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

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.

Airborne observations of IEPOX-derived isoprene SOA in the Amazon during SAMBBA

“…The role of aerosol acidity has long been seen as necessary for uptake (Surratt et al, 2007b;Eddingsaas et al, 2010) and this conclusion was supported by observational data from this region presented by Claeys et al (2010), although recent laboratory results from Nguyen et al (2014) show that it is possible to form SOA with the m/z = 82 marker from IEPOX on a neutral ammonium sulfate seed, providing it is aqueous, as opposed to solid. This occurs through ammonium ions (NH + 4 ) acting to catalyse the uptake in the absence of a proton donor.…”

“…Likewise, it could also be inferred that if B749 and B750 had been less humid, formation could have still taken place, due to the aerosols' acidity. While there is considerable interest in the literature concerning the role of sulfur and nitrogen in the formation mechanisms (Nguyen et al, 2014;Gomez-Gonzalez et al, 2008;Surratt et al, 2007a), it should be noted that only very limited conclusions can be drawn here. In addition to the M 30 /M 46 data possibly indicating some role of organic nitrogen, the sulfate to rBC ratio shows a positive gradient with altitude within the boundary layer in B749 (albeit a weaker gradient than the equivalent organic ratio).…”

“…The former humidity range is within the metastable regime for pure ammonium sulfate and according to the work of Song et al (2013), internally mixed ammonium sulfate and organic particles could effloresce under these conditions. As such, it is possible that the inorganic matter during B749 and B750 existed as a solid or as a non-aqueous organic liquid, which would inhibit the formation through the neutral mechanism of Nguyen et al (2014). Note that the acidic mechanism explored by Surratt et al (2007b) was shown to be effective at 30 % RH, so it could be inferred that had the aerosol in B744 and B745 been acidic, formation would have taken place, regardless of humidity.…”

“…As well as the phase effect on inorganic ion chemistry described by Nguyen et al (2014), the organic fraction is known to potentially exhibit exotic behaviours such as forming amorphous states and separations from the aqueous phase (Virtanen et al, 2010;Song et al, 2013), which may affect how the particles interact with the gas phase. This is a difficult concept to test with the data available, as there are no measurement data on the hygroscopicity of the particles or the exact phase of the organic fraction.…”

The Caltech Photooxidation Flow Tube reactor: design, fluid dynamics and characterization

“…The CPOT has been constructed as a complement to the Caltech 24 m 3 batch chambers (Bates et al, , 2016Schilling et al, 2015;Hodas et al, 2015;Loza et al, 2013Loza et al, , 2014McVay et al, 2014McVay et al, , 2016Nguyen et al, 2014Nguyen et al, , 2015Schwantes et al, 2015;Yee et al, 2013;Zhang et al, 2014; in carrying out studies of SOA formation resulting from the oxidation of volatile organic compounds (VOCs) by oxidants OH, O 3 and NO 3 over timescales not accessible in a batch chamber. Due to its steady-state operation, the CPOT also affords the capability to collect sufficient quantities of SOA generated in the reactor for comprehensive composition determination by offline mass spectrometry.…”