Isoprene has a strong effect on the oxidative capacity of the troposphere due to its abundance. Under low-NOx conditions, isoprene oxidizes to form isoprene-derived epoxydiols (IEPOX), contributing significantly to secondary organic aerosol (SOA) through heterogeneous reactions. In particular, organosulfates (OSs) can form from acid-driven reactive uptake of IEPOX onto preexisting particles followed by nucleophilic addition of inorganic sulfate, and they are an important component of SOA mass, primarily in submicron particles with long atmospheric lifetimes. Fundamental understanding of SOA and OS evolution in particles, including the formation of new compounds by oxidation as well as corresponding viscosity changes, is limited, particularly across relative humidity (RH) conditions above and below the deliquescence of typical sulfate aerosol particles. In a 2-m3 indoor chamber held at various RH values (30 – 80%), SOA was generated from reactive uptake of gas-phase IEPOX onto acidic ammonium sulfate aerosols (pH = 0.5 – 2.5) and then aged in an oxidation flow reactor (OFR) for 0 – 24 days of equivalent atmospheric ·OH exposure. We investigated the extent of inorganic sulfate conversion to organosulfate, formation of oligomers, single-particle physicochemical properties, such as viscosity and phase state, and oxidation kinetics. Chemical composition of particle-phase species, as well as aerosol morphological changes, are analyzed as a function of RH, oxidant exposure times, and particle acidity to better understand SOA and OS formation and destruction mechanisms in the ambient atmosphere.
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Organosulfates from Dark Aqueous Reactions of Isoprene-Derived Epoxydiols Under Cloud and Fog Conditions: Kinetics, Mechanism, and Effect of Reaction Environment on Regioselectivity of Sulfate Addition
Atmospheric oxidation of isoprene yields large quantities of highly water-soluble isoprene epoxydiols (IEPOX) that partition into fogs, clouds, and wet aerosols. In aqueous aerosols, the acid-catalyzed ring-opening of IEPOX followed by nucleophilic addition of inorganic sulfate or water forms organosulfates and 2-methyltetrols, respectively, contributing substantially to secondary organic aerosol (SOA). However, the fate of IEPOX in clouds, fogs, and evaporating hydrometeors is not well understood. Here we investigate the rates, product branching ratios, and stereochemistry of organosulfates from reactions of dilute IEPOX (5–10 mM) under a range of sulfate concentrations (0.3–50 mM) and pH values (1.83–3.38) in order to better understand the fate of IEPOX in clouds and fogs. From these aqueous dark reactions of β-IEPOX isomers (trans- and cis-2-methyl-2,3-epoxybutane-1,4-diols), which are the predominant IEPOX isomers, products were identified and quantified using hydrophilic interaction liquid chromatography coupled to an electrospray ionization high-resolution quadrupole time-of-flight mass spectrometer operated in negative ion mode (HILIC/(−)ESI-HR-QTOFMS). We found that the regiochemistry and stereochemistry were affected by pH, and the tertiary methyltetrol sulfate (C5H12O7S) was promoted by increasing solution acidity. Furthermore, the rate constants for the reaction of IEPOX under cloud-relevant conditions are up to 1 order of magnitude lower than reported in the literature for aerosol-relevant conditions due to a markedly different solution activities. Nevertheless, the contribution of cloud and fog water reactions to IEPOX SOA may be significant in cases of lower aqueous-phase pH (model estimate) or during droplet evaporation (not studied).
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- NSF-PAR ID:
- 10214927
- Editor(s):
- Blum, Joel
- Date Published:
- Journal Name:
- ACS Earth and Space Chemistry
- ISSN:
- 2472-3452
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Kinetics and Products of Heterogeneous Hydroxyl Radical Oxidation of Isoprene Epoxydiol‐Derived SOA.In isoprene‐rich regions, acid‐catalyzed multiphase reactions of isoprene epoxydiols (IEPOX) with inorganic sulfate (Sulfinorg) particles form secondary organic aerosol (IEPOX‐SOA), extensively converting Sulfinorg to lowervolatility particulate organosulfates (OSs), including 2‐ methyltetrol sulfates (2‐MTSs) and their dimers. Recently, we showed that heterogeneous hydroxyl radical (OH) oxidation of particulate 2‐MTSs generated multifunctional OS products. However, atmospheric models assume that OS‐rich IEPOX‐SOA particles remain unreactive towards heterogeneous OH oxidation, and limited laboratory studies have been conducted to examine the heterogeneous OH oxidation kinetics of full IEPOX‐SOA mixtures. Hence, this study investigated the kinetics and products resulting from heterogeneous OH oxidation of freshly‐generated IEPOXSOA in order to help derive model‐ready parameterizations. First, gas‐phase IEPOX was reacted with acidic Sulfinorg particles under dark conditions in order to form fresh IEPOX‐SOA particles. These particles were then subsequently aged at RH of 56% in an oxidation flow reactor at OH exposures ranging from 0~15 days of equivalent atmospheric exposure. Aged IEPOX‐SOA particles were sampled by an online aerosol chemical speciation monitor (ACSM) and collected onto Teflon filters for off‐line molecular‐level chemical analyses by hydrophilic liquid interaction chromatography method interfaced to electrospray ionization high‐resolution quadrupole time‐offlight mass spectrometry (HILIC/ESI‐HR‐QTOFMS). Our results show that heterogeneous OH oxidation only caused a 7% decay of IEPOX‐SOA by 10 days exposure, likely owing to the inhibition of reactive uptake of OH as fresh IEPOXSOA particles have an inorganic core‐organic shell morphology. A significantly higher fraction of IEPOX‐SOA (~37%) decayed by 15 days exposure, likely due to the increasing reactive uptake of OH as IEPOX‐SOA become more liquid‐like with aging. Freshly‐generated IEPOX‐SOA constituents exhibited varying degrees of aging with 2‐MTSdimers being the most reactive, followed by 2‐MTSs and 2‐ methyltetrols (2‐MTs), respectively. Notably, extensive amounts of previously characterized particle‐phase products in ambient fine aerosols were detected in our laboratory‐aged IEPOX‐SOA samples.more » « less
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1021/acsearthspacechem.0c00293
