Abstract
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 (SO42-), 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.
Highlights
A significant portion of the organic aerosol (OA) production from isoprene, a non-methane hydrocarbon emitted to the atmosphere in vast amounts, is attributed to the heterogeneous chemistry of isoprene epoxydiols (IEPOX) (Froyd et al, 2010; Chan et al, 2010; McNeill et al, 2012; Surratt et al, 2010; Hatch et al, 2011; Budisulistiorini et al, 2013; Pye et al, 2013)
For relative humidity (RH) conditions above the ammonium sulfate (AS) efflorescence point tested in this work (ERH ∼35 %, (Biskos et al, 2006)), prompt and efficient OA growth onto AS seeds was observed for both IEPOX isomers
The weak dependence on pH and the strong dependence on nucleophile activity and particle liquid water suggest that the IEPOX reactions in hydrated ammonium salts are nucleophile-limited, rather than catalyst-limited
Summary
A significant portion of the organic aerosol (OA) production from isoprene, a non-methane hydrocarbon emitted to the atmosphere in vast amounts, is attributed to the heterogeneous chemistry of isoprene epoxydiols (IEPOX) (Froyd et al, 2010; Chan et al, 2010; McNeill et al, 2012; Surratt et al, 2010; Hatch et al, 2011; Budisulistiorini et al, 2013; Pye et al, 2013). The mechanism for OA production from IEPOX has been proposed as ring-opening of the epoxide group, activated by proton transfer from a strong acid such as sulfuric acid (H2SO4), followed by nucleophilic addition of available nucleophiles in the condensed phase, e.g., addition of water to produce tetrols, sulfate to produce organosulfates, and so on (Minerath et al, 2008; Eddingsaas et al, 2010; Surratt et al, 2010) This proposed mechanism has been corroborated by chamber investigations of particle acidity effects on OA formation (Surratt et al, 2007; Lin et al, 2012), wherein dry acidic seeds (MgSO4 : H2SO4, 1 : 1) prompted strong reactive uptake behavior from epoxides (Paulot et al, 2009b), compared to negligible uptake for dry, non-acidified seeds. Nguyen et al.: Organic aerosol formation from the reactive uptake of IEPOX
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