Abstract. Aromatic carbonyls (e.g., methoxybenzaldehydes), an important class of photosensitizers, are abundant in the atmosphere. This study compared non-phenolic (3,4-dimethoxybenzaldehyde, DMB) and phenolic (vanillin, VL) methoxybenzaldehydes as photosensitizers for aqueous secondary organic aerosol (aqSOA) formation via guaiacol (GUA) oxidation under atmospherically relevant cloud and fog conditions. The effects of ammonium nitrate (AN) on these reactions were also explored. GUA oxidation by triplet excited states of DMB (³DMB*) (GUA+DMB) was ~4 times faster and exhibited greater light absorption than oxidation by ³VL* (GUA+VL). Both GUA+DMB and GUA+VL formed aqSOA composed of oligomers, functionalized monomers, oxygenated ring-opening species, and N-containing products in the presence of AN. The observation of N-heterocycles such as imidazoles indicates the participation of ammonium in the reactions. The majority of generated aqSOA are potential brown carbon (BrC) chromophores. Oligomerization and functionalization dominated in GUA+DMB and GUA+VL, but functionalization appeared to be more important in GUA+VL due to contributions from VL itself. AN did not significantly affect the oxidation kinetics, but it had distinct effects on the product distributions, likely due to differences in the photosensitizing abilities and structural features of DMB and VL. In particular, the more extensive fragmentation in GUA+DMB than in GUA+VL likely generated more N-containing products in GUA+DMB+AN. In GUA+VL+AN, the increased oligomers may be due to VL-derived phenoxy radicals induced by ^•OH or ^•NO₂ from nitrate photolysis. Furthermore, increased nitrated products observed in the presence of both DMB or VL and AN than in AN alone implies that photosensitized reactions may promote nitration. This work demonstrates how the structural features of photosensitizers affect aqSOA formation via non-carbonyl phenol oxidation. Potential interactions between photosensitization and AN photolysis were also elucidated. These findings facilitate a better understanding of photosensitized aqSOA formation and highlight the importance of ammonium nitrate photolysis in these reactions.

中文翻译：

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光敏愈创木酚氧化形成水性 SOA：在硝酸铵存在和不存在的情况下，非酚类和酚类甲氧基苯甲醛作为光敏剂的比较

摘要。芳香族羰基化合物（如甲氧基苯甲醛）是一类重要的光敏剂，在大气中含量丰富。本研究比较了非酚类（3,4-二甲氧基苯甲醛，DMB）和酚类（香草醛，VL）甲氧基苯甲醛作为光敏剂，在大气相关云雾条件下通过愈创木酚 (GUA) 氧化形成水性二次有机气溶胶 (aqSOA)。还探讨了硝酸铵 (AN) 对这些反应的影响。^{通过 DMB ( 3} DMB*) (GUA+DMB)的三重激发态氧化GUA 比氧化³快约 4 倍，并且表现出更大的光吸收VL* (GUA+VL)。在 AN 存在下，GUA+DMB 和 GUA+VL 均形成由低聚物、官能化单体、含氧开环物质和含 N 产物组成的 aqSOA。N-杂环如咪唑的观察表明铵参与了反应。大多数生成的 aqSOA 是潜在的棕碳 (BrC) 生色团。在GUA+DMB和GUA+VL中，低聚和功能化占主导地位，但由于VL本身的贡献，功能化在GUA+VL中似乎更为重要。AN 对氧化动力学没有显着影响，但对产物分布有明显影响，这可能是由于 DMB 和 VL 的光敏能力和结构特征的差异。尤其是，GUA+DMB 中比 GUA+VL 中更广泛的碎片可能在 GUA+DMB+AN 中产生更多的含氮产物。在 GUA+VL+AN 中，增加的低聚物可能是由于 VL 衍生的苯氧基自由基引起的^• OH 或^• NO ₂来自硝酸盐光解。此外，在存在 DMB 或 VL 和 AN 的情况下观察到比单独存在 AN 时增加的硝化产物意味着光敏反应可能促进硝化。这项工作展示了光敏剂的结构特征如何通过非羰基苯酚氧化影响 aqSOA 的形成。还阐明了光敏化和 AN 光解之间的潜在相互作用。这些发现有助于更好地理解光敏化 aqSOA 的形成，并强调硝酸铵光解在这些反应中的重要性。