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Abstract Number: 327 Working Group: Aerosol Chemistry Abstract Low-pH aerosols comprise a large fraction of atmospheric fine particulate matter. The effects of pH on secondary organic aerosol (SOA) formation are not well understood, in part because of the difficulty of accurately measuring aerosol pH. Of particular interest are the atmospherically-abundant isoprene epoxydiols (IEPOX), which undergo acid-driven reactions to form SOA. Models have assumed no upper limit for IEPOX-SOA formation rates as acidity increases. However, recent work has shown that organosulfate formation from IEPOX slows as the equilibrium of inorganic sulfate (Sulfinorg) shifts from sulfate (SO42-) towards bisulfate (HSO4-), which is a weaker nucleophile. We performed a series of trans-ß-IEPOX uptake experiments with ammonium sulfate seed solutions acidified to between pH 0 and 3, and modelled time-resolved methyltetrol (MT) and methyltetrol sulfate (MTS) formation and Sulfinorg consumption (kMT = 0.018 M-2 s-1, kMTS = 0.28 M-2 s-1). We found an inflection point between pH 1 and 1.4, below which MT formation dominates and above which MTS formation dominates, consistent with a changing balance of protonated and deprotonated Sulfinorg. Modelled MT and MTS formation fit the experimental data well both above and below the inflection point except at pH 1.4, where it significantly underpredicted the data at low initial IEPOX/Sulfinorg ratios. This indicates multi-phase chemical dynamics beyond those represented in our model, leading to very efficient IEPOX-SOA formation at pH 1.4. Further investigation is warranted into the connection of IEPOX-SOA formation with initial IEPOX/Sulfinorg ratio and aerosol pH.
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Decreases in Epoxide-Driven Secondary Organic Aerosol Production under Highly Acidic Conditions: The Importance of Acid–Base Equilibria
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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- PAR ID:
- 10594824
- Publisher / Repository:
- ACS
- Date Published:
- Journal Name:
- Environmental Science & Technology
- Volume:
- 58
- Issue:
- 24
- ISSN:
- 0013-936X
- Page Range / eLocation ID:
- 10675 to 10684
- Subject(s) / Keyword(s):
- atmospheric chemistry aerosol acidity multiphase chemistry air pollution climate change
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Low-pH aerosols comprise a large fraction of atmospheric fine particulate matter. The effects of pH on secondary organic aerosol (SOA) formation are not well understood, in part because of the difficulty of accurately measuring aerosol pH. Of particular interest are the atmospherically-abundant isoprene epoxydiols (IEPOX), which undergo acid-driven reactions to form SOA. Models have assumed no upper limit for IEPOX-SOA formation rates as acidity increases. However, recent work has shown that organosulfate formation from IEPOX slows as the equilibrium of inorganic sulfate (Sulfinorg) shifts from sulfate (SO42-) towards bisulfate (HSO4-), which is a weaker nucleophile. We performed a series of trans-ß-IEPOX uptake experiments with ammonium sulfate seed solutions acidified to between pH 0 and 3, and modelled time-resolved methyltetrol (MT) and methyltetrol sulfate (MTS) formation and Sulfinorg consumption (kMT = 0.018 M-2 s-1, kMTS = 0.28 M-2 s-1). We found an inflection point between pH 1 and 1.4, below which MT formation dominates and above which MTS formation dominates, consistent with a changing balance of protonated and deprotonated Sulfinorg. Modelled MT and MTS formation fit the experimental data well both above and below the inflection point except at pH 1.4, where it significantly underpredicted the data at low initial IEPOX/Sulfinorg ratios. This indicates multi-phase chemical dynamics beyond those represented in our model, leading to very efficient IEPOX-SOA formation at pH 1.4. Further investigation is warranted into the connection of IEPOX-SOA formation with initial IEPOX/Sulfinorg ratio and aerosol pH.more » « less
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Accepted Manuscript
- Journal Article:
- https://doi.org/10.1021/acs.est.3c10851
