Eugene Alexis J, Xia Sha-Sha, Guzman Marcelo I
Department of Chemistry, University of Kentucky , Lexington, Kentucky 40506, United States.
J Phys Chem A. 2016 Jun 2;120(21):3817-26. doi: 10.1021/acs.jpca.6b00225. Epub 2016 May 24.
Aerosols affect climate change, the energy balance of the atmosphere, and public health due to their variable chemical composition, size, and shape. While the formation of secondary organic aerosols (SOA) from gas phase precursors is relatively well understood, studying aqueous chemical reactions contributing to the total SOA budget is the current focus of major attention. Field measurements have revealed that mono-, di-, and oxo-carboxylic acids are abundant species present in SOA and atmospheric waters. This work explores the fate of one of these 2-oxocarboxylic acids, glyoxylic acid, which can photogenerate reactive species under solar irradiation. Additionally, the dark thermal aging of photoproducts is studied by UV-visible and fluorescence spectroscopies to reveal that the optical properties are altered by the glyoxal produced. The optical properties display periodicity in the time domain of the UV-visible spectrum of chromophores with absorption enhancement (thermochromism) or loss (photobleaching) during nighttime and daytime cycles, respectively. During irradiation, excited state glyoxylic acid can undergo α-cleavage or participate in hydrogen abstractions. The use of (13)C nuclear magnetic resonance spectroscopy (NMR) analysis shows that glyoxal is an important intermediate produced during direct photolysis. Glyoxal quickly reaches a quasi-steady state as confirmed by UHPLC-MS analysis of its corresponding (E) and (Z) 2,4-dinitrophenylhydrazones. The homolytic cleavage of glyoxylic acid is proposed as a fundamental step for the production of glyoxal. Both carbon oxides, CO2(g) and CO(g) evolving to the gas-phase, are quantified by FTIR spectroscopy. Finally, formic acid, oxalic acid, and tartaric acid photoproducts are identified by ion chromatography (IC) with conductivity and electrospray (ESI) mass spectrometry (MS) detection and (1)H NMR spectroscopy. A reaction mechanism is proposed based on all experimental observations.
气溶胶因其化学成分、大小和形状各异，会影响气候变化、大气能量平衡以及公众健康。虽然气相前体形成二次有机气溶胶（SOA）的过程相对已为人熟知，但研究对SOA总量有贡献的水相化学反应是当前主要关注的焦点。实地测量表明，一元、二元和氧代羧酸是SOA和大气水体中大量存在的物种。本研究探讨了这些氧代羧酸之一的乙醛酸的归宿，乙醛酸在太阳辐射下可光生成活性物种。此外，通过紫外可见光谱和荧光光谱研究了光产物的暗热老化，结果表明所产生的乙二醛会改变光学性质。在发色团的紫外可见光谱时域中，光学性质呈现周期性变化，在夜间和白天循环期间分别出现吸收增强（热致变色）或损失（光漂白）。在辐照过程中，激发态乙醛酸可发生α-裂解或参与氢提取反应。使用（13）C核磁共振光谱（NMR）分析表明，乙二醛是直接光解过程中产生的重要中间体。通过对其相应的（E）和（Z）2,4-二硝基苯腙的超高效液相色谱-质谱（UHPLC-MS）分析证实，乙二醛迅速达到准稳态。乙醛酸的均裂被认为是生成乙二醛的基本步骤。通过傅里叶变换红外光谱（FTIR）对释放到气相中的两种碳氧化物CO2（g）和CO（g）进行了定量分析。最后，通过离子色谱（IC）结合电导检测、电喷雾（ESI）质谱（MS）检测以及（1）H NMR光谱鉴定了甲酸、草酸和酒石酸光产物。基于所有实验观察结果提出了一个反应机理。
