Modeling the Influence of Carbon Branching Structure on SOA Formation of Alkanes

AZAD MADHU, Myoseon Jang, University of Florida

     Abstract Number: 332
     Working Group: Urban Aerosols

Abstract
Branched alkanes represent significant proportions of hydrocarbon emissions in urban environments and are important to be considered with regard to Secondary Organic Aerosol (SOA) formation. However, the prediction of the SOA formation from branched alkanes is complex due to the variety in carbon lengths and branching structures. In this study, the Unified Partitioning Aerosol Phase Reaction (UNIPAR) model, which can predict SOA mass via multiphase reactions of hydrocarbons, is extended to simulate the SOA formation of various branched alkanes. SOA simulations with the UNIPAR model require a product distribution which is created by lumping explicitly predicted products according to their volatility and reactivity. In order to account for the large variation in branched alkane structures, the lumping array was predicted as a function of carbon lengths and branching ratios in the model. The product distribution of each branched alkane was constructed using an existing array of the linear alkane with the closest vapor pressure. In addition to causing a decrease in vapor pressure, increasing the branching ratio of an alkane can also decrease the ability of alkanes to undergo autoxidation reactions, which serves to reduce both the formation of low-volatility products and SOA yields. An autoxidation reduction factor, as a function of branching ratio, is applied to the lumped groups within the product distribution array which include autoxidation products. The resulting product distributions are then applied to predict branched alkane SOA formation using the UNIPAR model and compared to SOA data generated in an outdoor photochemical smog chamber. Additionally, the SOA formation from branched and linear alkanes in diesel fuel was simulated to show the relative importance of anthropogenically emitted branched and linear alkanes within a wide range of carbon numbers.