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1. Organosulfate Formation in Proxies for Aged Sea Spray Aerosol: Reactive Uptake of Isoprene Epoxydiols to Acidic Sodium Sulfate

   Cooke, Madeline; Armstrong, N. Cazimir; Fankhauser, Alison; Kolozsvari, Katherine; Waters, Cara; Parham, Rebecca; Yan, Jin; Lei, Ziying; Chen, Yuzhi; Zhang, Yue; et al (December 2022, 2022 Fall AGU Meeting)

   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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2. Decreases in Epoxide-Driven Secondary Organic Aerosol Production under Highly Acidic Conditions: The Importance of Acid–Base Equilibria

   https://doi.org/10.1021/acs.est.3c10851  

   Cooke, Madeline E; Armstrong, N Cazimir; Fankhauser, Alison M; Chen, Yuzhi; Lei, Ziying; Zhang, Yue; Ledsky, Isabel R; Turpin, Barbara J; Zhang, Zhenfa; Gold, Avram; et al (June 2024, Environmental Science & Technology)

   ABSTRACT: Isoprene has the highest atmospheric emissions of any nonmethane hydrocarbon, and isoprene epoxydiols (IEPOX) are well-established oxidation products and the primary contributors forming isoprene-derived secondary organic aerosol (SOA). Highly acidic particles (pH 0−3) widespread across the lower troposphere enable acid-driven multiphase chemistry of IEPOX, such as epoxide ring-opening reactions forming methyltetrol sulfates through nucleophilic attack of sulfate (SO4 2−). Herein, we systematically demonstrate an unexpected decrease in SOA formation from IEPOX on highly acidic particles (pH < 1). While IEPOX-SOA formation is commonly assumed to increase at low pH when more [H+] is available to protonate epoxides, we observe maximum SOA formation at pH 1 and less SOA formation at pH 0.0 and 0.4. This is attributed to limited availability of SO4 2− at pH values below the acid dissociation constant (pKa) of SO42− and bisulfate (HSO4−). The nucleophilicity of HSO4− is 100× lower than SO42−, decreasing SOA formation and shifting particulate products from low-volatility organosulfates to higher-volatility polyols. Current model parameterizations predicting SOA yields for IEPOX-SOA do not properly account for the SO42−/HSO4 − equilibrium, leading to overpredictions of SOA formation at low pH. Accounting for this underexplored acidity-dependent behavior is critical for accurately predicting SOA concentrations and resolving SOA impacts on air quality. 

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3. Effect of Initial Seed Aerosol Acidity on the Kinetics and Products of Heterogeneous Hydroxyl Radical Oxidation of Isoprene Epoxydiol-Derived Secondary Organic Aerosol

   https://doi.org/10.1021/acs.jpca.4c08082  

   Yan, Jin; Armstrong, N Cazimir; Kolozsvari, Katherine R; Waters, Cara M; Xiao, Yao; Fankhauser, Alison M; Cooke, Madeline E; Frauenheim, Molly; Buchenau, Nicolas A; Parham, Rebecca L; et al (May 2025, The Journal of Physical Chemistry A)

   ABSTRACT: At fixed aerosol acidity, we recently demonstrated that dimers in isoprene epoxydiol-derived secondary organic aerosol (IEPOX-SOA) can heterogeneously react with hydroxyl radical (·OH) at faster rates than monomers. Aerosol acidity influences this aging process by enhancing the formation of oligomers in freshly generated IEPOX-SOA. Therefore, we systematically examined the role of aerosol acidity on kinetics and products resulting from heterogeneous ·OH oxidation of freshly generated IEPOX-SOA. IEPOX reacted with inorganic sulfate aerosol of varying initial pH (0.5, 1.5, and 2.5) in a steady state smog chamber to yield a constant source of freshly generated IEPOX-SOA, which was aged in an oxidation flow reactor for 0−22 equiv days of atmospheric ·OH exposure. Molecular-level chemical analyses revealed that the most acidic sulfate aerosol (pH 0.5) formed the largest oligomeric mass fraction, causing the slowest IEPOX-SOA mass decay with aging. Reactive uptake coefficients of ·OH (γOH) were 0.24 ± 0.06, 0.40 ± 0.05, and 0.49 ± 0.20 for IEPOX-SOA generated at pH 0.5, 1.5, and 2.5, respectively. IEPOXSOA became more liquid-like for pH 1.5 and 2.5, while exhibiting an irregular pattern for pH 0.5 with aging. Using kinetic and physicochemical data derived for a single aerosol pH in atmospheric models could inaccurately predict the fate of the IEPOX-SOA. 

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4. Acidity-Driven Shifts in Aerosolized Sodium Sulfate Crystal Structure via Raman Microspectroscopy and Implications for Isoprene-Derived Secondary Organic Aerosol Formation

   https://doi.org/10.1021/acs.jpca.5c02222  

   Cooke, Madeline E; Alotbi, Ali R; Waters, Cara M; Heagle, Lindsay J; Armstrong, N Cazimir; Poworoznek, Carlie J; Zhang, Zhenfa; Gold, Avram; Surratt, Jason D; Ault, Andrew P (August 2025, The Journal of Physical Chemistry A)

   Sea spray aerosol (SSA) contributes significantly to global aerosol budgets and the NaCl present often heterogeneously reacts with H2SO4 to form Na2SO4 within particles of varying acidity. The acidic particles can subsequently undergo reactive uptake of isoprene-derived oxidation products, such as isoprene epoxydiols (IEPOX), to form significant amounts of secondary organic aerosol (SOA). While the complex pH-dependence of IEPOX-SOA formation with ammonium sulfate aerosol has been widely explored, there has been minimal research on pH-dependent SOA formation with sodium sulfate. Herein, aerosolized solutions of Na2SO4 mixed with different amounts of H2SO4 (pH = 1, 1.3, and 3) were used as seed aerosol to take up IEPOX and form SOA. We unexpectedly observed a distinct crystal structure (phase III) in more acidic particles using Raman spectroscopy, based on characteristic peaks for νas(SO4 2−) at 1077, 1130, and 1199 cm−1 versus 1101, 1131, and 1152 cm−1 for νas(SO4 2−) in typical crystalline Na2SO4 (phase V). The presence of phase III in iSOA indicates a more acidic environment. Our results show that reactive uptake of IEPOX to seed particles increases acidity (i.e., pH decreases) over time. This has important implications for IEPOX-SOA formation, particularly on aged SSA in marine and coastal environments. 

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5. Observation-constrained kinetic modeling of isoprene SOA formation in the atmosphere

   https://doi.org/10.5194/acp-24-6153-2024  

   Shen, Chuanyang; Yang, Xiaoyan; Thornton, Joel; Shilling, John; Bi, Chenyang; Isaacman-VanWertz, Gabriel; Zhang, Haofei (January 2024, Atmospheric Chemistry and Physics)

   Abstract. Isoprene has the largest global non-methane hydrocarbon emission, and the oxidation of isoprene plays a crucial role in the formation of secondary organic aerosol (SOA). Two primary processes are known to contribute to SOA formation from isoprene oxidation: (1) the reactive uptake of isoprene-derived epoxides on acidic or aqueous particle surfaces and (2) the absorptive gas–particle partitioning of low-volatility oxidation products. In this study, we developed a new multiphase condensed isoprene oxidation mechanism that includes these processes with key molecular intermediates and products. The new mechanism was applied to simulate isoprene gas-phase oxidation products and SOA formation from previously published chamber experiments under a variety of conditions and atmospheric observations during the Southern Oxidant and Aerosol Studies (SOAS) field campaign. Our results show that SOA formation from most of the chamber experiments is reasonably reproduced using our mechanism, except when the concentration ratios of initial nitric oxide to isoprene exceed ∼ 2, the formed SOA is significantly underpredicted. The SOAS simulations also reasonably agree with the measurements regarding the diurnal pattern and concentrations of different product categories, while the total isoprene SOA remains underestimated. The molecular compositions of the modeled SOA indicate that multifunctional low-volatility products contribute to isoprene SOA more significantly than previously thought, with a median mass contribution of ∼ 57 % to the total modeled isoprene SOA. However, this contribution is intricately intertwined with IEPOX-derived SOA (IEPOX: isoprene-derived epoxydiols), posing challenges for their differentiation using bulk aerosol composition analysis (e.g., the aerosol mass spectrometer with positive matrix factorization). Furthermore, the SOA from these pathways may vary greatly, mainly dependent on the volatility estimation and treatment of particle-phase processes (i.e., photolysis and hydrolysis). Our findings emphasize that the various pathways to produce these low-volatility species should be considered in models to more accurately predict isoprene SOA formation. The new condensed isoprene chemical mechanism can be further incorporated into regional-scale air quality models, such as the Community Multiscale Air Quality Modelling System (CMAQ), to assess isoprene SOA formation on a larger scale. 

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