Simulating the SOA formation of isoprene from partitioning and aerosol phase reactions in the presence of inorganics

Abstract. The secondary organic aerosol (SOA) produced by the photooxidation of isoprene with and without inorganic seed is simulated using the Unified Partitioning Aerosol Phase Reaction (UNIPAR) model. Recent work has found the SOA formation of isoprene to be sensitive to both aerosol acidity ([H⁺], mol L⁻¹) and aerosol liquid water content (LWC) with the presence of either leading to significant aerosol phase organic mass generation and large growth in SOA yields (Y_SOA). Classical partitioning models alone are insufficient to predict isoprene SOA formation due to the high volatility of photooxidation products and sensitivity of their mass yields to variations in inorganic aerosol composition. UNIPAR utilizes the chemical structures provided by a near-explicit chemical mechanism to estimate the thermodynamic properties of the gas phase products, which are lumped based on their calculated vapor pressure (eight groups) and aerosol phase reactivity (six groups). UNIPAR then determines the SOA formation of each lumping group from both partitioning and aerosol phase reactions (oligomerization, acid-catalyzed reactions and organosulfate formation) assuming a single homogeneously mixed organic–inorganic phase as a function of inorganic composition and VOC ∕ NO_x (VOC – volatile organic compound). The model is validated using isoprene photooxidation experiments performed in the dual, outdoor University of Florida Atmospheric PHotochemical Outdoor Reactor (UF APHOR) chambers. UNIPAR is able to predict the experimental SOA formation of isoprene without seed, with H₂SO₄ seed gradually titrated by ammonia, and with the acidic seed generated by SO₂ oxidation. Oligomeric mass is predicted to account for more than 65 % of the total organic mass formed in all cases and over 85 % in the presence of strongly acidic seed. The model is run to determine the sensitivity of Y_SOA to [H⁺], LWC and VOC ∕ NO_x, and it is determined that the SOA formation of isoprene is most strongly related to [H⁺] but is dynamically related to all three parameters. For VOC ∕ NO_x  >  10, with increasing NO_x both experimental and simulated Y_SOA increase and are found to be more sensitive to [H⁺] and LWC. For atmospherically relevant conditions, Y_SOA is found to be more than 150 % higher in partially titrated acidic seeds (NH₄HSO₄) than in effloresced inorganics or in isoprene only.