2-Dimensional Model of SOA Formation from the Photooxidation of Linear Alkanes Using Volatility and Reactivity
AZAD MADHU, Myoseon Jang, David Deacon,
University of Florida Abstract Number: 423
Working Group: Urban Aerosols
AbstractAlkanes are a significant component of emissions from diesel fuel and they must be considered when predicting secondary organic aerosol (SOA) formation in urban environments. In this study, SOA formation is predicted using the Unified Partitioning Aerosol Phase Reaction (UNIPAR) model via the multiphase reactions of various linear alkanes. The model used explicit gas kinetic mechanisms (Master Chemical Mechanism v3.3.1, MCM) of linear alkanes, with autoxidation pathways of alkoxy radical species added, to predict the formation of oxidized products. The resulting oxidized products from the gas mechanism were lumped into an array based on their volatility and reactivity. This product distribution was used to predict the formation of SOA with the three major pathways used by the UNIPAR model: gas-particle partitioning, organic phase oligomerization, and acid catalyzed oligomerization and organosulfate formation in the inorganic phase. Autoxidation reactions served to increase the production of low volatility compounds in the gas phase which consequently increased partitioning to the particle-phase. The model predictions are compared to data collected from an outdoor photochemical smog chamber for linear alkanes C9-C15 under varying environmental conditions (NO
x levels, temperature, and inorganic seed conditions). Because MCM does not currently have explicit gas kinetic mechanisms for linear alkanes larger than C12, the shift in volatility between product distributions for alkanes C9-C12 was mathematically characterized as a function of carbon number using an incremental volatility coefficient. This incremental volatility coefficient was used to predict product distribution of alkanes C13-C15 which were subsequently used to predict SOA formation and compared to chamber data. Further extrapolation was performed to predict the possible SOA formation of larger linear alkanes (>C15) that can be found in diesel. Alkane SOA was found to be insignificantly impacted by particle-phase reactions but significantly impacted by gas-particle partitioning.