Aqueous reactions may turn precursors into light-absorbing and toxic products, leading to air quality deterioration and adverse health effects. In this study, we comprehensively investigated eugenol photooxidation (a representative biomass-burning-emitted, highly substituted phenolic compound) in the bulk aqueous phase with direct photolysis, a hydroxyl radical (OH), and an organic triplet excited state (³C^∗). Results show that the degradation rates of eugenol followed the order of ³C${}^{\ast }>$ OH > direct photolysis. During the ³C^∗-initiated oxidation, different reactive oxygen species (ROS), including ³C^∗, OH, ¹O₂, and O${}_{\mathrm{2}}^{\mathrm{•}-}$, can participate in the oxidation of eugenol, quenching experiments verified ³C^∗ was the most important one, while, during OH-initiated oxidation, O${}_{\mathrm{2}}^{\mathrm{•}-}$ was a more important ROS than OH for degrading eugenol. The rate constants under saturated O₂, air, and N₂ followed the order of $k_{{\mathrm{O}}_{\mathrm{2}}}>k_{\mathrm{Air}}>k_{{\mathrm{N}}_{\mathrm{2}}}$ for both direct photolysis and OH-initiated oxidation but changed to $k_{\mathrm{Air}}>k_{{\mathrm{N}}_{\mathrm{2}}}>k_{{\mathrm{O}}_{\mathrm{2}}}$ for ³C^∗-mediated oxidation. pH and dissolved oxygen (DO) levels both decreased during oxidation, indicating the formation of acids and the participation of DO in oxidation. Ultraviolet and visible (UV-vis) light absorption spectra of the reaction products showed a clear absorbance enhancement in the 300–400 nm range for all three sets of experiments, and new fluorescence at excitation/emission $=\mathrm{250}/$ (400–500) nm appeared, suggesting the formation of new chromophores and fluorophores (brown carbon species). These species were likely attributed to humic-like substances (HULIS), as shown by the increases in HULIS concentrations during oxidation. Large mass yields of products (140 %–197 %) after 23 h of illumination were obtained, and high oxidation degrees of these products were also observed. Correspondingly, a series of oxygenated compounds were identified, and a detailed reaction mechanism with functionalization as a dominant pathway was proposed. At last, the dithiothreitol (DTT) assay was applied to assess the oxidation potential of the reaction products, and the end products of all three sets of experiments showed higher DTT consumption rates than those of eugenol, indicating that more toxic species were produced upon aqueous oxidation. Overall, our results from using eugenol as a model compound, underscore the potential importance of the aqueous processing of biomass burning emissions in secondary organic aerosol (SOA) formation.

中文翻译：

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OH和3C*引发丁香酚光氧化反应的水相产物的光学和化学性质及氧化电位

水反应可能会将前体转化为吸光和有毒产物，导致空气质量恶化和对健康产生不利影响。在这项研究中，我们通过直接光解、羟基自由基 (OH) 和有机三重激发态 ( ³ C ^* ) 在本体水相中全面研究了丁香酚光氧化（一种具有代表性的生物质燃烧发射的高度取代的酚类化合物） . 结果表明，丁香酚的降解速率遵循³ C的顺序。${}^{*}>$ OH  > 直接光解。在³ C ^*引发的氧化过程中，不同的活性氧 (ROS)，包括³ C ^*、OH、¹ O ₂和 O${}_{\mathrm{2}}^{\mathrm{•}-}$, 可以参与丁香酚的氧化, 淬火实验证实³ C ^*是最重要的, 而在 OH 引发的氧化过程中, O${}_{\mathrm{2}}^{\mathrm{•}-}$是比 OH 更重要的降解丁香酚的活性氧。_{饱和O 2}、空气和N ₂下的速率常数依次为${ķ}_{{\mathrm{○}}_{\mathrm{2}}}>{ķ}_{\mathrm{空气}}>{ķ}_{{\mathrm{ñ}}_{\mathrm{2}}}$对于直接光解和 OH 引发的氧化，但改为${ķ}_{\mathrm{空气}}>{ķ}_{{\mathrm{ñ}}_{\mathrm{2}}}>{ķ}_{{\mathrm{○}}_{\mathrm{2}}}$对于³ C ^{* -}介导的氧化。氧化过程中 pH 值和溶解氧 (DO) 水平均下降，表明酸的形成和 DO 参与氧化。对于所有三组实验，反应产物的紫外和可见 (UV-vis) 光吸收光谱在 300-400 nm 范围内显示出明显的吸光度增强，并且在激发/发射处有新的荧光 $=\mathrm{250}/$ (400–500) nm 出现，表明形成了新的发色团和荧光团（棕碳物种）。这些物种可能归因于腐殖质类物质 (HULIS)，如氧化过程中 HULIS 浓度的增加所示。光照 23 小时后，产物的质量产率很高（140 %–197 %），并且这些产物的氧化程度也很高。相应地，鉴定了一系列含氧化合物，并提出了以功能化为主要途径的详细反应机理。最后，应用二硫苏糖醇（DTT）测定来评估反应产物的氧化电位，三组实验的最终产物都显示出比丁香酚更高的DTT消耗率，表明在水溶液中产生了更多的有毒物质。氧化。