| 摘要: |
| 棕碳（吸光性有机物）气溶胶对地表太阳辐射平衡有重要影响，溶剂萃取法是测量其光学性质的主要手段之一，但是该方法的结果很大程度上取决于所采用的溶剂。本研究分别采用甲醇和有机混合溶剂（乙腈∶二氯甲烷∶正己烷=2∶2∶1）提取大气总悬浮颗粒物样品（TSP）中的棕碳组分，并通过长光程（0.94 m）液芯波导毛细管流通池/紫外—可见分光光度计测量了两种溶剂所提取棕碳的吸收光谱（250—600 nm）。结果表明：300—600 nm甲醇提取棕碳的吸收系数均值是混合溶剂的(1.8±0.1)倍。而对于质量吸收效率，在280—600 nm甲醇提取的棕碳稍高于混合溶剂(1.2±0.1)倍，在250—280 nm混合溶剂提取的棕碳则明显高于甲醇(1.4±0.5)倍。为了研究这两种溶剂萃取棕碳光学性质的差异，本研究对比了两种溶剂对有机化合物的提取效率以及主要发色团组分（硝基芳香族化合物和多环芳烃类物质）在两种溶剂中的吸收光谱。实验结果表明：甲醇和混合溶剂所萃取棕碳的吸收光谱出现差异的主要原因包括以下三个方面：（1）与混合溶剂相比，甲醇对有机物提取效率更高。甲醇的提取效率为63.1%±3.4%，混合溶剂为42.9%±3.6%；（2）甲醇和混合溶剂所萃取棕碳的化学组成存在差异；（3）部分棕碳类物质（如硝基芳香族化合物）在两种溶剂中的吸收光谱存在差异。 |
| 关键词: 棕碳 光学性质 溶剂效应 发色团 |
| DOI：10.7515/JEE212017 |
| CSTR：32259.14.JEE212017 |
| 分类号: |
| 基金项目:国家自然科学基金项目（41877408，41925015）；中国科学院“从0到1”原始创新项目（ZDBS-LY-DQC001） |
| 英文基金项目:National Natural Science Foundation of China (41877408, 41925015); The Chinese Academy of Sciences (ZDBS-LY-DQC001) |
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| Effects of extraction solvents on the optical properties of brown carbon aerosols |
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ZHAO Meiling, HUANG Rujin, YANG Lu
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1. State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, Chinese Academy of Science, Xi’an 710061, China
2. University of Chinese Academy of Sciences, Beijing 100049, China
3. CAS Center for Excellence in Quaternary Science and Global Change, Xi’an 710061, China
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| Abstract: |
| Background, aim, and scope Brown carbon (BrC), the light-absorbing organic matter, has significant impacts on Earth’s radiative balance due to its strong absorption capability in the ultraviolet-visible region. Solvent extraction has been widely used to characterize the chemical chromophores and optical properties of BrC. However, the effects of solvents on BrC optical properties are not well understood. In this study, we compare the optical properties of BrC extracted by methanol and organic mixed solvent (acetonitrile∶dichloromethane∶hexane=2∶2∶1, v∶v), which are commonly used for BrC extraction, and the absorption spectra of main chromophores in these two solvents to investigate the effects of solvents on BrC optical properties. Materials and methods The ambient samples are the total suspended particulate collected in Xi’an. The sampling time was 12 h for each sample, and the sampling flow rate was 0.3 m3·min−1. The samples were measured for organic carbon (OC) and absorption coefficient. The absorption spectra of nitroaromatic compounds and polycyclic aromatic hydrocarbons in these two solvents were also measured. Results At 300—600 nm, the average absorption coefficient of the samples extracted by methanol and the organic mixed solvent are (7.6±4.7) Mm−1 and (14.6±9.5) Mm−1, respectively. The AAE are (5.7±0.3) for BrC extracted by methanol and (5.5±0.3) for BrC extracted by organic mixed solvent. The total organic carbon extracted by methanol is about 20% higher than by the organic mixed solvent. The absorption spectra of some nitroaromatic compounds in these two solvents are quite different, but no obvious difference for polycyclic aromatic hydrocarbons. Discussion Compared with methanol, the BrC extracted by the organic mixed solvent exhibits stronger light absorption ability at short wavelengths (250—280 nm) but weaker at long wavelengths (280—600 nm). The sum of the absorption intensity from multiple extractions by these two solvents are different, suggesting the differences in BrC composition extracted by methanol and organic mixed solvent. Meanwhile, the differences in absorption spectra between nitroaromatic compounds and polycyclic aromatic hydrocarbons in these two solvents reflect that polar chromophores may have different forms in these two solvents. Conclusions The optical properties of BrC extracted by methanol and organic mixed solvent are diverse and the possible reasons are as follows: (1) Methanol can extract more brown carbon compounds compared with organic mixed solvent. (2) Some chromophores extracted by methanol and organic mixed solvents are different. (3) Some brown carbon compounds have different forms in different solvents. Recommendations and perspectives This work investigated the effects of extraction solvents on the optical properties of brown carbon and the possible reasons for the difference in absorption. Future studies should focus more on the chemical forms of BrC chromophores and the link with their optical properties. |
| Key words: brown carbon optical properties solvent effect chromophore |
