Secondary organic aerosol formation from the photooxidation of isoprene, 1,3-butadiene, and 2,3-dimethyl-1,3-butadiene under high NO_x conditions

Abstract. Secondary organic aerosol (SOA) formation from atmospheric oxidation of isoprene has been the subject of multiple studies in recent years; however, reactions of other conjugated dienes emitted from anthropogenic sources remain poorly understood. SOA formation from the photooxidation of isoprene, isoprene-1-¹³C, 1,3-butadiene, and 2,3-dimethyl-1,3-butadiene is investigated for high NO_x conditions. The SOA yield measured in the 1,3-butadiene/NO_x/H₂O₂ irradiation system (0.089–0.178) was close to or slightly higher than that measured with isoprene under similar NO_x conditions (0.077–0.103), suggesting that the photooxidation of 1,3-butadiene is a possible source of SOA in urban air. In contrast, a very small amount of SOA particles was produced in experiments with 2,3-dimethyl-1,3-butadiene. Off-line liquid chromatography – mass spectrometry analysis revealed that the signals of oligoesters comprise a major fraction (0.10–0.33) of the signals of the SOA products observed from all dienes investigated. The oligoesters originate from the unsaturated aldehyde gas phase diene reaction products; namely, semi-volatile compounds produced by the oxidation of the unsaturated aldehyde undergo particle-phase oligoester formation. Oligoesters produced by the dehydration reaction between nitrooxypolyol and 2-methylglyceric acid monomer or its oligomer were also characterized in these experiments with isoprene as the starting diene. These oligomers are possible sources of the 2-methyltetrols found in ambient aerosol samples collected under high NO_x conditions. Furthermore, in low-temperature experiments also conducted in this study, the SOA yield measured with isoprene at 278 K was 2–3 times as high as that measured at 300 K under similar concentration conditions. Although oligomerization plays an important role in SOA formation from isoprene photooxidation, the observed temperature dependence of SOA yield is largely explained by gas/particle partitioning of semi-volatile compounds.