This study provides molecular insights into the light absorption properties of biomass burning (BB) brown carbon (BrC) through the chemical characterization of tar condensates generated from heated wood pellets at oxidative and pyrolysis conditions. Both liquid tar condensates separated into “darker oily” and “lighter aqueous” immiscible phases. The molecular composition of these samples was investigated using reversed-phase liquid chromatography coupled with a photodiode array detector and a high-resolution mass spectrometer. The results revealed two sets of BrC chromophores: (1) common to all four samples and (2) specific to the “oily” fractions. The common BrC chromophores consist of polar, monoaromatic species. The oil-specific BrC chromophores include less-polar and nonpolar polyaromatic compounds. The most-light-absorbing pyrolysis oily phase (PO) was aerosolized and size-separated using a cascade impactor to compare the composition and optical properties of the bulk versus the aerosolized BrC. The mass absorption coefficient (MAC_{300–500 nm}) of aerosolized PO increased compared to that of the bulk, due to gas-phase partitioning of more volatile and less absorbing chromophores. The optical properties of the aerosolized PO were consistent with previously reported ambient BB BrC measurements. These results suggest the darkening of atmospheric BrC following non-reactive evaporation that transforms the optical properties and composition of aged BrC aerosols.

中文翻译：

---

与生物质燃烧有关的大气棕碳的化学组成和分子特异性光学性质

这项研究通过氧化和热解条件下从加热的木质颗粒产生的焦油冷凝物的化学表征，提供了对生物质燃烧（BB）棕碳（BrC）的光吸收特性的分子见解。液态焦油冷凝物分为“较暗的油性”和“较轻的水相”不混溶相。这些样品的分子组成使用反相液相色谱与光电二极管阵列检测器和高分辨率质谱仪联用进行了研究。结果显示了两组BrC生色团：（1）所有四个样品共有，（2）特定于“油性”馏分。常见的BrC生色团由极性单芳族物质组成。油特异性BrC生色团包括极性较小和非极性的多芳族化合物。使用级联撞击器对吸光度最高的热解油相（PO）进行雾化和尺寸分离，以比较块状和雾化的BrC的组成和光学性质。质量吸收系数（MAC相较于大部分，_{300-500 nm}）的雾化PO增加了，这是由于气相分配了更多易挥发和吸收较少的生色团。雾化PO的光学性质与先前报道的环境BB BrC测量结果一致。这些结果表明，非反应性蒸发后大气BrC变黑，这会改变老化的BrC气溶胶的光学性质和组成。