LABORATORY STUDIES OF THE RESPONSE OF A PEROXY RADICAL CHEMICAL AMPLIFIER TO HO2 AND A SERIES OF ORGANIC PEROXY-RADICALS

The response of a peroxy radical chemical amplifier, supplied by the U niversity of East Anglia (the UEA-PERCA), to HO2 and seven organic per oxy radicals (CH3O2, C2H5O2, neo-C5H11O2, HOCH2CH2O2, CH3CH(OH)CH(O-2) CH3, (CH3)(2)C(OH)C(O-2)(CH3)(2) and CH3C(= =O)O-2) has been investiga ted. The peroxy radicals were produced in air at typical ambient level s (ca. 20-30 pptv) by reaction of CO or an appropriate organic precurs or with OH radicals, generated from the near UV photolysis of nitrous acid (HONO) in a flow reactor. Experiments carried out at room tempera ture and atmospheric pressure in dry air, allowed measurement of the r esponse of the UEA-PERCA to the organic peroxy radicals relative to th e response to HO2 (denoted Psi(obs)), for reagent NO concentrations in the range 1-8 ppmv. The results indicate that HO2, CH3O2 and larger p eroxy radicals containing polar functional groups are removed to a cer tain 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 rem oval in the inlet zone appears to be minor. With the assumption that n eo-C5H11O2 is not removed heterogeneously, the results are used to der ive the following fractional responses (denoted Psi) 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(O-2)CH3, (77 +/- 6)%; (CH3)(2)C(OH)C(O-2)(CH3)(2 ), (81 +/- 6)%; and CH3C(=O)O-2, (76 +/- 5)%. Further experiments esta blished 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 interpr eted in terms of competitive adsorption of H2O and HO2 on the pyrex su rfaces. A similar influence was observed when the inlet zone was coate d with teflon. For the majority of peroxy radicals studied, the reduct ion of Psi to less than 100% is almost entirely due to heterogeneous r emoval in the inlet zone. In the case of neo-C5H11O2, however, gas-pha se 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 an di scussed, and it is shown that calculations of the fractional conversio n of neo-C5H11O2 to HO2 (i.e. Psi) under idealised, well mixed conditi ons, do not give a good description of the observed dependence of Psi on the variation of the reagent NO concentration. The results are disc ussed in terms of the interpretation of field measurements of peroxy r adicals made using the chemical amplification technique.