Abstract. Highly oxygenated molecules (HOMs) from the atmospheric oxidation of biogenic volatile organic compounds are important contributors to secondary organic aerosol (SOA). Organic peroxy radicals (RO2) and hydroperoxy radicals (HO2) are key species influencing the HOM product distribution. In laboratory studies, experimental requirements often result in overemphasis on RO2 cross-reactions compared to reactions of RO2 with HO2. We analyzed the photochemical formation of HOMs from α-pinene and their potential to contribute to SOA formation under high (≈1/1) and low (≈1/100) HO2/RO2 conditions. As HO2/RO2 > 1 is prevalent in the daytime atmosphere, sufficiently high HO2/RO2 is crucial to mimic atmospheric conditions and to prevent biases by low HO2/RO2 on the HOM product distribution and thus SOA yield. Experiments were performed under steady-state conditions in the new, continuously stirred tank reactor SAPHIR-STAR at Forschungszentrum Jülich. The HO2/RO2 ratio was increased by adding CO while keeping the OH concentration constant. We determined the HOM's SOA formation potential, considering its fraction remaining in the gas phase after seeding with (NH4)2SO4 aerosol. An increase in HO2/RO2 led to a reduction in SOA formation potential, with the main driver being a ∼ 60 % reduction in HOM-accretion products. We also observed a shift in HOM-monomer functionalization from carbonyl to hydroperoxide groups. We determined a reduction of the HOM's SOA formation potential by ∼ 30 % at HO2/RO2 ≈1/1 compared to HO2/RO2 ≈ 1/100. Particle-phase observations measured a similar decrease in SOA mass and yield. Our study shows that too low HO2/RO2 ratios compared to the atmosphere can lead to an overestimation of SOA yields.

中文翻译：

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HO2∕RO2比例对高氧α-蒎烯光氧化产物和二次有机气溶胶形成潜力的影响

摘要。生物挥发性有机化合物大气氧化产生的高含氧分子 (HOM) 是二次有机气溶胶 (SOA) 的重要贡献者。有机过氧自由基（RO2）和氢过氧自由基（HO2）是影响HOM产物分布的关键物质。在实验室研究中，与 RO2 与 HO2 的反应相比，实验要求常常导致过分强调 RO2 交叉反应。我们分析了 α-蒎烯光化学形成 HOM 的过程，以及它们在高（约 1/1）和低（约 1/100）HO2/RO2 条件下促进 SOA 形成的潜力。由于 HO2/RO2 > 1 在白天大气中普遍存在，因此足够高的 HO2/RO2 对于模拟大气条件并防止低 HO2/RO2 对 HOM 产品分布和 SOA 产率的偏差至关重要。实验在于利希研究中心的新型连续搅拌釜反应器 SAPHIR-STAR 中在稳态条件下进行。通过添加 CO 来增加 HO2/RO2 比率，同时保持 OH 浓度恒定。我们确定了 HOM 的 SOA 形成潜力，考虑到在用 (NH4)2SO4 气溶胶接种后残留在气相中的部分。 HO2/RO2 的增加导致 SOA 形成潜力降低，主要驱动因素是 HOM 吸积产物减少约 60%。我们还观察到 HOM 单体官能化从羰基到氢过氧化物基团的转变。我们确定，与 HO2/RO2 ≈ 1/100 相比，HO2/RO2 ≈1/1 时 HOM 的 SOA 形成潜力降低了 ∼ 30%。粒子相观测测量到 SOA 质量和产量也有类似的下降。我们的研究表明，与大气相比，HO2/RO2 比率过低会导致 SOA 产量的高估。