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

Madeline Cooke, N. Cazimir Armstrong, Alison Fankhauser, Yuzhi Chen, Ziying Lei, Yue Zhang, Isabel Ledsky, Barbara Turpin, Zhenfa Zhang, Avram Gold, V. Faye McNeill, Jason Surratt, ANDREW AULT, University of Michigan

     Abstract Number: 207
     Working Group: Aerosol Chemistry

Abstract
Organic aerosol (OA) is ubiquitous globally, with a majority formed through secondary processes (SOA). Isoprene has the highest emissions to the atmosphere of any non-methane hydrocarbon, and isoprene epoxydiols (IEPOX) are well-established oxidation products and the primary reaction pathway forming isoprene-derived 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 leading to organosulfate and polyol formation in fine particles. Herein, we systematically demonstrate an unexpected decrease in SOA formation from IEPOX on highly acidic particles (pH < 1). After reactive uptake, IEPOX undergoes nucleophilic attack by sulfate (SO₄^2-) to form methyltetrol sulfates and H₂O to form 2-methyltetrols. While 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. Lower SOA formation is attributed to limited availability of SO₄^2- at pH values below the acid dissociation constant (pK_a) of SO₄^2- and bisulfate (HSO₄^-). The nucleophilicity of HSO₄^- is 100× lower than SO₄^2-, decreasing SOA formation and shifting the products from organosulfates to polyols based on HILIC/ESI-HR-QTOFMS. The observed sulfate-limited regime (pH ≤ 0.4) is difficult to replicate with thermodynamic and mechanistically-focused atmospheric models due to high aerosol ionic strengths (> 12 moles/kg), and the SO₄^2-/HSO₄^- equilibrium. Accounting for this underexplored acidity-dependent behavior is critical for accurately predicting SOA concentrations and resolving the impacts of SOA-driven poor air quality in many regions globally.