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.