Modeling SOA Formation from Monocyclic, Bicyclic, and Branched-Cyclic Alkanes

Cycloalkanes, which represent a significant proportion of hydrocarbons released through fuel evaporation and exhaust, produce considerable amounts of secondary organic aerosol (SOA). However, cycloalkanes vary in the number of rings and alkyl branch lengths, leading to high complexity in SOA predictions. In this study, the Unified Partitioning Aerosol Phase Reaction (UNIPAR) model is extended to predict SOA formation via multiphase reactions of cyclohexane, decalin, and various branched cyclohexanes. UNIPAR employs a lumped product distribution, originating from an explicit gas mechanism, to process multiphase partitioning and in-particle chemistry for SOA formation. The product distributions of cyclohexane and decalin are created using explicit oxidation mechanisms. Product distributions of alkyl-branched cyclohexanes are created as a composite of those of cyclohexane and linear alkanes. UNIPAR is applied to predict SOA formation for cyclohexane, decalin, butylcyclohexane, and octylcyclohexane, under various NO_x levels and seed conditions, and compared to chamber data. Cyclohexane and decalin SOA formation occurs exclusively through particle-phase reactions due to ring-opening aldehydic products. When the NO_x level (HC ppbC/NO_x ppb) = 3 at given conditions, the oligomeric SOA fraction represents 99.8, 82.2, and 1.0% of mass formed from decalin, butylcyclohexane, and n-decane, respectively. Except cyclohexane, SOA yields of cyclic alkanes are insensitive to seed types. With increasing alkyl branch length, cycloalkanes have increased SOA yields, and their SOA formation behaves more similarly to linear alkanes. Due to insensitivity to seed and NO_x conditions, temperature is the key environmental parameter influencing the SOA yields of large cyclic alkanes.