Dithiothreitol Activity by Particulate Oxidizers of SOA Produced from Photooxidation of Hydrocarbons under Varied NOx Levels
Huanhuan Jiang, Myoseon Jang, and Zechen Yu
Department of Environmental Engineering Sciences, Engineering School of Sustainable Infrastructure and Environment, University of Florida, Gainesville, FL 32608, USA
Received: 09 Mar 2017 – Accepted for review: 09 Mar 2017 – Discussion started: 10 Mar 2017
Abstract. When hydrocarbons 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 (SOA) to the oxidative potential was investigated. SOA were generated from the photooxidation of toluene, 1,3,5-trimethylbenzene, isoprene, and α-pinene under varied NOx levels. Oxidative potential was determined from the typical mass-normalized consumption rate (reaction time t = 30 min) of dithiothreitol (DTTt), a surrogate for biological reducing agents. At high NOx conditions, the DTTt of toluene SOA was 2–5 times higher than that of other types of SOA. Isoprene DTTt significantly decreased with increasing NOx (up to 69 % reduction by changing the hydrocarbon / NOx ratio from 30 to 5). The DTTt of 1,3,5-trimethylbenzene and α-pinene SOA was insensitive to NOx 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 SOA of this study. For toluene and isoprene, mass-normalized DTT consumption (DTTm) 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. The amount of organic hydroperoxides was substantial, while PANs were found to be insignificant for both SOA. Isoprene DTTm was almost exclusively attributable to organic hydroperoxides, while toluene DTTm was partially attributable to organic hydroperoxides. The results of the model compound study suggest that electron-deficient alkenes, which are abundant in toluene SOA, could also modulate DTTm.
Jiang, H., Jang, M., and Yu, Z.: Dithiothreitol Activity by Particulate Oxidizers of SOA Produced from Photooxidation of Hydrocarbons under Varied NOx Levels, Atmos. Chem. Phys. Discuss., doi:10.5194/acp-2017-214, in review, 2017.