Effects of Relative Humidity and Photoaging on the Formation, Composition, and Aging of Ethylbenzene SOA: Insights from Chamber Experiments on Chlorine Radical-Initiated Oxidation of Ethylbenzene

Abstract

The oxidation of alkyl-substituted aromatic molecules produces oxygenated volatile organic compounds (OVOCs) and secondary organic aerosols (SOA) that are major components of ambient urban air. Despite their ubiquity, the impacts of variable ambient conditions, such as relative humidity (RH) and actinic exposure, on the physicochemical processes that contribute to SOA formation are still the subject of ongoing research and refinement. In this work, we perform laboratory environmental chamber experiments and use an I^– FIGAERO–CIMS to examine the molecular composition of high-NO_x ethylbenzene oxidation products and SOA in response to varied relative humidity (dry conditions, 40% RH, and 60% RH) during either dark aging or photoaging (with ∼354 nm UV-A lights). Experiments are performed in a mixed Cl and OH radical environment. Compared to OH chemistry, Cl chemistry forms a greater amount of nitroaromatic products by enhancing benzaldehyde formation and phenolic H abstraction while also forming several organochlorine molecules that may serve as tracers for Cl chemistry, of which C₂H₃ClO₂ (presumably chloroacetic acid) appears to be the most consistent and stable. Organonitrate (ON) molecules undergo hydrolytic and photolytic losses. Nitroaromatic molecules condense more efficiently under humid conditions, presumably due to the relatively high solubility of hydroxy and dihydroxy aromatic molecules, but do not appear stable in the condensed phase during either dark or photoaging. Small oxygenates make up a substantial portion of SOA that increases at high RH (due to increased uptake) and during photoaging (due to SOA photolysis and fragmentation). Photoaging initially leads to a degree of oligomerization in the condensed phase before continued photoaging leads to an eventual loss of these and other compounds. Our results show that RH and photoaging substantially impact the composition and evolution of many gas- and particle-phase species produced during ethylbenzene oxidation and suggest that these environmental factors can exert strong control over SOA formation and evolution, particularly in urban regions.