Abbaumechanismen biogener Kohlenwasserstoffe in der Troposphaere. Laboruntersuchungen am Beispiel des Isoprens

Investigations on three topics relevant to the atmospheric oxidation of isoprene have been carried out in this work: 1. Product study of the OH-initiated oxidation of isoprene 2. Product study of the reaction of isoprene with O_3 3. Kinetic study of the reactions of isoprene primary oxidation products with OH radicals and O_3 (1.) 2- and 3-methyl-3-butene-1,2-diol have been identified as products of the OH-initiated oxidation of isoprene in the absence of NO_x. From the yields of these dihydroxy compounds the branching ratio k_2_a/k_2=#alpha#=0.7#+-#0.1 was obtained. In the presence of NO_x, a total yield of 11% of organic nitrates was measured. The intermediate formation of peroxy nitrates was observed. The thermal lifetime at 298 K (1 atm) of these compounds was #propor to#1 s. (2.) The formation yields of products from the ozonolysis of isoprene have been determined under different experimental conditions. The yields of the main products, methacrolein, methyl vinyl ketone, and formaldehyde, were found to increase upon addition of cyclohexane (#propor to#400 ppm) as well as of water vapour (1-10 mbar). This can be explained by scavenging of OH-radicals and of Criegee-intermediates by cyclohexane and H_2O, respectively. A number of products could be identified, which are thought to be formed by the decomposition of Criegee-intermediates: CO, CO_2, H_2, methanol, methyl hydroperoxide, propene, acetone, ketene, parts of the formaldehyde and eventually methylglyoxal. Some of these products are not formed directly, but via further reactions of the initially built alkyl or alkoxy radicals. (3.) Rate constants for the reactions of the isoprene oxidation products with OH have been measured at room temperature by means of a relative rare method. All substances exhibit high reactivity towards OH, especially the dihydroxy compounds where the rate constants are comparable to that of isoprene itself. (orig./SR)SIGLEAvailable from FIZ Karlsruhe / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman