Brown carbon (BrC) exhibits highly variable light-absorption properties, with the imaginary part of the refractive indices (k) varying over several orders of magnitude. This poorly understood variability poses a challenge to accurately determining the BrC climate effect. Here, we present a framework to explain the variability in BrC k. We hypothesize that a fraction of BrC is composed of black-carbon (BC) precursors whose transformation to BC is not complete, and that there is a continuum of light-absorption properties along which BC and BrC lie. To test this hypothesis, we performed controlled-combustion experiments using benzene and toluene. By systematically varying the combustion conditions, we isolated BrC components along the brown–black continuum progressing from light (k = 0.004 at 550 nm) to dark (k = 0.25 at 550 nm). Using laser-desorption-ionization mass spectrometry and thermodenuder measurements, we show that the BrC progression from light to dark is associated with an increase in molecular size and decrease in volatility. The darkest BrC has molecular sizes of several 1000 Da, is refractory, and is optically more similar to BC than the lighter BrC, blurring the lines between the optical properties of BrC and BC.

中文翻译：

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吸光燃烧气溶胶的棕黑连续体

褐碳（BrC）表现出高度可变的光吸收特性，折射率（k）的虚部在几个数量级上变化。这种对变化的了解不足，对准确确定BrC气候影响提出了挑战。在这里，我们提出一个框架来解释BrC k的变异性。我们假设一部分BrC由黑碳（BC）前驱体组成，这些前驱体向BC的转化不完全，并且存在BC和BrC沿着其连续的光吸收特性。为了验证这一假设，我们使用苯和甲苯进行了燃烧实验。通过系统地改变燃烧条件，我们从光（在550 nm处k = 0.004）至暗（在550 nm处k = 0.25）。使用激光解吸电离质谱和热剥蚀仪测量，我们显示BrC从亮到暗的进展与分子大小的增加和挥发性的降低有关。最暗的BrC的分子大小为几千Da，具有耐火性，并且在光学上与BC较浅的BrC更相似，从而模糊了BrC和BC光学特性之间的界线。