The role of relative humidity (RH) in secondary organic aerosol (SOA) formation from high-NO_x photooxidation of long-chain alkanes was investigated by performing simulation chamber experiments on n-dodecane (C₁₂H₂₆). This molecule was chosen as a model compound for the class of long-chain alkanes. The experiments span a wide range of RH conditions from <1 to 70%. The humidity was found to reduce the SOA production yield by a factor of 2, from extremely dry (<1%) to humid (RH ≥ 5%) conditions. Measurements of major oxidation products in the gaseous and aerosol phases revealed the effect of water on a series of multiphasic reaction mechanisms and SOA production. Under very low RH, an acid-catalyzed “dehydration” reaction of polyfunctional molecules leads to lower volatility products. This pathway was found to be inhibited from dry to ambient RH conditions, explaining the SOA yield reduction. In addition to the previously established carbonyls and hydroxycarbonyls, novel tracers, that is, lactone and carboxylic acid, were identified using authentic standards, providing evidence for water-sensitive multiphasic pathways. Among the species formed via the dehydration channel, we find not only volatile species but also very reactive intermediates (e.g., dihydrofurans) which partition back to the gas phase. Owing to its double bond, it can be further oxidized to lower volatility secondary products, which forms a complex multiphasic scheme involving many condensation reaction–evaporation steps.

中文翻译：

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相对湿度在长链烷烃高NO _x光氧化引起的次级有机气溶胶形成中的作用：正十二烷案例研究

通过在正十二烷（C ₁₂ H _26）上进行模拟室实验，研究了相对湿度（RH）在长链烷烃的高NO _x光氧化中形成次级有机气溶胶（SOA）中的作用。）。选择该分子作为长链烷烃类别的模型化合物。实验涵盖了从<1％到70％的各种RH条件。从极端干燥（<1％）到潮湿（RH≥5％）的条件下，发现湿度降低了SOA产量的2倍。对气态和气溶胶主要氧化产物的测量揭示了水对一系列多相反应机理和SOA产生的影响。在非常低的相对湿度下，多官能分子的酸催化“脱水”反应会导致挥发性降低。发现该途径在干燥至环境RH条件下均受到抑制，这说明SOA产量降低。除了先前建立的羰基和羟基羰基外，新型示踪剂，即内酯和羧酸，使用真实的标准进行鉴定，为水敏感的多相途径提供证据。在通过脱水通道形成的物质中，我们不仅发现挥发性物质，而且发现了非常活泼的中间体（例如，二氢呋喃），它们重新分配回气相。由于其双键，它可以被进一步氧化以降低挥发性副产物，从而形成一个复杂的多相方案，涉及许多缩合反应-蒸发步骤。