Influence of relative humidity on cyclohexene SOA formation from OH photooxidation

Secondary organic aerosol (SOA) is a complex mixture consisting of a variety of oxidation products. In this study, the role of relative humidity (RH) on SOA formation with different [H₂O₂]₀/[cyclohexene]₀ was investigated in a smog chamber. It was found that the cyclohexene SOA yield increases with increasing initial OH concentration at both high and low RH conditions. The increased rate of SOA formation was lower at wet conditions than that at dry conditions. For [H₂O₂]₀/[cyclohexene]₀ = 0.4 and 0.8, the SOA yield increased from 1.5% to 8% at dry condition to 7% and 12% at wet condition, respectively. In contrast, at high RH the SOA yield fell from 54% to 52% for [H₂O₂]₀/[cyclohexene]₀ = 1.3. The SOA mass loss was higher at high RH at the same OH exposure. The chemical composition of SOA was characterized using hybrid quadrupole-orbitrap mass spectrometer equipped with electrospray ionization (ESI-Q-Orbitrap-HRMS). Oligomers, which were responsible for the increase of the SOA yield, were detected in the SOA formed at high RH conditions. The esterification reaction between dicarboxylic acids and HOC₆H₁₀-O-O-C₆H₁₀OH was the pathway of oligomers formation. All the oligomers have cyclic molecular structures. For [H₂O₂]₀/[cyclohexene]₀ = 1.3, the relative intensity of both low and high molecular weight substances reduced more at wet conditions. This indicates that at sufficient OH level, the inhibition of oligomers formation and the further reaction of SOA with OH result in a slightly lower SOA yield at wet condition.