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Kinetics of the nitrate-mediated photooxidation of monocarboxylic acids in the aqueous phase
Environmental Science: Processes & Impacts ( IF 4.3 ) Pub Date : 2023-01-26 , DOI: 10.1039/d2em00458e Yuting Lyu 1, 2 , Jany Ting Chun Chow 1 , Theodora Nah 1, 2
Environmental Science: Processes & Impacts ( IF 4.3 ) Pub Date : 2023-01-26 , DOI: 10.1039/d2em00458e Yuting Lyu 1, 2 , Jany Ting Chun Chow 1 , Theodora Nah 1, 2
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The photooxidation of organic compounds by hydroxyl radicals (·OH) in atmospheric aqueous phases contributes to both the formation and aging of secondary organic aerosols (SOAs), which usually include carboxylic acids. Hydrogen peroxide (H2O2) and inorganic nitrate are two important ·OH photochemical sources in atmospheric aqueous phases. The aqueous phase pH is an important factor that not only controls the dissociation of carboxylic acids and consequently their ·OH reactivities, but also the production of ·OH and other reactive species from the photolysis of some ·OH photochemical precursors, particularly inorganic nitrate. While many studies have reported on the aqueous pH-dependent photodegradation rates of carboxylic acids with ·OH produced by H2O2 photolysis, the aqueous pH-dependent photodegradation rates of carboxylic acids with ·OH produced by inorganic nitrate photolysis have not been studied. In this work, we investigated the pH-dependent (pH 2 to 7) aqueous photooxidation of formic acid (FA), glycolic acid (GA), and pyruvic acid (PA) initiated by the photolysis of ammonium nitrate (NH4NO3). The observed reaction rates of the three carboxylic acids were controlled by the [NH4NO3]/[carboxylic acid] concentration ratio. Higher [NH4NO3]/[carboxylic acid] concentration ratios resulted in faster photodegradation rates, which could be attributed to the higher concentrations of ·OH produced from the photolysis of higher concentrations of NH4NO3. In addition, the observed photodegradation rates of the three carboxylic acids strongly depended on the pH. The highest photodegradation rate was observed at pH 4 for FA, whereas the highest photodegradation rates were observed at pH 2 for GA and PA. The observed pH-dependent FA and GA photodegradation rates were due to the combined effects of the pH-dependent ·OH formation from NH4NO3 photolysis and the differences in ·OH reactivities of dissociated vs. undissociated FA and GA. In contrast, the observed pH-dependent PA photodegradation rate was due primarily to the pH-dependent decarboxylation of PA initiated by light. These results highlight how the aqueous phase pH and inorganic nitrate photolysis can combine to influence the degradation rates of carboxylic acids, which can have significant implications for how the atmospheric fates of carboxylic acids are modeled for regions with substantial concentrations of inorganic nitrate in cloud water and aqueous aerosols.
中文翻译：
水相中硝酸盐介导的单羧酸光氧化动力学
大气水相中羟基自由基 (·OH) 对有机化合物的光氧化作用会导致二次有机气溶胶 (SOA) 的形成和老化，其中通常包括羧酸。过氧化氢 (H 2 O 2 ) 和无机硝酸盐是大气水相中两个重要的·OH 光化学来源。水相 pH 值是一个重要因素，它不仅控制羧酸的解离，进而控制它们的·OH 反应性，而且还控制一些·OH 光化学前体，特别是无机硝酸盐的光解产生·OH 和其他活性物质。虽然许多研究报道了羧酸与 H 2 O产生的·OH 的水溶液 pH 依赖性光降解速率2光解中，无机硝酸盐光解产生的·OH 对羧酸的水溶液 pH 依赖性光降解速率尚未研究。在这项工作中，我们研究了由硝酸铵 (NH 4 NO 3 ) 光解引发的甲酸 (FA)、乙醇酸 (GA) 和丙酮酸 (PA) 的 pH 依赖性（pH 2 至 7）水溶液光氧化. 观察到的三种羧酸的反应速率由[NH 4 NO 3 ]/[羧酸]浓度比控制。高[NH 4 NO 3]/[羧酸]浓度比导致更快的光降解速率，这可归因于较高浓度的NH 4 NO 3的光解产生较高浓度的·OH 。此外，观察到的三种羧酸的光降解率在很大程度上取决于 pH 值。对于 FA，在 pH 4 时观察到最高的光降解率，而对于 GA 和 PA，在 pH 2 时观察到最高的光降解率。观察到的 pH 依赖性 FA 和 GA 光降解速率是由于 NH 4 NO 3光解中 pH 依赖性·OH 形成的综合影响以及离解与未分解的·OH 反应性差异。未解离的 FA 和 GA。相反，观察到的 pH 依赖性 PA 光降解速率主要是由于光引发的 PA 的 pH 依赖性脱羧作用。这些结果强调了水相 pH 值和无机硝酸盐光解如何结合起来影响羧酸的降解速率，这可能对如何针对云水中无机硝酸盐浓度较高的地区模拟羧酸的大气命运具有重要意义，并且水性气溶胶。
更新日期：2023-01-26
中文翻译：
水相中硝酸盐介导的单羧酸光氧化动力学
大气水相中羟基自由基 (·OH) 对有机化合物的光氧化作用会导致二次有机气溶胶 (SOA) 的形成和老化，其中通常包括羧酸。过氧化氢 (H 2 O 2 ) 和无机硝酸盐是大气水相中两个重要的·OH 光化学来源。水相 pH 值是一个重要因素，它不仅控制羧酸的解离，进而控制它们的·OH 反应性，而且还控制一些·OH 光化学前体，特别是无机硝酸盐的光解产生·OH 和其他活性物质。虽然许多研究报道了羧酸与 H 2 O产生的·OH 的水溶液 pH 依赖性光降解速率2光解中，无机硝酸盐光解产生的·OH 对羧酸的水溶液 pH 依赖性光降解速率尚未研究。在这项工作中，我们研究了由硝酸铵 (NH 4 NO 3 ) 光解引发的甲酸 (FA)、乙醇酸 (GA) 和丙酮酸 (PA) 的 pH 依赖性（pH 2 至 7）水溶液光氧化. 观察到的三种羧酸的反应速率由[NH 4 NO 3 ]/[羧酸]浓度比控制。高[NH 4 NO 3]/[羧酸]浓度比导致更快的光降解速率，这可归因于较高浓度的NH 4 NO 3的光解产生较高浓度的·OH 。此外，观察到的三种羧酸的光降解率在很大程度上取决于 pH 值。对于 FA，在 pH 4 时观察到最高的光降解率，而对于 GA 和 PA，在 pH 2 时观察到最高的光降解率。观察到的 pH 依赖性 FA 和 GA 光降解速率是由于 NH 4 NO 3光解中 pH 依赖性·OH 形成的综合影响以及离解与未分解的·OH 反应性差异。未解离的 FA 和 GA。相反，观察到的 pH 依赖性 PA 光降解速率主要是由于光引发的 PA 的 pH 依赖性脱羧作用。这些结果强调了水相 pH 值和无机硝酸盐光解如何结合起来影响羧酸的降解速率，这可能对如何针对云水中无机硝酸盐浓度较高的地区模拟羧酸的大气命运具有重要意义，并且水性气溶胶。
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