The Impact of Structure on Alkane SOA Formation

AZAD MADHU, Myoseon Jang, Yujin Jo, University of Florida

     Abstract Number: 408
     Working Group: Urban Aerosols

Abstract
Alkanes represent significant proportions of hydrocarbon emissions in urban environments. Alkanes emissions are primarily anthropogenic, released from sources such as fossil fuels, pesticides, and various volatile chemical products. Previous literature has demonstrated that the inclusion of intermediate volatility organic compounds, such as alkanes, can significantly improve predictions of regional organic aerosol formation. In this work, parameters are developed for the simulation of secondary organic aerosol (SOA) formation from linear, branched, and cyclic alkanes using the UNIPAR model. The UNIPAR model simulates SOA formation, for a given hydrocarbon precursor, via multiphase partitioning and considers heterogeneous particle-phase reactions. To do so, UNIPAR relies on a product distribution representing each SOA precursor HC, created by lumping the products from a semi-explicit gas oxidation mechanism into an array based on their volatility and reactivity. The addition of autoxidation reactions to the MCM mechanisms of linear alkanes C9-C12 is demonstrated to significantly improve SOA predictions. To simulate SOA formation from larger linear alkanes without MCM mechanisms available, an incremental volatility coefficient is used to unify and extrapolate existing product distributions. Additionally, branched alkane product distributions are represented with the product distribution of the linear alkane with the nearest vapor pressure. To account for the impact of methyl branches on autoxidation, the representative linear alkane product distribution is augmented by an autoxidation reduction factor which is a function of the degree and position of branching. Due to the unique oxidation products of cyclic alkanes, cyclohexane and decalin gas oxidation mechanisms are used to create their respective product distributions. The product distribution of cyclohexane and decalin are then extended to cyclic-branched species which feature cyclic structures with attached branches of varying lengths. Parameters developed for linear, branched, and cyclic species are validated against data collected from the UF-APHOR outdoor photochemical smog chamber.