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- Experimental and theoretical examination of the chemical kinetics of a pollutant coating on porous particles (2009)
- The persistence in the different compartments and in the atmospheric long-range transportation is important property of pesticides as representatives of the semivolatile substances. These compounds could be distributed dependent on the air pressure as well as the temperature - between gas and particle phase. In an aerosol smog chamber could be simulated the atmospheric degradation of airborne substances through hydroxyl radicals. The smog-chamber was cooled on 2 and -10°C and the degradation kinetic of semivolatile substance (Aldrin) coated on fine quartz particles (Aerosil 380) was researched. The coated Aerosil was mixed with water in ration 1:1000 and the suspension was sprayed into the chamber. Fine agglomerates were formed during the spaying with mean diameter approximately 1 µm. The precursors for the production of hydroxyl radicals were either reacting mixtures of hydrazine and ozone in absence of light source or photolysis of methylntrite. The concentration of hydroxyl radicals was varied over two powers of ten, from approximately 5•105 to 7•107 cm-3. The concentration of that OH–radicals was calculated over the degradation rate of hydrocarbons (n-octane, n-hexane, 2,2,3-trimthylbutane, 2,2-dimethylbutane and 2,2,3,3-tetramethylbutane). The hydrocarbons were cryofocussed in a glass-coated steel capillary at -110°C (using liquid nitrogen and a magnetic valve to control the flow) and analyzed gas chromatographically. Two products (Photoaldrin and Dieldrin) could be detected from the chemical reaction of Aldrin with hydroxyl radicals. A high concentration of the products were produced with additional experiments with coated Aerosil and glass balloons (d = 70 µm, unporous) and production of hydroxyl radicals from the methylnitrite photolysis in an irradiated rotating evaporator. The products were extracted from the carrier material and were identified with GC–MS. Photoaldrin was formed faster than Dieldrin and reacts also faster then Dieldrin. The temperature gradient between the top and the bottom of the smog-chamber was measured. The temperature difference is important for the air mixing of the chamber content. In the presence of a light-source (the fluorescence lamps are under the the smog camber) the temperature difference is 1.0 °C and ensures a fast mixing in the chamber. This difference of the not irradiated chamber is about 0.3 °C and causes a insufficient mixing, that is noticeable through strong fluctuations of the aerosol density. The structure of the aerosol agglomerates was imaged according to the ion etching method with FESEM (Field Emission Scanning Electron Microscopy) in the Fraunhofer-IKTS. The imagines were evaluated in this work with the program “Lince”. A maximum of the agglomerate diameter was obtained by 0,5 µm. The pore size distribution has a maximum by approximately 20 nm diameter. The life-time of Aldrin and respectively the rate constant of the reaction with hydroxyl radicals could be calculated directly from the experiments. The observed rate constant had a dependence on the OH–concentration in approaching form 3.5•10-5•[OH]-0.88 (the function yields a straight in double logarithmic scale). On the basis of the structure of the agglomerates, a mathematical model was applied from the literature in order to take the influence of the agglomerate structure into account. The observed concentration of Aldrin decreases because of the chemical reaction, the radial diffusion from the agglomerate center to the periphery and because of the evaporation of the substance from the agglomerate surface. It is considered also in the model that the concentration of the hydroxyl radicals alters with the penetration in the agglomerate. The penetration depth can not be determined experimentally. This value, as well as the life-time and the diffusion coefficient could be estimated from the experiments. If the experiment is made by low temperature, the evaporation could be neglect. The evaporated part of Aldrin decreases with the increasing OH concentration. The reciprocal life-time or the reaction rate of Aldrin increases linearly with the increase of the OH concentration. The rate constant of the reaction of Aldrin and OH radicals could be calculated from the reaction rate and the OH concentration. The OH rate constant was kOH = 6.2•10-11 ± 1.3•10-11 cm-3s-1. The effective diffusion coefficient was calculated by -10°C and yielded a value of Deff = 4.6•10-11 ± 2.2•10-11 cm2s-1.