Laboratory studies of stable carbon isotope ratio of secondary particulate organic matter in the gas-phase.

摘要

Laboratory studies of stable carbon isotope ratio of secondary particulate organic matter (POM) were carried out. In the studies, secondary POM was generated by the reaction of toluene with OH radical in the gas-phase using a flow reactor or a smog chamber. The generated POM was collected on fiber filters for quantitative and isotope analysis. The measurements were made for both total POM carbon and specific compounds.;A novel method of compound specific stable carbon analysis for nitrohydroxy aromatic compounds, which are the major compounds often found in the secondary POM produced in the experiments, was developed. Using this method, the isotope ratios of7 nitrohydroxy aromatic products were determined. The results show that many of the nitrohydroxy aromatic products found in the particulate phase have almost the same isotope ratio as the total POM carbon.;Using these findings and stable carbon isotope measurements, novel concepts to study secondary POM from ambient measurements are presented.;The results demonstrate that the isotope ratio of total POM carbon was significantly lighter than the isotope ratio of the initial toluene. The difference was in the range from 3‰ to 6‰. This opens the possibility to distinguish between primary and emissions and secondary formation in the atmosphere for fossil fuel derived POM. The isotope ratio of total POM carbon systematically depended on the degree of toluene consumption, and the isotope ratio was predictable using the kinetic isotope effect for the reaction of toluene with OH radicals, the initial isotope ratio of toluene, and the fraction of toluene consumed. This finding indicates that the attack of an OH radical on a toluene molecule determines the carbon isotope fractionation of the formed POM, although the postulated formation mechanisms of condensable products by others proceed through a reaction sequence consisting of several steps. It was also found that under the experimental conditions of the studies here, the yield of total POM carbon was proportional to the fraction of toluene consumed.