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 (i5 13 q of the secondary POM was in the range of -32.2%0 to -32.9%0 (VPDB scale), with some indication for a slight dependence on the extent of toluene consumption. These measured i513 C values were, on average, 5.8%0 lighter than those of the parent toluene. Those observations are slightly lower than the i513 C values of the sum of all toluene oxidation products (from -31.6%0 to -32.3%0) that are predicted using the kinetic isotope effect for the reaction of toluene with OH-radical under these reaction conditions and the initial i5 13 C 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 0.60/00 ±0.2%0 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.

Method for determination of stable carbon isotope ratio of methylnitrophenols in atmospheric particulate matter

“…More recently it also has been suggested that compound specific isotope ratio measurements can be used to study the formation of secondary organic pollutants from VOC degradation in the atmosphere (Irei et al, 2006(Irei et al, , 2011Irei, 2008, Fisseha et al, 2009Li et al, 2010;Iannone et al, 2010).…”

“…Based on the kinetic isotope effect and mass budget considerations the products of toluene oxidation in the initial phase of the reaction will be 5 ‰-6 ‰ lighter than the parent compound. Indeed, Irei et al (2006) observed that secondary POM formed by the OH-radical initiated oxidation of toluene in a flow reactor is on average 5.6 ‰ lighter under conditions where less than 30 % of toluene has reacted (Fig. 4).…”

“…Phase transitions such as evaporation typically only cause very small isotope effects. For example Irei et al (2006) found a difference of (0.33 ± 0.02 ‰) between gas and liquid phase during partial evaporation of toluene. It therefore is very likely that for isotope ratio measurements the bias from changes in distribution between particle-phase and gas-phase during sampling will be below the estimated overall uncertainty of the isotope ratio measurements of 0.5 ‰.…”

“…Individual data points represent measurements of the carbon isotope ratio of total POM. The flow reactor and smog chamber data are taken from Irei et al (2006, 2011) and Irei (2008. It should be noted that the position of the ambient 2-methyl-4-nitrophenol data along the x-axis does not indicate the extent of toluene processing.…”

Origin Identification of Carbonaceous Aerosol Particles by Carbon Isotope Ratio Analysis

“…Rudolph (2007) has demonstrated that isotope fractionation can occur during reactions of volatile organic compounds with oxidants. Few studies have examined isotope fractionation during SOA formation (Irei et al, 2006;Fisseha et al, 2009;Irei et al, 2011). Wang and Kawamura (2006) showed that aging removes lighter carbon isotopes from particulate phase in the atmospheric oxidation processes.…”