Organic aerosol formation from the reactive uptake of isoprene epoxydiols (IEPOX) onto non-acidified inorganic seeds

Abstract. The reactive partitioning of cis and trans β-IEPOX was investigated on hydrated inorganic seed particles, without the addition of acids. No organic aerosol (OA) formation was observed on dry ammonium sulfate (AS); however, prompt and efficient OA growth was observed for the cis and trans β-IEPOX on AS seeds at liquid water contents of 40–75% of the total particle mass. OA formation from IEPOX is a kinetically limited process, thus the OA growth continues if there is a reservoir of gas-phase IEPOX. There appears to be no differences, within error, in the OA growth or composition attributable to the cis / trans isomeric structures. Reactive uptake of IEPOX onto hydrated AS seeds with added base (NaOH) also produced high OA loadings, suggesting the pH dependence for OA formation from IEPOX is weak for AS particles. No OA formation, after particle drying, was observed on seed particles where Na⁺ was substituted for NH₄⁺. The Henry's Law partitioning of IEPOX was measured on NaCl particles (ionic strength ~9 M) to be 3 × 10⁷ M atm⁻¹ (−50 / +100%). A small quantity of OA was produced when NH₄⁺ was present in the particles, but the chloride (Cl^-) anion was substituted for sulfate (SO₄^2-), possibly suggesting differences in nucleophilic strength of the anions. Online time-of-flight aerosol mass spectrometry and offline filter analysis provide evidence of oxygenated hydrocarbons, organosulfates, and amines in the particle organic composition. The results are consistent with weak correlations between IEPOX-derived OA and particle acidity or liquid water observed in field studies, as the chemical system is nucleophile-limited and not limited in water or catalyst activity.