Modeling the Influence of Chain Length on SOA Formation via Multiphase Reactions of Alkanes

Abstract

Abstract. Secondary Organic Aerosol (SOA) from diesel fuel is known to be significantly sourced from the atmospheric oxidation of aliphatic hydrocarbons. In this study, the formation of linear-alkane SOA was predicted using the Unified Partitioning Aerosol Phase Reaction (UNIPAR) model that simulated multiphase reactions of hydrocarbons. In the model, the formation of oxygenated products from the photooxidation of linear alkanes was simulated using a near-explicit gas kinetic mechanism. Autoxidation paths integrated with alkyl peroxy radicals were added to the Master Chemical Mechanismv3.3.1 to improve the formation of low volatility products in the gas phase and better predict SOA mass. The resulting gas products were then classified into volatility-reactivity based lumping groups that are linked to mass-based stoichiometric coefficients. The SOA mass in the UNIPAR model is produced via three major pathways: partitioning of gaseous oxidized products onto both the organic and wet inorganic phases; oligomerization in organic phase; and reactions in the wet inorganic phase (acid-catalyzed oligomerization and organosulfate formation). The model performance was demonstrated for SOA data that were produced through the photooxidation of a homologous series of linear alkanes ranging from C9 to C15 under varying environments (NOx levels, temperature, and inorganic seed conditions) in a large outdoor photochemical smog chamber. The product distributions of linear alkanes were mathematically predicted as a function of carbon number using an incremental volatility coefficient (IVC) to cover a wide range of alkane lengths. The prediction of alkane SOA using the incremental volatility-based product distributions, which were obtained with C9–C12 alkanes, was evaluated for prediction of C13 and C15 chamber data and further extrapolated to predict the SOA from longer chain alkanes (C15) that can be found in diesel. The model simulation of linear alkanes in diesel fuel suggests that SOA mass is mainly produced by alkanes C15 and higher. Alkane SOA is insignificantly impacted by the reactions of organic species in the wet inorganic phase due to the hydrophobicity of products, but significantly influenced by gas-particle partitioning.

CAMx-UNIPAR Simulation of SOA Mass Formed from Multiphase Reactions of Hydrocarbons under the Central Valley Urban Atmospheres of California

“…For example, chamber studies showed that aromatic SOA yields increased with decreasing the NOx level within the NOx-rich region (Yang et al, 2020). A 415 gradual increase in alkane SOA was reported in the recent chamber study by Madhu et al (2022) with decreasing the NOx level over both NOx-rich and NOx-poor regions. The reduction of NOx in the NOx-rich region dramatically decreases nighttime biogenic SOA yields owing to the reaction of biogenic HCs with decreased nitrate radicals, but the reduced NOx increases daytime biogenic SOA yields (Han and Jang, 2023).…”

“…The structure of the UNIPAR SOA model has been described previously (Cao and Jang, 2010;Im et al, 2014;Beardsley and Jang, 2016;Zhou et al, 2019;Yu et al, 2021c;Yu et al, 2022;Choi and Jang, 2022;Madhu et al, 2022;Han and Jang, 2023). Briefly, the key components of the UNIPAR model are described as follow:…”

“…Thus, their solubility in the aqueous phase is low due to their large activity coefficients. Evidently, α-pinene SOA and alkane SOA have a low O:C ratio (~0.55) (Zhang et al, 2015;Galeazzo 365 et al, 2021;Madhu et al, 2022) compared with isoprene SOA (~1.1) (Song et al, 2015). In order to evaluate the sensitivity of different SOA species to aerosol hygroscopicity, the isoprene to alkane SOA mass ratios were calculated during the wet and dry periods as shown in Fig.…”

“…Long-chain alkanes are regarded as essential precursors for SOA formation (Aumont et al, 2012;Madhu et al, 2022). Studies have shown that the formation of low volatility products via peroxy radical autoxidation significantly attributed to terpene SOA mass (Xavier et al, 2019;Bianchi et al, 2019).…”

“…Studies have shown that the formation of low volatility products via peroxy radical autoxidation significantly attributed to terpene SOA mass (Xavier et al, 2019;Bianchi et al, 2019). Madhu et al (2022) recently extended an 85 autoxidation mechanism to alkane semi-explicit oxidation mechanisms, improving the predictability of alkane SOA using the UNIPAR model against their chamber study. In addition, the UNIPAR model has recently been expanded to simulate biogenic SOA based on three major paths (i.e., OH radicals, ozone and nitrate radicals) being capable of nighttime SOA formation (Han and Jang, 2023) that was dominated by oxidation with ozone and nitrate radicals.…”