Abstract
The secondary organic aerosol (SOA) formation from photooxidation of gasoline vapor was simulated by using the UNIfied Partitioning Aerosol phase Reaction (UNIPAR) model, which predicted SOA growth via multiphase reactions of hydrocarbons. The Carbon Bond 6 (CB6r3) mechanism was incorporated with the SOA model to estimate the hydrocarbon consumption and the concentration of radicals (i.e., RO2 and HO2), which were closely related to atmospheric aging of gas products. Oxygenated products were lumped according to their volatilities and reactivity and linked to stoichiometric coefficients and their physicochemical parameters, which were dynamically constructed at different NOx levels and degrees of gas aging. To assess the gasoline SOA potential in ambient air, model parameters were corrected for gas–wall partitioning (GWP), which was predicted by a qualitative structure activity relationship for explicit products. The simulated gasoline SOA mass was evaluated against observed data obtained in the UF-APHOR chamber under ambient sunlight. The influence of environmental conditions on gasoline SOA was characterized under varying NOx levels, aerosol acidity, humidity, temperature, and concentrations of aqueous salts and gasoline vapor. Both the measured and simulated gasoline SOA formation was sensitive to seeded conditions (acidity and hygroscopicity) and NOx levels. A considerable difference in SOA mass appeared before and after efflorescence relative humidity in the presence of salted aqueous solution. SOA growth in the presence of aqueous reactions was more impacted by temperature than that in absence of seed. The impact of GWP on SOA formation was generally significant, and it appeared to be higher in the absence of wet salts. We conclude that the SOA model in the corpus with both heterogeneous reactions and the model parameters corrected for GWP is essential to accurately predict SOA mass in ambient air.
Highlights
25 The atmospheric oxidation of hydrocarbons (HCs) produces ozone in the troposphere through a photochemical cycle of nitrogen oxides
To assess the gasoline secondary organic aerosol (SOA) potential in ambient air, model parameters were corrected for gas–wall partitioning (GWP), which was predicted by a qualitative structure activity relationship for explicit products
The oligomers in SOA formed from photooxidation of precursor HCs in chambers and the ambient air has been identified as 25-70% of the SOA mass (Kalberer et al, 2004;Tolocka et al, 2004;Gross et al, 2006;Kalberer et al, 2006;Hallquist et al, 2009)
Summary
25 The atmospheric oxidation of hydrocarbons (HCs) produces ozone in the troposphere through a photochemical cycle of nitrogen oxides. Pye et al (2017) evaluate the importance 50 of aerosol-water-organic interactions in the CMAQ model accounting for the uptake of water onto the hydrophilic organics (Pye et al, 2017) Despite such efforts, the performance of SOA formation in representing spatial and seasonal variation in ambient aerosol tends to underestimate total aerosol mass in the southern and western US (Appel et al, 2021). Many model studies reported that the significance of GWP varied with precursor HCs, gas oxidation process (different NOx levels or oxidants), chamber dimension and size, seed conditions, and meteorological conditions (temperature and humidity) (Zhang et al, 2014;Krechmer et al, 2020;Brune, 2019;Huang et al, 60 2018). The important feature of the 65 UNIPAR model is to simulate SOA formation via aqueous phase reactions of organic species in the absence of GWP. The sensitivity of gasoline SOA formation to various environmental conditions, such as temperature, relative humidity (RH), seed conditions, and the concentration of HC, was investigated
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