Abstract. Branched alkanes represent a significant proportion of hydrocarbons emitted in urban environments. To accurately predict the secondary organic aerosol (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 a volatility- and reactivity-based αi 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 αi array of each branched alkane was primarily constructed using an existing αi 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 number. In addition, increasing the number of alkyl branches 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 that 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 with a wide range of carbon numbers. Overall, branched alkanes accounted for a higher proportion of SOA mass than linear alkanes due to their higher contribution to diesel fuel.

中文翻译：

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通过烷烃的多相反应模拟碳支化结构对二次有机气溶胶形成的影响

摘要。支链烷烃占城市环境中排放的碳氢化合物的很大一部分。为了准确预测城市环境中的二次有机气溶胶 (SOA) 预算，这些支链烷烃应被视为 SOA 前体。然而，目前尚不清楚在不同环境条件下由不同支链烷烃形成 SOA 的潜力。在这项研究中，统一分配气溶胶相反应 (UNIPAR) 模型被扩展为通过各种支链烷烃的多相反应来预测 SOA 的形成。 UNIPAR 模型处理多相分配和气溶胶相反应以形成 SOA，需要根据明确的气体动力学机制预测产物分布，其含氧产物用于创建基于挥发性和反应性的 αi 物种阵列。由于缺乏实际适用的明确气体机理，因此采用考虑碳长度和支化结构的创新方法来预测各种支链烷烃的产物分布。每个支链烷烃的 αi 阵列主要使用现有的具有最接近蒸气压的直链烷烃 αi 阵列构建。一般来说，支链烷烃及其氧化产物的蒸气压低于相同碳数的直链烷烃。此外，增加烷基支链的数量还会降低烷烃进行自氧化反应的能力，该反应往往会形成低挥发性产物，并显着促进烷烃 SOA 的形成。为了解释这一点，将作为支化程度和位置函数的自氧化还原因子应用于包含自氧化产物的集总基团。然后将所得的产物分布应用于 UNIPAR 模型，以预测支链烷烃 SOA 的形成。将模拟的 SOA 质量与室外光化学烟雾室中不同实验条件（即 NOx 水平、种子条件和湿度）下生成的 SOA 数据进行比较。支链烷烃 SOA 产量受 NOx 水平的显着影响，但受种子条件或湿度的影响不显着。对柴油燃料中支链烷烃和直链烷烃形成 SOA 进行了模拟，以了解具有宽碳数范围的支链烷烃和直链烷烃的相对重要性。总体而言，支链烷烃在 SOA 质量中所占的比例高于直链烷烃，因为它们对柴油的贡献更大。