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Photochemistry of Products of the Aqueous Reaction of Methylglyoxal with Ammonium Sulfate
ACS Earth and Space Chemistry ( IF 2.9 ) Pub Date : 2017-09-28 00:00:00 , DOI: 10.1021/acsearthspacechem.7b00075 Paige K. Aiona 1 , Hyun Ji Lee 1 , Renee Leslie 1 , Peng Lin 2 , Alexander Laskin 2 , Julia Laskin 2 , Sergey A. Nizkorodov 1
ACS Earth and Space Chemistry ( IF 2.9 ) Pub Date : 2017-09-28 00:00:00 , DOI: 10.1021/acsearthspacechem.7b00075 Paige K. Aiona 1 , Hyun Ji Lee 1 , Renee Leslie 1 , Peng Lin 2 , Alexander Laskin 2 , Julia Laskin 2 , Sergey A. Nizkorodov 1
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Aqueous reactions of methylglyoxal (MG) and glyoxal with ammonium sulfate (AS) produce light-absorbing compounds (chromophores) and may serve as a source of atmospheric secondary “brown carbon” (BrC). The molecular composition of these chromophores is ambiguous, and their transformation due to exposure to solar UV radiation is not well understood. We examined the molecular composition, mass absorption coefficients, and fluorescence spectra of BrC samples produced by the evaporation of aqueous MG/AS solutions. Chromatograms of BrC produced by evaporation were different from those of BrC produced by slow MG/AS reaction in water, highlighting the substantial sensitivity of BrC to its formation conditions. The BrC samples were characterized before and after their exposure to broadband (270–390 nm) UV radiation. Irradiation led to rapid photobleaching, a decrease in the characteristic 280 nm absorption band, a complete loss of fluorescence, and a dramatic change in molecular composition. By comparing the composition before and after the irradiation, we identified several structural motifs that may contribute to the light-absorbing properties of MG/AS BrC. For example, a family of oligomers built from an imidazole carbonyl and repetitive MG units was prominent in the initial sample and decreased in abundance after photolysis. More complex oligomers containing both imidazole and pyrrole rings in their structures also appeared to contribute to the pool of BrC chromophores. The selective reduction of carbonyl functional groups by sodium borohydride diminished the absorption but had little effect on the fluorescence of MG/AS BrC samples, suggesting that absorption in this system is dominated by individual chromophores as opposed to supramolecular charge-transfer complexes. Due to the efficient photolysis of the BrC chromophores, this MG/AS BrC system has limited impact on the direct radiative forcing of climate but may have an effect on atmospheric photochemistry in aerosol particles.
中文翻译：
甲基乙二醛与硫酸铵水反应产物的光化学
甲基乙二醛（MG）和乙二醛与硫酸铵（AS）的水反应产生吸光化合物（发色团），并可作为大气中次级“棕色碳”（BrC）的来源。这些生色团的分子组成是模棱两可的，并且由于暴露于太阳紫外线辐射而导致的它们的转化还不是很清楚。我们检查了由MG / AS水溶液蒸发产生的BrC样品的分子组成，质量吸收系数和荧光光谱。蒸发产生的BrC的色谱图与水中缓慢MG / AS反应产生的BrC的色谱图不同，突显了BrC对其形成条件的实质敏感性。BrC样品在暴露于宽带（270–390 nm）紫外线辐射之前和之后进行表征。辐射导致光漂白迅速，280 nm特征吸收带的减少，荧光的完全丧失以及分子组成的巨大变化。通过比较辐照前后的成分，我们确定了可能有助于MG / AS BrC的光吸收性能的几个结构图案。例如，由咪唑羰基和重复的MG单元构建的一族低聚物在初始样品中突出，光解后的丰度降低。在其结构中同时包含咪唑环和吡咯环的更复杂的低聚物似乎也有助于BrC发色团的聚集。硼氢化钠选择性还原羰基官能团可减少吸收，但对MG / AS BrC样品的荧光影响很小，这表明该系统中的吸收主要由单个发色团决定，而不是超分子电荷转移络合物。由于BrC生色团的有效光解作用，该MG / AS BrC系统对气候的直接辐射强迫影响有限，但可能对气溶胶颗粒中的大气光化学产生影响。
更新日期：2017-09-28
中文翻译：
甲基乙二醛与硫酸铵水反应产物的光化学
甲基乙二醛（MG）和乙二醛与硫酸铵（AS）的水反应产生吸光化合物（发色团），并可作为大气中次级“棕色碳”（BrC）的来源。这些生色团的分子组成是模棱两可的，并且由于暴露于太阳紫外线辐射而导致的它们的转化还不是很清楚。我们检查了由MG / AS水溶液蒸发产生的BrC样品的分子组成，质量吸收系数和荧光光谱。蒸发产生的BrC的色谱图与水中缓慢MG / AS反应产生的BrC的色谱图不同，突显了BrC对其形成条件的实质敏感性。BrC样品在暴露于宽带（270–390 nm）紫外线辐射之前和之后进行表征。辐射导致光漂白迅速，280 nm特征吸收带的减少，荧光的完全丧失以及分子组成的巨大变化。通过比较辐照前后的成分，我们确定了可能有助于MG / AS BrC的光吸收性能的几个结构图案。例如，由咪唑羰基和重复的MG单元构建的一族低聚物在初始样品中突出，光解后的丰度降低。在其结构中同时包含咪唑环和吡咯环的更复杂的低聚物似乎也有助于BrC发色团的聚集。硼氢化钠选择性还原羰基官能团可减少吸收，但对MG / AS BrC样品的荧光影响很小，这表明该系统中的吸收主要由单个发色团决定，而不是超分子电荷转移络合物。由于BrC生色团的有效光解作用，该MG / AS BrC系统对气候的直接辐射强迫影响有限，但可能对气溶胶颗粒中的大气光化学产生影响。
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