Heterogeneous reactions of tropospheric trace-gases on solid model aerosol surface : a laboratory study

The first part of this laboratory study deals with heterogeneous reactions of bromine- and chlorine-containing species relevant to the atmosphere with solid model alkali salt substrates. The second part of this work treats the heterogeneous interaction of acidic compounds with various mineral dust substrates. The aim of this study is to investigate the role of adsorbed water, H2O(a) and the influence of defect surface sites on the reactions. Furthermore, reaction mechanisms are proposed for the different interactions. Steady state and pulsed valve uptake experiments have been performed in a low-pressure flow reactor. In order to characterise the solid samples images have been taken using a scanning electron microscope (SEM). Furthermore, a few scans using a photoacoustic cell coupled to a FTIR spectrometer have been performed. First, the interaction of gaseous Cl2, HCl, BrCl, Br2, Cl2O and HOCl with solid KBr and furthermore, the reaction of Cl2O and HOCl on solid NaCl have been investigated. The reaction products of the reactions of Cl2, Cl2O and HOCl with KBr have been Br2 and BrCl. The reaction of Cl2O with KBr leads to slow additional formation of HOCl, BrOCl and Cl2, and the reaction of HOCl with KBr leads to slow formation of HOBr, Cl2O, Br2O and BrOCl. The reaction of Cl2O on solid NaCl leads to formation of Cl2 but also a slow formation of HOCl has been observed which is attributed to the reaction with adsorbed H2O(a). The interaction of HOCl with NaCl was immeasurably slow under our conditions. In order to measure the amount of adsorbed H2O(a) desorption experiments have been carried out and a calibration curve has been obtained. While performing desorption experiments on ground KBr grains and thin KBr films sprayed on a gold-coated substrate desorption of HOBr has been observed which we attributed to the presence of surface adsorbed molecular bromine. Using the SEM we found that pumping and heating the substrate lead to a better-ordered surface. The exposure of the salt sample to an excess of chlorine or bromine leads also to a recrystallisation of the surface. Furthermore, all reactions on alkali salt samples have shown that adsorbed halogen species are retained on the surface. These adsorbed species play a crucial role for the reactions of halogens with alkali salt. In the case of HOCl on KBr we explain the autocatalytic behaviour of the reaction by the presence of adsorbed halogen species, particularly Br2. In the Cl2/KBr system a regeneration of the surface has been observed which we explain with desorption of adsorbed intermediates, but also by crystallisation of KCl that may lead to a recycling of reactive sites. In contrast to Cl2/KBr no regeneration of the substrate towards formation of gaseous bromine-containing species has been observed in the HOCl/KBr system. We explain this with the fact that KOH thermodynamically does not tend to crystallise in the presence of H2O as does the alkali halide. In the second part the reactions