Abstract We compared the imaginary part of the refractive indices (k) of brown carbon (BrC) retrieved from online and solvent-extraction offline light-absorption measurements. BrC with variable light-absorption properties was generated from the controlled combustion of three structurally different fuels: toluene (aromatic), isooctane (branched alkane), and cyclohexane (cyclic alkane). The online retrieval method involved combining real-time measurements of aerosol absorption coefficients (at 422, 532, and 781 nm) and size distributions with Mie calculations. The offline method involved extracting BrC samples with two organic solvents, methanol and dichloromethane, followed by light-absorption measurements of the extracts using a UV-vis spectrophotometer. For the least absorbing BrC, k values of the extracts in both solvents were similar to those of the aerosol. However, for darker BrC, k values of the BrC extracts in dichloromethane were smaller than those of the BrC aerosol, and k values of the BrC extracts in methanol were the smallest, with the discrepancy among the three increasing with increasing BrC darkness. These results indicate that BrC produced in this study was more soluble in dichloromethane than methanol, and the BrC solubility in both solvents decreased with increasing BrC darkness. Finally, k of BrC aerosol and extracts followed the same trend of decreasing wavelength dependence with increasing k as previous data. This further supports the brown-black continuum of light-absorption properties and that the differences observed between k of the BrC aerosol and extracts are due to the inefficiency of solvent extraction and not due to intrinsic differences between the online and offline methods. Copyright © 2020 American Association for Aerosol Research

中文翻译：

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从在线和离线测量中检索到的棕碳吸光特性之间的差异

摘要 我们比较了从在线和溶剂提取离线光吸收测量中获取的褐碳 (BrC) 折射率 (k) 的虚部。具有可变光吸收特性的 BrC 是由三种结构不同的燃料的受控燃烧产生的：甲苯（芳香族）、异辛烷（支链烷烃）和环己烷（环状烷烃）。在线检索方法包括将实时测量的气溶胶吸收系数（422、532 和 781 nm）和尺寸分布与 Mie 计算相结合。离线方法包括用两种有机溶剂甲醇和二氯甲烷提取 BrC 样品，然后使用紫外-可见分光光度计对提取物进行光吸收测量。对于吸收最少的 BrC，两种溶剂中提取物的 k 值与气溶胶的相似。然而，对于较深的 BrC，二氯甲烷中 BrC 提取物的 k 值小于 BrC 气溶胶的 k 值，甲醇中 BrC 提取物的 k 值最小，三者之间的差异随着 BrC 暗度的增加而增加。这些结果表明，本研究中产生的 BrC 在二氯甲烷中的溶解度高于甲醇，并且 BrC 在两种溶剂中的溶解度都随着 BrC 暗度的增加而降低。最后，BrC 气溶胶和提取物的 k 遵循与先前数据相同的随着 k 增加而降低波长依赖性的趋势。这进一步支持了光吸收特性的棕黑色连续体，并且观察到 BrC 气溶胶和提取物的 k 之间的差异是由于溶剂提取效率低下，而不是由于在线和离线方法之间的内在差异。版权所有 © 2020 美国气溶胶研究协会