Experimentally determined yields of secondary organic aerosol (SOA) from the photochemical oxidation of isoprene in the absence of aqueous acidic aerosol vary substantially, both within a given experiment and across different environmental chamber conditions. The underlying mechanisms driving this variation remain poorly evaluated, leading to significant uncertainty in how to extrapolate laboratory chamber results to the atmosphere. Herein, we compare SOA predictions from a near-explicit gas-phase chemical mechanism of isoprene oxidation by the hydroxyl radical (OH) in the presence and absence of nitrogen oxide radicals (NO_x), to multiple chamber experiments on non-aqueous isoprene photochemical SOA (ipSOA) conducted by different groups in different chambers. SOA is predicted by volatility-driven gas-particle partitioning of hundreds of individual reaction products. The mechanism includes simplified descriptions of particle-phase organic chemistry, including organic hydroperoxide photolysis, and organic nitrate hydrolysis and accretion reactions. The model has good skill (mean normalized bias typically within 25%) at predicting the observed formation and evolution of ipSOA across a range of chambers and conditions at low NO_x. The model has much less skill at describing the observed non-linear response of ipSOA to elevated NO_x. Organic nitrate hydrolysis is unable to explain significant ipSOA at high NO_x, whereas particle-phase accretion reactions of tertiary nitrates may play a role. Uncertainties in the chamber radical environment and fate of key organic peroxy radicals (RO₂) remain as or even more important than vapor losses to chamber walls in determining how best to extrapolate chamber-based yields to the atmosphere. Implications for likely atmospheric yields of ipSOA and recommendations for future chamber experiments are discussed.

中文翻译：

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异戊二烯光化学次级有机气溶胶形成和演化的近乎明确的机械评估：有和没有添加NO _x的多室实验的模拟

在给定的实验中以及在不同的环境腔室条件下，在不存在酸性酸性气溶胶的情况下，由异戊二烯的光化学氧化实验确定的次要有机气溶胶（SOA）的收率变化很大。导致这种变化的潜在机制仍未得到很好的评估，从而导致在如何将实验室结果推算到大气方面存在很大的不确定性。在这里，我们比较了在存在和不存在氮氧化物自由基（NO _x）的情况下，异戊二烯被羟基自由基（OH）氧化的近乎明确的气相化学机理的SOA预测。），然后由不同小组在不同的小组中对非水异戊二烯光化学SOA（ipSOA）进行多小组实验。SOA是由数百种独立反应产物的挥发性驱动的气体颗粒分配预测的。该机制包括对颗粒相有机化学的简化描述，包括有机氢过氧化物光解，有机硝酸盐水解和积聚反应。该模型具有在低NO预测ipSOA在一系列腔室和条件所观察到的形成和演化好的技能（通常在25％的平均归一化的偏压）_X。该模型在描述观察到的ipSOA对升高的NO _x的非线性响应方面的技能要差得多。有机硝酸盐水解无法解释高NO下明显的ipSOA_x，而硝酸叔硝酸盐的颗粒相积聚反应可能起作用。在确定如何最佳地将基于腔室的产率外推到大气中时，腔室自由基环境的不确定性和关键有机过氧自由基（RO ₂）的命运仍然与腔室壁蒸气损失相同，甚至比蒸汽损失更为重要。讨论了可能的ipSOA大气产量​​的含义以及对未来腔室实验的建议。