Showing papers by "Michael E. Jenkin published in 1998"

Peroxy Radical Kinetics Resulting from the OH-Initiated Oxidation of 1,3-Butadiene, 2,3-Dimethyl-1,3-Butadiene and Isoprene

Abstract: The laser flash photolysis/UV absorption spectrometry technique has been used to investigate the kinetics of the peroxy radical permutation reactions (i.e. self and cross reactions) arising from the OH-initiated oxidation of isoprene (2-methyl-1,3-butadiene), and of the simpler, but related conjugated dienes, 1,3-butadiene and 2,3-dimethyl-1,3-butadiene. The results of the two simpler systems are analysed to provide values of the rate coefficients for the 6 peroxy radical permutation reactions of the three types of isomeric peroxy radical produced in each system (T = 298 K, P = 760 Torr). The rate coefficients are all significantly larger than values estimated previously by extrapolation of structure-reactivity relationships based on the kinetics of a limited dataset of simpler radicals containing similar structural features. The results are discussed in terms of trends in self and cross reaction reactivity of primary, secondary and tertiary peroxy radicals containing combinations of allyl, β-hydroxy and δ-hydroxy functionalities. Since the peroxy radicals formed in these systems are structurally very similar to those formed in the isoprene system, the kinetic parameters derived from the results of the simpler systems are used to assist the assignment of kinetic parameters to the 21 permutation reactions of the six types of isomeric peroxy radical generated in the isoprene system. Kinetic models describing the OH-initiated degradation of all three conjugated dienes to first generation products in the absence of NOx are recommended, which are also consistent with available end product studies. The model for isoprene is considered to be a further improvement on that suggested previously for its OH-initiated oxidation in the absence of NOx. The mechanism is further extended to include chemistry applicable to ‘NOx-present’ conditions, and calculated product yields are compared with those reported in the literature.

Laboratory Studies of the Response of a Peroxy Radical Chemical Amplifier to HO2 and a Series of Organic Peroxy Radicals

Abstract: The response of a peroxy radical chemical amplifier, supplied by the University of East Anglia (the UEA-PERCA), to HO2 and seven organic peroxy radicals (CH3O2, C2H5O2, neo-C5H11O2, HOCH2CH2O2, CH3CH(OH)CH(O2)CH3, (CH3)2C(OH)C(O2)(CH3)2 and CH3C(=O)O2) has been investigated. The peroxy radicals were produced in air at typical ambient levels (ca. 20–30 pptv) by reaction of CO or an appropriate organic precursor with OH radicals, generated from the near UV photolysis of nitrous acid (HONO) in a flow reactor. Experiments carried out at room temperature and atmospheric pressure in dry air, allowed measurement of the response of the UEA-PERCA to the organic peroxy radicals relative to the response to HO2 (denoted Ψobs), for reagent NO concentrations in the range 1–8 ppmv. The results indicate that HO2, CH3O2 and larger peroxy radicals containing polar functional groups are removed to a certain extent on the pyrex surfaces of the inlet zone of the UEA-PERCA, prior to reaching the reaction zone where the amplification chemistry occurs. For C2H5O2 and larger alkyl peroxy radicals, heterogeneous removal in the inlet zone appears to be minor. With the assumption that neo-C5H11O2 is not removed heterogeneously, the results are used to derive the following fractional responses (denoted Ψ) of the UEA-PERCA to the peroxy radicals ([NO] = 3 ppmv): HO2, (69 ± 5)%; CH3O2, (78 ± 5)%; C2H5O2, (95 ± 7)%; neo-C5H11O2, (74 ± 5)%; HOCH2CH2O2, (73 ± 5)%; CH3CH(OH)CH(O2)CH3, (77 ± 6)%; (CH3)2C(OH)C(O2)(CH3)2, (81 ± 6)%; and CH3C(=O)O2, (76 ± 5)%. Further experiments established that the response of the UEA-PERCA to HO2 was increased by ca. 10% when the air is moist (20–30% relative humidity). This is interpreted in terms of competitive adsorption of H2O and HO2 on the pyrex surfaces. A similar influence was observed when the inlet zone was coated with teflon. For the majority of peroxy radicals studied, the reduction of Ψ to less than 100% is almost entirely due to heterogeneous removal in the inlet zone. In the case of neo-C5H11O2, however, gas-phase reactions in the reaction zone have a significant influence on the response, due in part to the formation of t-C4H9O2 as an intermediate in its conversion to HO2. The influence of reactions of the RO2 and RO intermediates which limit the yield of HO2 in the reaction zone are discussed, and it is shown that calculations of the fractional conversion of neo-C5H11O2 to HO2 (i.e. Ψ) under idealised, well mixed conditions, do not give a good description of the observed dependence of Ψ on the variation of the reagent NO concentration. The results are discussed in terms of the interpretation of field measurements of peroxy radicals made using the chemical amplification technique.