Department of Chemistry, University of Michigan, Ann Arbor, Michigan 48109, United States.
Department of Environmental Sciences and Engineering, Gillings School of Global Public Health, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27516, United States.
Environ Sci Technol. 2024 Jun 18;58(24):10675-10684. doi: 10.1021/acs.est.3c10851. Epub 2024 Jun 6.
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 (SO). 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 SO at pH values below the acid dissociation constant (p) of SO and bisulfate (HSO). The nucleophilicity of HSO is 100× lower than SO, 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 SO/HSO 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.
异戊二烯是大气中非甲烷碳氢化合物中排放最高的一种，而异戊二烯环氧化物（IEPOX）是一种已被充分证实的氧化产物，也是形成异戊二烯源次生有机气溶胶（SOA）的主要贡献物。在整个底层大气中广泛存在的高度酸性颗粒（pH 值 0-3）使 IEPOX 发生酸驱动的多相化学反应，例如通过硫酸根（SO ）的亲核攻击形成甲基四醇硫酸盐的环氧化物开环反应。在此，我们系统地证明了在高度酸性颗粒（pH < 1）上，IEPOX 形成 SOA 的情况出乎意料地减少。虽然通常认为在更多[H]可用于质子化环氧化物时，IEPOX-SOA 的形成会在低 pH 下增加，但我们在 pH 1 时观察到最大的 SOA 形成，而在 pH 0.0 和 0.4 时形成的 SOA 较少。这归因于在低于硫酸根（SO ）和亚硫酸氢根（HSO ）的酸离解常数（p）的 pH 值下，SO 的可用性有限。HSO 的亲核性比 SO 低 100 倍，从而减少 SOA 的形成，并将颗粒产物从低挥发性有机硫酸盐转移到高挥发性多元醇。目前预测 IEPOX-SOA 的 SOA 产率的模型参数化不能正确考虑 SO/HSO 平衡，从而导致在低 pH 值下对 SOA 形成的过度预测。考虑到这种未被充分探索的酸度依赖性行为对于准确预测 SOA 浓度和解决 SOA 对空气质量的影响至关重要。
