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Heterogeneous hydroxyl radical (•OH) oxidation is an important aging process for isoprene epoxydiol-derived secondary organic aerosol (IEPOX-SOA) that alters its chemical composition. It was recently demonstrated that heterogeneous •OH oxidation can age single-component particulate methyltetrol sulfates (MTSs), causing ∼55% of the SOA mass loss. However, our most recent study of freshly generated IEPOX-SOA particulate mixtures suggests that the lifetime of the complete IEPOX-SOA mixture against heterogeneous •OH oxidation can be prolonged through the fragmentation of higher-order oligomers. Published studies suggest that the heterogeneous •OH oxidation of IEPOX SOA could affect the organic atmospheric aerosol budget at varying rates, depending on aerosol chemical composition. However, heterogeneous •OH oxidation kinetics for the full IEPOX-SOA particulate mixture have not been reported. Here, we exposed freshly generated IEPOX-SOA particles to heterogeneous oxidation by •OH under humid conditions (relative humidity ∼57%) for 0−15 atmospheric-equivalent days of aging and derived an effective heterogeneous •OH rate coefficient (kOH) of 2.64 ± 0.4 × 10−13 cm^3 molecules−1 s−1. While ∼44% of particulate organic mass of nonoxidized IEPOX-SOA was consumed over the entire 15 day aging period, only <7% was consumed during the initial 10 aging days. By molecular-level chemical analysis, we determined oligomers were consumed at a faster rate (by a factor of 2−4) than monomers. Analysis of aerosol physicochemical properties shows that IEPOX-SOA has a core−shell morphology, and the shell becomes thinner with •OH oxidation. In summary, this study demonstrates that heterogeneous •OH oxidation of IEPOX-SOA particles is a dynamic process in which aerosol chemical composition and physicochemical properties play important roles. 

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. 

Isoprene is one of the most common biogenic volatile organic compounds (BVOC) in the atmosphere, produced by many plants. Isoprene undergoes oxidation to form gaseous isoprene epoxydiols (IEPOX) under low-NOx conditions, which can lead to the formation of secondary organic aerosol (SOA) particles. SOA-containing particles affect climate by scattering and absorbing solar radiation or acting as cloud condensation nuclei (CCN). High concentrations of SOA are also associated with adverse health impacts in people. While in the atmosphere, IEPOX SOA particles continue to undergo reactions with atmospheric oxidants, including hydroxyl radical (OH). To isolate and probe this process, we studied atmospheric chemical processes in an aerosol chamber to better understand the evolution of heterogeneous OH oxidation of IEPOX-derived SOA particles. Since very little is understood about the structural and spectroscopic properties because of the complexity of their many sources and atmospheric processing, individual particle measurements are necessary to provide better understanding of the composition of IEPOX SOA. We injected particles composed of mixtures of ammonium sulfate and sulfuric acid across a range of acidities(PH = 0.5 – 2.5) and gas-phase IEPOX into the chamber to generate SOA. The SOA particles were then sent to an oxidation flow reactor, and exposed to different OH concentrations representative of aging of a number of days. We kept relative humidity (RH) constant at ~65%, the temperature was ~23 °C, and levels of oxidation were controlled by adjusting lamp intensity. After oxidized SOA was impacted on quartz substrates, we used single-particle Raman microspectroscopy to identify their functional group compositions. From the Raman vibrational spectra of submicron particles (~500-1000 nm aerodynamic diameter), we observed a distinct difference in core-shell morphology and composition: an organic outer layer and an aqueous-inorganic core. The core also has significantly more CH-stretch than the shell. Small changes were also observed with increasing oxidation, which are important to consider when predicting SOA particle evolution in the atmosphere. 

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. 

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. 

Organosulfates (OSs) formed from heterogeneous reactions of organic-derived oxidation products with sulfate ions are an important component of secondary organic aerosol (SOA) mass, primarily in submicron particles with long atmospheric lifetimes. Fundamental understanding of OS evolution in particles, including the formation of new compounds via oxidation, is limited, particularly across relative humidities above and below the deliquescence of typical sulfate aerosol particles. By generating aqueous particulate OSs and other SOA products from the acid-driven reactive uptake of isoprene epoxydiols (IEPOX) onto inorganic sulfate aerosols in a 2-m3 indoor chamber at various relative humidities (30 – 80%) and injecting them into an oxidation flow reactor under the presence of hydroxyl radicals (·OH), we investigate the modification of particle size distributions, extent of inorganic sulfate conversion to organosulfates, and single-particle physicochemical properties. Chemical composition of particle-phase species, as well as aerosol morphological changes, are analyzed as a function of relative humidity and oxidant exposure times to better understand OS formation and destruction mechanisms in the ambient atmosphere. 

The ability of an atmospheric aerosol particle to take up water or to participate in heterogeneous reactions is highly influenced by its phase state – solid, semi-solid, or liquid. The changes in phase state can be predicted by glass transition temperature (Tg), as particles at temperatures below their Tg will show solid properties, while increasing the temperature above their Tg will allow for semi-solid and eventually liquid properties. Historically, measurements of the Tg of bulk materials have been studied in order to model the phase states of aerosols in the atmosphere; however, these methods only permit an estimation of aerosol Tg based on their bulk chemical composition. Determining the Tg of individual particles will allow for more accurate model predictions of aerosol phase state. Herein, we apply a recently developed method utilizing a nano-thermal analysis (nanoTA) module coupled to an atomic force microscope (AFM), to determine the Tg of individual secondary organic aerosol (SOA) particles generated from the reactive uptake of isoprene-derived epoxydiols (IEPOX) onto acidic ammonium sulfate aerosol particles. NanoTA works by using a specialized AFM probe which can be heated while in contact with a particle of interest. As the temperature increases, the probe deflection will first increase due to thermal expansion of the particle followed by a decrease at its melting temperature (Tm). The Tg of the particle can then be determined from Tm using the Boyer–Beaman rule. We compare the Tg of the SOA particles formed from IEPOX uptake onto ammonium sulfate particles with different initial aerosol pH values, as well as under a range of oxidant exposure conditions. Our measurements will allow for more accurate representations of the phase state of aerosols under a range of atmospheric conditions. 

Abstract Secondary organic aerosol (SOA) is key to our climate, affecting Earth’s radiative balance both indirectly and directly. Understanding the chemical composition and properties of SOA are crucial to accurately predict their concentrations and ultimately their impact on climate in models. Multiphase chemical reactions in the atmosphere have been found to form a variety of low-volatility, high-molecular-weight species, or oligomers. Although oligomers may constitute a large portion of SOA, they are not well understood. Most analytical techniques are unable to detect such high-mass organic species, so their formation and degradation mechanisms are still in need of investigation. Herein, we present a method using matrix-assisted laser desorption ionization mass spectrometry (MALDI-MS) to determine the oligomeric content of aerosol particles. We apply the method to analyze SOA particles formed from reactive uptake of IEPOX onto acidic ammonium sulfate seed particles during atmospheric chamber experiments. We compare the oligomeric content of the particles based on key properties, including particle acidity and exposure to oxidants. We compared multiple sample collection methods, including impaction into deionized water using a Liquid Spot Sampler (Aerosol Devices) and direct impaction onto a sampling plate. Our work will provide insight about the formation of 

Oxidation of isoprene, the biogenic volatile organic compound with the highest emissions globally, is a large source of secondary organic aerosol (SOA) in the atmosphere. Organosulfates, particularly methyltetrol sulfates, formed from acid-driven reactions of isoprene epoxydiols (IEPOX), a key oxidation product, are important contributors to SOA mass. To date, most studies have focused on organosulfate formation on ammonium sulfate particles at low pH. However, recent work has shown sea spray aerosol (SSA) in the accumulation mode (~100 nm) is often quite acidic (pH ~ 2) and IEPOX-derived organosulfates have been identified in marine environments. Herein, we demonstrate that substantial SOA, including organosulfates, are formed on acidic sodium sulfate particles (pH = 1.3), representative of marine aerosol heterogeneously reacting with H2SO4 to form Na2SO4. For acidic sodium and ammonium sulfate particles, 31 and 28% (±1%), respectively, of inorganic sulfate is incorporated into organosulfate species, even though acidic particles with sodium versus ammonium as the primary cation formed 5% (±0.2) less SOA volume and 45% (±6%) less methyltetrol sulfates, suggesting other organosulfates may form. Even though both exhibited core-shell morphology after IEPOX uptake, physicochemical differences were observed via Raman microspectroscopy, with organosulfates identified in both the core and shell of acidic ammonium sulfate SOA particles, but only in the core for acidic sodium sulfate SOA via Raman microspectroscopy. Our results suggest that isoprene-derived SOA formed on aged SSA is potentially an important, but underappreciated, source of SOA and organosulfates in marine and coastal regions and could modify SOA budgets in these environments.