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Abstract No.: TuP-269
Session: Reaction Mechanisms
Presentation date: Tue, Aug 29, 2006
Presentation time: 09:50 – 11:20

Laboratory Studies of Heterogeneous Reactions Responsible for Chlorine Activation of the Troposphere

Vladislav V. Zelenov1, Elena V. Aparina1, Denis V. Shestakov2, Yulii M. Gershenzon2

1 Institute for Energy Problems of Chemical Physics, Chernogolovka, Russian Federation
2 Semenov Institute of Chemical Physics, Moscow, Russian Federation

Correspondence address: Vladislav V. Zelenov, Institute for Energy Problems of Chemical Physics, Russian Academy of Sciences, Academician Semenov St., 2/10, Chernogolovka, Moscow Region, 142432 Russian Federation.

Keywords: Environmental; Kinetics; Reaction Mechanisms; Reaction, Surface.

Novel aspect: A new approach in chemistry of the troposphere to take into account heterogeneous reactions leading to chlorine liberation.

 

Sea salt is the main source of halogens in the lower troposphere in coastal industrial regions with high NOx emission. Waves produce small droplets of seawater in marine boundary layer. When the droplets are transported to higher altitude or inland where the relative humidity is lower, they are turned to crystallites and form the marine aerosols. An interaction between the aerosol particles and nitrogen-containing minor constituents leads to halogen liberation into the gas phase. The generated halogen-containing gases in turn effectively oxidize hydrocarbon pollutants. In modern databases for the troposphere, heterogeneous reactions are introduced in terms of uptake coefficients by analogy with the rate constants for elementary gas-phase reactions. In contrast to the latter, the uptake coefficients are non-elementary ones. In general, they depend on the surface representation, a volume concentration of the reactants, relative humidity, and temperature. The problem of the uptake coefficient measurement is to extrapolate laboratory data to the real troposphere characterized by much lower reactant concentration, variable humidity, and specific surface of the aerosol particles.
Kinetic studies and the mechanism determination of NO3 and ClONO2 uptake on polycrystalline films of individual salts NaCl, NaBr, and MgCl26H2O at different humidity were carried out, using a coated-insert flow tube reactor combined with high-resolution, low-energy electron-impact mass spectrometer. Temporal dependences of the uptake coefficient and the partial uptake coefficients leading to a formation of the primary products were measured for different NO3 and ClONO2 concentrations. These dependences are established to be described by parametric form γ=γ0 exp(-t/τ)+γs, the parameters γ0, γs and τ being dependent on concentration of the gas-phase reactant and water. As a rule, the initial γ0 far exceeds the steady-state γs value. In the framework of the proposed kinetic model, the data are explained and the main elementary kinetic parameters of the uptake are evaluated. The model is based on combination of Langmuir competitive adsorption, formation of surface complexes on initial active sites followed by their unimolecular decomposition. The model gives an analytic expression for the experimental parameters γ0, γs and τ in terms of elementary rate constants and the reactant volume concentration. The final objective of the proposed model is extrapolation of parameters γ0, γs and τ to real tropospheric conditions. In particular, the extrapolated characteristic time τ proves to be comparable or more than that of dry aerosol deposition. This gives an opportunity to choose which of the probabilities γ0 or γs has to be used for tropospheric modeling.