Abstract We recently demonstrated that the heterogeneous hydroxyl radical (·OH) oxidation is an important aging process for isoprene epoxydiol-derived secondary organic aerosol (IEPOX-SOA) that alters its chemical composition, and thus, aerosol physicochemical properties. Notably, dimeric species in IEPOX-SOA were found to heterogeneously react with ·OH at a much faster rate than monomers, suggesting that the initial oligomeric content of freshly-generated IEPOX-SOA particles may affect its subsequent atmospheric oxidation. Aerosol acidity could in principle influence this aging process by enhancing the formation of sulfated and non-sulfated oligomers in freshly-generated IEPOX-SOA. Many multifunctional organosulfate (OS) products derived from heterogeneous ·OH oxidation of sulfur-containing IEPOX-SOA have been observed in cloud water residues and ice nucleating particles and could affect the ability of aged IEPOX-SOA particles to act as cloud condensation nuclei. Hence, this study systematically investigated the effect of aerosol acidity on the kinetics and products resulting from heterogeneous ·OH oxidation of IEPOX-SOA particles. Gas-phase IEPOX was reacted with inorganic sulfate particles of varying pH (0.5 to 2.0) in an indoor smog chamber operated under dark, steady-state conditions to form freshly-generated IEPOX-SOA particles. These particles were then aged at a relative humidity of 60% in an oxidation flow reactor (OFR) for 0-15 days of equivalent atmospheric ·OH exposure. Aged IEPOX-SOA particles were sampled by an online aerosol chemical speciation monitor (ACSM) to measure real-time aerosol mass and chemical changes of the SOA particles, and were also collected onto Teflon filters and into PILS vials for molecular-level chemical analyses by hydrophilic liquid interaction chromatography method interfaced to electrospray ionization high-resolution quadrupole time-of-flight mass spectrometry (HILIC/ESI-HR-QTOFMS), ion chromatography, and total OS mass amounts.
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Isoprene Epoxydiol-Derived Sulfated and Non-Sulfated Oligomers Suppress Particulate Mass Loss during Oxidative Aging of Secondary Organic Aerosol
Acid-driven multiphase chemistry of isoprene epoxydiols (IEPOX) with inorganic sulfate aerosols contributes substantially to formation of secondary organic aerosol (SOA), which constitutes a large mass fraction of atmospheric fine particulate matter (PM2.5). However, atmospheric chemical sinks of freshly generated IEPOX-SOA particles remain unclear. We examined the role of heterogeneous oxidation of freshly-generated IEPOX-SOA particles by gas-phase hydroxyl radical (•OH) under dark conditions as one potential atmospheric sink. After 4 h of gas-phase •OH exposure (~3x108 molecules cm-3), chemical changes in smog chamber-generated IEPOX-SOA particles were assessed by hydrophilic interaction liquid chromatography coupled with electrospray ionization high-resolution quadrupole time-of-flight mass spectrometry (HILIC/ESI-HR-QTOFMS). Comparison of molecular-level compositional changes in IEPOX-SOA particles during aging with or without •OH revealed that decomposition of oligomers by heterogeneous •OH oxidation acts as a sink for •OH and maintains a reservoir of low-volatility compounds including monomeric sulfate esters and oligomer fragments. We propose tentative structures and formation mechanisms for previously uncharacterized SOA constituents in PM2.5. Our results suggest that this •OH-driven renewal of low-volatility products may extend atmospheric lifetimes of IEPOX-SOA particles by slowing production of low-molecular weight, high-volatility organic fragments, and likely contributes to large quantities of 2-methyltetrols and methyltetrol sulfates reported in PM2.5.
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- Award ID(s):
- 2001027
- NSF-PAR ID:
- 10392011
- Date Published:
- Journal Name:
- AAAR 40th Annual Conference
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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We recently demonstrated that the heterogeneous hydroxyl radical (OH) oxidation is an important aging process for isoprene epoxydiol-derived secondary organic aerosol (IEPOX-SOA) that alters its chemical composition, and thus, aerosol physicochemical properties. Notably, dimeric species in IEPOX-SOA were found to heterogeneously react with OH at a much faster rate than monomers, suggesting that the initial oligomeric content of freshly-generated IEPOX-SOA particles may affect its subsequent atmospheric oxidation. Aerosol acidity could in principle influence this aging process by enhancing the formation of sulfated and non-sulfated oligomers in freshly-generated IEPOX-SOA. Many multifunctional organosulfate (OS) products derived from heterogeneous OH oxidation of sulfur-containing IEPOX-SOA have been observed in cloud water residues and ice nucleating particles and could affect the ability of aged IEPOX-SOA particles to act as cloud condensation nuclei. Hence, this study systematically investigated the effect of aerosol acidity on the kinetics and products resulting from heterogeneous OH oxidation of IEPOX-SOA particles. We reacted gas-phase IEPOX with inorganic sulfate particles of varying pH (0.5 to 2.5) in an indoor smog chamber operated under dark, steady-state conditions to form freshly-generated IEPOX-SOA particles. These particles were aged at a relative humidity of 65% in an oxidation flow reactor (OFR) for 0-21 days of equivalent atmospheric OH exposure. Through molecular-level chemical analyses by hydrophilic interaction liquid chromatography method interfaced to electrospray ionization high-resolution quadrupole time-of-flight mass spectrometry (HILIC/ESI-HR-QTOFMS), we observed that highly acidic aerosol has higher oligomer ratio and exhibit much slower mass decay with OH oxidation (pH=0.5, lifetime = 56 days) as compared to less acidic aerosols (pH=2.5, lifetime=17 days). Based on atomic force microscopy (AFM) analysis, aerosol acidity could also affect the morphology and viscosity of IEPOX-SOA during OH oxidation process.more » « less
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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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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.more » « less
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Secondary organic aerosol (SOA) is composed of a significant fraction of low-volatility high-molecular-weight oligomer products. These species can affect particle viscosity, morphology, and mixing timescales, yet they are not very well understood. While strides have been made in elucidating oligomer formation mechanisms, their degradation is less studied. Previous work suggests that the presence of oligomers may suppress particle mass loss during atmospheric aging by slowing the production high-volatility fragments from monomers. Our work investigates the effects of relative humidity (RH) on oligomer formation in SOA and the effects of hydroxyl radical (·OH) exposure on oligomer degradation. To probe these questions, SOA is generated by the reactive uptake of isoprene epoxydiols (IEPOX) onto acidic ammonium sulfate aerosol in a 2-m3 steady-state chamber, followed by exposure to ·OH in an oxidation flow reactor. We investigate SOA formation at 30-80% RH, which is above and below the deliquescence point of ammonium sulfate. We examine the evolution of SOA bulk chemical composition as well as single-particle physicochemical properties over the course of aging using mass spectrometry-, spectroscopy-, and microscopy-based techniques. An optimized matrix-assisted laser desorption ionization mass spectrometry (MALDI-MS) method is used to identify and track the presence of oligomers in SOA over the course of aging. Our research will provide insight about the formation and degradation of oligomers in the atmosphere, which will allow better modeling of their impact on climate.more » « less
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