28 Aug 2023
 | 28 Aug 2023

Modeling the influence of carbon branching structure on SOA formation via multiphase reactions of alkanes

Azad Madhu, Myoseon Jang, and Yujin Jo

Abstract. Branched alkanes represent a significant proportion of hydrocarbons emitted in urban environments. To accurately predict the SOA budgets in urban environments, these branched alkanes should be considered as SOA precursors. However, the potential to form SOA from diverse branched alkanes under varying environmental conditions is currently not well understood. In this study, the Unified Partitioning Aerosol Phase Reaction (UNIPAR) model is extended to predict SOA formation via the multiphase reactions of various branched alkanes. Simulations with the UNIPAR model, which processes multiphase partitioning and aerosol phase reactions to form SOA, require a product distribution predicted from an explicit gas kinetic mechanism, whose oxygenated products are applied to create volatility-reactivity based lumping species array. Due to a lack of practically applicable explicit gas mechanisms, the prediction of the product distributions of various branched alkanes was approached with an innovative method that considers carbon lengths and branching structures. The lumping array of each branched alkane was primarily constructed using an existing lumping array of the linear alkane with the nearest vapor pressure. Generally, the vapor pressures of branched alkanes and their oxidation products are lower than those of linear alkanes with the same carbon length. In addition, increasing the branching of an alkane can also decrease the ability of alkanes to undergo autoxidation reactions that tend to form low-volatility products and significantly contribute to alkane SOA formation. To account for this, an autoxidation reduction factor, as a function of the degree and position of branching, was applied to the lumped groups which contain autoxidation products. The resulting product distributions were then applied to the UNIPAR model for predicting branched alkane SOA formation. The simulated SOA mass was compared to SOA data generated under varying experimental conditions (i.e., NOx levels, seed conditions, and humidity) in an outdoor photochemical smog chamber. Branched alkane SOA yields were significantly impacted by NOx levels but insignificantly impacted by seed conditions or humidity. The SOA formation from branched and linear alkanes in diesel fuel was simulated to understand the relative importance of branched and linear alkanes in a wide range of carbon numbers. Overall, branched alkanes account for more SOA mass than linear alkanes due to their higher contribution to diesel fuel. As anthropogenic emissions of hydrocarbons decrease, biogenic precursors tend to become increasingly important with regard to atmospheric organic aerosol. Unlike aromatics, which are almost exclusively sourced from anthropogenic emissions, alkanes can also be emitted from biogenic sources (i.e. plant wax) and they will remain a significant source of SOA.

Azad Madhu, Myoseon Jang, and Yujin Jo

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on egusphere-2023-1500', Anonymous Referee #1, 19 Sep 2023
  • RC2: 'Comment on egusphere-2023-1500', Anonymous Referee #2, 05 Nov 2023
  • AC1: 'Comment on egusphere-2023-1500', Myoseon Jang, 18 Dec 2023
Azad Madhu, Myoseon Jang, and Yujin Jo
Azad Madhu, Myoseon Jang, and Yujin Jo


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Short summary
SOA formation from branched alkanes is simulated using the UNIPAR model which predicted SOA growth via multiphase reactions of hydrocarbons and compared with chamber data. Product distributions of branched alkanes were created by extrapolating product distributions of linear alkanes. To account for methyl branching, an autoxidation reduction factor was applied to product distributions. Branched alkanes in diesel fuel were shown to produce a higher proportion of SOA compared to linear alkanes.