Flow reactor studies of the stable carbon isotope composition of secondary particulate organic matter generated by OH-radical-induced reactions of toluene

Abstract

Secondary particulate organic matter (POM) formed in a flow reactor by the OH-radical-induced reactions of toluene was collected on quartz fiber filters, and its stable carbon isotope ratio was analyzed by off-line combustion and subsequent dual-inlet isotope ratio mass spectrometry. The toluene consumption in these experiments ranged from 7% to 29%. The stable carbon isotope composition ( δ 13C) of the secondary POM was in the range of −32.2‰ to −32.9‰ (VPDB scale), with some indication for a slight dependence on the extent of toluene consumption. These measured δ 13C values were, on average, 5.8‰ lighter than those of the parent toluene. Those observations are slightly lower than the δ 13C values of the sum of all toluene oxidation products (from −31.6‰ to −32.3‰) that are predicted using the kinetic isotope effect for the reaction of toluene with OH-radical under these reaction conditions and the initial δ 13C of the parent toluene. Therefore, mass balance dictates that the fractionation between gas-phase and particle-phase products is small. On average, the particle-phase products are 0.6‰±0.2‰ lighter than the gas-phase products. This is in agreement with the concept that the initial reaction of toluene with the OH-radical is the slowest step in the reaction sequence resulting in POM formation.