Dithiothreitol activity by particulate oxidizers of SOA produced from photooxidation of hydrocarbons under varied NO_x levels

Abstract. When hydrocarbons (HCs) are atmospherically oxidized, they form particulate oxidizers, including quinones, organic hydroperoxides, and peroxyacyl nitrates (PANs). These particulate oxidizers can modify cellular materials (e.g., proteins and enzymes) and adversely modulate cell functions. In this study, the contribution of particulate oxidizers in secondary organic aerosols (SOAs) to the oxidative potential was investigated. SOAs were generated from the photooxidation of toluene, 1,3,5-trimethylbenzene, isoprene, and α-pinene under varied NO_x levels. Oxidative potential was determined from the typical mass-normalized consumption rate (reaction time t =  30 min) of dithiothreitol (DTT_t), a surrogate for biological reducing agents. Under high-NO_x conditions, the DTT_t of toluene SOA was 2–5 times higher than that of the other types of SOA. Isoprene DTT_t significantly decreased with increasing NO_x (up to 69 % reduction by changing the HC ∕ NO_x ratio from 30 to 5). The DTT_t of 1,3,5-trimethylbenzene and α-pinene SOA was insensitive to NO_x under the experimental conditions of this study. The significance of quinones to the oxidative potential of SOA was tested through the enhancement of DTT consumption in the presence of 2,4-dimethylimidazole, a co-catalyst for the redox cycling of quinones; however, no significant effect of 2,4-dimethylimidazole on modulation of DTT consumption was observed for all SOA, suggesting that a negligible amount of quinones was present in the SOA of this study. For toluene and isoprene, mass-normalized DTT consumption (DTT_m) was determined over an extended period of reaction time (t =  2 h) to quantify their maximum capacity to consume DTT. The total quantities of PANs and organic hydroperoxides in toluene SOA and isoprene SOA were also measured using the Griess assay and the 4-nitrophenylboronic acid assay, respectively. Under the NO_x conditions (HC ∕ NO_x ratio: 5–36 ppbC ppb⁻¹) applied in this study, the amount of organic hydroperoxides was substantial, while PANs were found to be insignificant for both SOAs. Isoprene DTT_m was almost exclusively attributable to organic hydroperoxides, while toluene DTT_m was partially attributable to organic hydroperoxides. The DTT assay results of the model compound study suggested that electron-deficient alkenes, which are abundant in toluene SOA, could also modulate DTT_m.