Discrepancies between brown carbon light-absorption properties retrieved from online and offline measurements
Discrepancies between brown carbon light-absorption properties retrieved from online and offline measurements
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DOI：
10.1080/02786826.2020.1820940
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发表时间：
2020-10-01
影响因子：
5.2
通讯作者：
Saleh, Rawad
中科院分区：
文献类型：
--
作者：
Cheng, Zezhen;Atwi, Khairallah;Saleh, Rawad
我们比较了从在线和溶剂萃取离线光吸收测量中提取的棕色碳(BRC)的折射率(K)的虚部。三种结构不同的燃料甲苯(芳香烃)、异辛烷(支链烷烃)和环己烷(环烷烃)的受控燃烧生成了具有不同光吸收性能的BRC。在线检索方法包括将实时测量的气溶胶吸收系数(在422、532和781纳米处)和粒子分布与MIE计算相结合。离线方法包括用甲醇和二氯甲烷两种有机溶剂提取BRC样品，然后使用UV-Vis分光光度计测量提取物的光吸收。对于吸收最少的BRC，两种溶剂中提取物的k值与气雾剂的k值相似。然而，对于较暗的BRC，BRC在二氯甲烷中的K值小于BRC气溶胶的K值，而在甲醇中的K值最小，三者之间的差异随着BRC暗度的增加而增大。这些结果表明，本研究生产的BRC在二氯甲烷中的溶解度高于在甲醇中的溶解度，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，KOF BRC气溶胶和萃取物遵循与先前数据相同的趋势，即随着kA的增加而减小对波长的依赖性。这进一步支持了棕黑色连续光吸收特性，并且观察到的BRC气雾剂和萃取物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(C)2020美国气溶胶研究协会
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,kvalues of the extracts in both solvents were similar to those of the aerosol. However, for darker BrC,kvalues of the BrC extracts in dichloromethane were smaller than those of the BrC aerosol, andkvalues 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,kof BrC aerosol and extracts followed the same trend of decreasing wavelength dependence with increasingkas previous data. This further supports the brown-black continuum of light-absorption properties and that the differences observed betweenkof 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 (c) 2020 American Association for Aerosol Research