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

Abstract

Abstract. 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.

Multiphase reactions of hydrocarbons into an air quality model with CAMx-UNIPAR: Impacts of humidity and NOx on secondary organic aerosol formation in the Southern USA

“…The contribution of isoprene to the total SOA gradually increased from February to April. The dominance of alkane and terpene SOA has been previously reported in CAMx-UNIPAR simulations for the Central Valley, CA (Jo et al, 2023).…”

“…For example, biogenic HCs are rapidly consumed through reactions with various oxidants, such as OH radicals, nitrate radicals, and ozone, and thus quickly form SOA near their sources day and night. The spatial distribution of biogenic SOA also coincided with that of biogenic HCs emissions over the East Asia and the Central Valley (Jo et al, 2023;Yu et al, 2022). In contrast, anthropogenic HCs are relatively slow to be oxidized with OH radicals, mainly during the daytime, so that they can travel further from their sources by winds.…”

“…CAMx-UNIPAR simulations indicated that SOA formation in the Southern USA was dominated by alkanes, terpene, and isoprene (Figure 4). The significance of isoprene SOA is a unique feature in the Southern USA compared to that in the West and Mid-USA (Hu et al, 2015;Jo et al, 2023). The contribution of isoprene to the total SOA gradually increased from February to April.…”

“…For example, emissions of aromatic HCs were high in North and East China (Wang et al, 2021), but high aromatic SOA appeared in the Yellow Sea (Yu et al, 2022), indicating an eastward shift of aromatic HCs due to prevailing westerlies. In the Central Valley in CA, both alkane and aromatic SOA shifted from HCs sources toward the South of the Central Valley owing to the southward wind (Jo et al, 2023).…”

“…For example, SOA formation from long-chain alkanes, which are regarded as essential precursors for SOA formation (Aumont et al, 2012;Madhu, Jang, & Deacon, 2023), was missing in CAMx-UNIPAR v1.1. In addition, nighttime biogenic SOA, which is dominated by oxidation with ozone or nitrate radicals, was not addressed in manuscript submitted to Journal of Advances in Modeling Earth Systems Furthermore, Jo et al (2023) recently predicted SOA formation in the Central Valley, California (CA), USA, during the winter of 2018 using the updated CAMx-UNIPAR v1.2 . For air quality simulations, long-chain alkanes with different carbon lengths and nighttime chemistry of biogenic HCs (terpene, sesquiterpene, and isoprene) were added to CAMx-UNIPAR v1.2.…”