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Abstract. Secondary organic aerosol (SOA) from diesel fuel is known to besignificantly sourced from the atmospheric oxidation of aliphatichydrocarbons. In this study, the formation of linear alkane SOA waspredicted using the Unified Partitioning Aerosol Phase Reaction (UNIPAR)model that simulated multiphase reactions of hydrocarbons. In the model, theformation of oxygenated products from the photooxidation of linear alkaneswas simulated using a nearly explicit gas kinetic mechanism. Autoxidationpaths integrated with alkyl peroxy radicals were added to the MasterChemical Mechanism v3.3.1 to improve the prediction of low-volatilityproducts in the gas phase and SOA mass. The resulting gas products were thenlumped into volatility- and reactivity-based groups that are linked to mass-basedstoichiometric coefficients. The SOA mass in the UNIPAR model is producedvia three major pathways: partitioning of gaseous oxidized products ontoboth the organic and wet inorganic phases, oligomerization in the organic phase,and reactions in the wet inorganic phase (acid-catalyzed oligomerization andorganosulfate formation). The model performance was demonstrated for SOAdata that were produced through the photooxidation of a homologous series oflinear alkanes ranging from C9–C15 under varying environments (NOxlevels and inorganic seed conditions) in a large outdoor photochemical smogchamber. The product distributions of linear alkanes were mathematicallypredicted as a function of carbon number using an incremental volatilitycoefficient (IVC) to cover a wide range of alkane lengths. The prediction ofalkane SOA using the incremental volatility-based product distributions,which were obtained with C9–C12 alkanes, was evaluated for C13and C15 chamber data and further extrapolated to predict the SOA from longer-chain alkanes (≥ C15) that can be found in diesel. The model simulationof linear alkanes in diesel fuel suggests that SOA mass is mainly producedby alkanes C15 and higher. Alkane SOA is insignificantly impacted by thereactions of organic species in the wet inorganic phase due to thehydrophobicity of products but significantly influenced by gas–particlepartitioning. 

The UNIfied Partitioning-Aerosol phase Reaction (UNIPAR) model was established on the Comprehensive Air quality Model with extensions (CAMx) to process Secondary Organic Aerosol (SOA) formation by capturing multiphase reactions of hydrocarbons (HCs) in regional scales. SOA growth was simulated using a wide range of anthropogenic HCs including ten aromatics and linear alkanes with different carbon-lengths. The atmospheric processes of biogenic HCs (isoprene, terpenes, and sesquiterpene) were simulated for the major oxidation paths (ozone, OH radicals, and nitrate radicals) to predict day and night SOA formation. The UNIPAR model streamlined the multiphase partitioning of the lumping species originating from semi-explicitly predicted gas products and their heterogeneous chemistry to form non-volatile oligomeric species in both organic aerosol and inorganic aqueous phase. The CAMx-UNIPAR model predicted SOA formation at four ground urban sites (San Jose, Sacramento, Fresno, and Bakersfield) in California, United States during wintertime 2018. Overall, the simulated mass concentrations of the total organic matter, consisting of primary OA (POA) and SOA, showed a good agreement with the observations. The simulated SOA mass in the urban areas of California was predominated by alkane and terpene. During the daytime, low-volatile products originating from the autoxidation of long-chain alkanes considerably contributed to the SOA mass. In contrast, a significant amount of nighttime SOA was produced by the reaction of terpene with ozone or nitrate radicals. The spatial distributions of anthropogenic SOA associated with aromatic and alkane HCs were noticeably affected by the southward wind direction owing to the relatively long lifetime of their atmospheric oxidation, whereas those of biogenic SOA were nearly insensitive to wind direction. During wintertime 2018, the impact of inorganic aerosol hygroscopicity on the total SOA budget was not evident because of the small contribution of aromatic and isoprene products that are hydrophilic and reactive in the inorganic aqueous phase. However, an increased isoprene SOA mass was predicted during the wet periods, although its contribution to the total SOA was little.