Department of Chemistry, University of California, Irvine, California 92697, USA.
Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, Washington 99352, USA.
Faraday Discuss. 2013;165:473-94. doi: 10.1039/c3fd00036b.
Sources and chemical composition of brown carbon are poorly understood, and even less is known about the mechanisms of its atmospheric transformations. This work presents molecular-level investigations of the reactive compound ketolimononaldehyde (KLA, C9H14O3), a second-generation ozonolysis product of limonene (C10H16), as a potent brown carbon precursor in secondary organic aerosol (SOA) through its reactions with reduced nitrogen compounds, such as ammonium ion (NH4+), ammonia, and amino acids. The reactions of synthesized and purified KLA with NH4+ and glycine resulted in the formation of chromophores nearly identical in spectral properties and formation rates to those found in similarly-aged limonene/O3 SOA. Similar chemical reaction processes of limononaldehyde (LA, C10H16O2) and pinonaldehyde (PA, C10H16O2), the first-generation ozonolysis products of limonene and alpha-pinene, respectively, were also studied, but the resulting products did not exhibit the light absorption properties of brown carbon, suggesting that the unique molecular structure of KLA produces visible-light-absorbing compounds. The KLA/NH4+ and KLA/GLY reactions produce water-soluble, hydrolysis-resilient chromophores with high mass absorption coefficients (MAC = 2000-4000 cm2 g(-1)) at lambda - 500 nm, precisely at the maximum of the solar emission spectrum. Liquid chromatography was used to isolate the light-absorbing fraction, and UV-Vis, FTIR, NMR and high-resolution mass spectrometry (HR-MS) techniques were used to investigate the structures and chemical properties of the light-absorbing compounds. The KLA browning reaction generates a diverse mixture of light-absorbing compounds, with the majority of the observable products containing 1-4 units of KLA and 0-2 nitrogen atoms. Based on the HR-MS product distribution, conjugated aldol condensates, secondary imines (Schiff bases), and N-heterocycles like pyrroles may contribute in varying degree to the light-absorbing properties of the KLA brown carbon. The results of this study demonstrate the high degree of selectivity of organic compound structures on the light-absorbing properties of SOA.
棕色碳的来源和化学成分了解甚少，其大气转化机制更是知之甚少。本工作通过其与还原氮化合物（如铵离子（NH4+）、氨和氨基酸）的反应，对作为二次有机气溶胶（SOA）中潜在棕色碳前体的柠檬烯（C10H16）第二代臭氧分解产物酮基柠檬醛（KLA，C9H14O3）进行了分子水平的研究。合成和纯化的 KLA 与 NH4+和甘氨酸的反应导致发色团的形成，其光谱性质和形成速率与在类似年龄的柠檬烯/O3 SOA 中发现的发色团几乎相同。还研究了柠檬醛（LA，C10H16O2）和 pinonaldehyde（PA，C10H16O2）的类似化学反应过程，分别为柠檬烯和α-蒎烯的第一代臭氧分解产物，但所得产物没有表现出棕色碳的光吸收特性，表明 KLA 的独特分子结构产生了可见光吸收化合物。KLA/NH4+和 KLA/GLY 反应产生水溶性、水解抗性的发色团，在 lambda - 500nm 处具有高质量吸收系数（MAC = 2000-4000cm2g-1），正好在太阳发射光谱的最大值处。使用液相色谱法分离吸光部分，并使用 UV-Vis、FTIR、NMR 和高分辨率质谱（HR-MS）技术研究吸光化合物的结构和化学性质。KLA 褐变反应生成了多种吸光化合物的混合物，其中大部分可观察到的产物含有 1-4 个 KLA 单元和 0-2 个氮原子。根据 HR-MS 产物分布，共轭羟醛缩合产物、次级亚胺（席夫碱）和吡咯等 N-杂环可能在不同程度上对 KLA 棕色碳的光吸收性质做出贡献。本研究结果表明，有机化合物结构对 SOA 光吸收性质具有高度选择性。
