Chemical reactions in atmospheric organic aerosol (OA) add large uncertainties to accurate predictions of the effect of aerosol on health, visibility, and climate. The acceleration of reaction rates of organic compounds in droplets compared to bulk solution has been reported; however, the mechanism and the principle of acceleration largely remains unknown. Malonic acid (MA) is a dicarboxylic acid that exhibits keto–enol tautomerization and is ubiquitous in organic aerosols found in the nature. An environment controlled electrodynamic balance (EDB) coupled with Mie scattering imaging (MSI) and Raman spectroscopy was used to levitate single charged MA droplets and investigate the effect of relative humidity (RH: 90%, 70%, 50%, and 30%) and size (28–91 μm diameter) on the reaction kinetics of keto–enol tautomerization of MA. Raman spectroscopy of hydrogen–deuterium isotopic exchange in MA droplets enabled quantitative analysis of MA tautomerization kinetics. The result showed slower reaction rates of MA droplets at lower RH as well as in larger-sized droplets. Application of a step-by-step isotopic exchange model to the MA droplets at 90% RH condition was used to determine the enolization rate of MA, yielding a value 10-fold higher than determined in bulk solution. Keto and enol forms of MA have distinctive physicochemical properties, such as reactivity and hydrogen bond structure. The result from our work suggests that keto–enol tautomerization can play an important role for aging of MA-containing OA in nature, as the enol form of MA can undergo accelerated chemical reactions due to the presence of the reactive carbon–carbon double bond.

中文翻译：

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水滴中丙二酸的加速酮-烯醇互变异构动力学

大气有机气溶胶 (OA) 中的化学反应为准确预测气溶胶对健康、能见度和气候的影响增加了很大的不确定性。据报道，与本体溶液相比，液滴中有机化合物的反应速率加快；然而，加速的机制和原理在很大程度上仍然未知。丙二酸 (MA) 是一种二元羧酸，具有酮-烯醇互变异构作用，普遍存在于自然界中的有机气溶胶中。环境控制的电动平衡 (EDB) 与 Mie 散射成像 (MSI) 和拉曼光谱相结合，用于悬浮单电荷 MA 液滴并研究相对湿度的影响（RH：90%、70%、50% 和 30%） MA 的酮-烯醇互变异构反应动力学的大小（28-91 μm 直径）。MA 液滴中氢-氘同位素交换的拉曼光谱能够定量分析 MA 互变异构化动力学。结果显示 MA 液滴在较低 RH 以及较大尺寸液滴中的反应速率较慢。在 90% RH 条件下，将逐步同位素交换模型应用于 MA 液滴以确定 MA 的烯醇化率，产生的值比散装溶液中确定的值高 10 倍。酮和烯醇形式的 MA 具有独特的物理化学性质，例如反应性和氢键结构。我们的工作结果表明，酮 - 烯醇互变异构可以在自然界中对含 MA 的 OA 的老化发挥重要作用，因为由于存在活性碳 - 碳双键，MA 的烯醇形式可以加速化学反应。