen
A number of semivolatile compounds and proxies of environmental compounds, such as oleic acid, long-chain alkanes, polycyclic aromatic hydrocarbons, plasticizers, PCBs, brominated flame retardants and various pesticides have been investigated for their atmospheric degradation rates by OH radicals and/or ozone. Some of them have been examined in the gas phase and some of them in the adsorbed state as thin films or sub-monolayers on appropriate solid materials in either flow reactors or aerosol chambers, by exposing them to OH radicals or ozone at known levels.
In the last decades, numerous studies were performed to investigate the reaction of OH radicals with compounds in the gas phase. However, due to its complex mechanisms, aerosol-borne reactions with OH radicals are poorly understood. Only recently, the Langmuir-Hinshelwood mechanism and the Eley-Rideal mechanism have been applied on the reaction of semivolatile compounds with O3, considering different adsorption processes of reactants and the gas-particle equilibrium characteristics.
In this work, we applied both mechanisms in the analysis of the reaction of aerosol-borne terbuthylazine with OH radicals in our simulation glass-smog chamber. Home made silica particles were used as carrier material. Compared to previous <10 nm particles (Aerosil 200), these larger particles (diameter about 160 nm) have less tendency to agglomerate. Compared to previous studies, a negative correlation with OH concentration was found for the second order reaction rate constant. At relatively low OH concentrations, the reaction could be explained by the Eley-Rideal mechanism, while at higher OH concentration, the Langmuir-Hinshelwood mechanism is more appropriate to explain concentration-time dependencies. Because the adsorption could be affected by gas-particle equilibria, results from different experimental setups (e.g. Chamber experiments and flow tube experiments) are assumed to be different.
Acknowledgements: Present work is supported by the EU within the infrastructure EUROCHAMP-2. We also acknowledge former support by the Umweltbundesamt, the companies CIBA-Geigy (now Syngenta), Bayer and BASF and by the EU in the project MOST.