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  1. Free, publicly-accessible full text available April 24, 2025
  2. 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. 
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    Free, publicly-accessible full text available October 19, 2024
  3. 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. 
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  4. 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 the oxidation product isoprene epoxydiol (IEPOX) onto particulate sulfate, 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 that sea spray aerosol (SSA) in the accumulation mode (~100 nm) is often quite acidic (pH ~ 2). Marine biota are well-established sources of isoprene, with annual global oceanic fluxes of isoprene estimated to range from 1-12 Tg, 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, representative of marine aerosol heterogeneously reacting with H2SO4 to form Na2SO4. We compare SOA formed from the reactive uptake of IEPOX onto particulate sulfate and find that the cation (sodium vs. ammonium) impacts the physical properties and chemical composition of the SOA formed. Additionally, we investigate the formation of SOA derived from sodium sulfate based on key properties including particle acidity and the extent of exposure to oxidation via OH radicals. 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 and composition in these environments. 
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  5. Secondary organic aerosol (SOA) is a significant component of atmospheric fine particulate matter (PM2.5) globally that can form through multiphase chemistry of oxidized volatile organic compounds (VOC) leading to lower‐volatility particulate species. Condensed phase reactions of certain SOA constituents with inorganic sulfate derived from SO2 oxidation will lead to the formation of organosulfates, which can account for up to 10 – 15% of the organic mass within PM2.5. Despite the ubiquitous presence of atmospheric fine particulate organosulfates, our fundamental understanding of the molecular structure of organosulfates is limited, including for 2‐methyltetrol organosulfates (2‐MTSs), which are typically the single most abundant organosulfates measured in PM2.5, formed from isoprene oxidation products. As atmospheric aerosol pH varies widely (0 – 6), it is important to know whether organosulfates exist primarily in their protonated (ROSO3H) or deprotonated (ROSO3 ‐) forms. In this study, vibrational modes of synthetically‐pure 2‐MTSs were spectroscopically probed using Raman and infrared (IR) spectroscopies, supported by density functional theory (DFT) of the protonated and deprotonated structures. Vibrational bands at 1035 and 1059 cm‐1 were seen in both the IR and Raman spectra, and were associated with the ROSO3 ‐ anion by comparison to DFT calculations. Analysis of Raman spectra across a range of acidities (pH = 0 – 10) shows that 2‐MTSs are deprotonated (ROSO3 ‐) at those pH values. Additional DFT calculations for organosulfates derived from isoprene, α‐pinene, β‐caryophyllene, and toluene suggest that most organosulfates exist in their deprotonated form (ROSO3 ‐) in atmospheric particles. These charged species may have significant implications for our understanding of aerosol acidity and should be considered in thermodynamic model calculations. 
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