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

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 (δ13C) 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 δ13C values were, on average, 5.8‰ lighter than those of the parent toluene. Those observations are slightly lower than the δ13C 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 δ13C 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.