Stable carbon isotope ratio of secondary particulate organic matter formed by photooxidation of toluene in indoor smog chamber

Laboratory studies were conducted for stable carbon isotope ratio for secondary particulate organic matter (POM) formed by photooxidation of toluene. Using an 8 m3 indoor YorkU smog chamber, three POM generation experiments were carried out under room temperature, atmospheric pressure, and RH < 6%. In this study, the different initial conditions were set from those in the previous flow reactor study for secondary POM formed by photooxidation of toluene: introduction of seed particles, and 40–80 times lower initial toluene mixing ratios (∼1900 μg m−3) and ∼60 times higher toluene to NOx ratios (∼7), respectively. The formation of secondary POM was confirmed by the size distribution measurements of particles and the POM was collected on quartz fiber filters for carbon mass and stable carbon isotope ratio analysis. The size distribution measurements showed that the POM mass yields were between 37% and 55% at the end of experiments, while the filter analysis showed that the yields of POM carbon collected then were between 7% and 21%, implying the substantial contribution of heteroatoms. The plot of the POM carbon yields from this study and the previous study indicated that the POM carbon yields were merged into a single profile. The results of isotope measurements demonstrated that the carbon isotope fractionation at the initial reaction step was predominant in the overall carbon isotope fractionations that occur in the sequence of the reactions leading to the formation of secondary POM carbon. Using the results, novel approach for estimation of secondary POM carbon yield with stable carbon isotope ratios is proposed.

Research highlights
► δ13C and carbon yield of secondary POM formed by toluene/OH reaction are presented.
► POM carbon yields were proportional to fraction of toluene consumed.
► The results imply contribution of heteroatoms and/or of inorganic to the PM formed.
► Dependence between isotope ratio and particulate yield is presented.