The light-absorbing organic aerosol referred to as brown carbon (BrC) affects the global radiative balance. The linkages between its molecular composition and light absorption properties and how environmental factors influence BrC composition are not well understood. In this study, atmospheric dissolved organic matter (ADOM) in 55 aerosol samples from Guangzhou was characterized using Fourier transform ion cyclotron resonance mass spectrometry and light absorption measurements. The abundant components in ADOM were aliphatics and peptide-likes (in structure), or nitrogen- and sulfur-containing compounds (in elemental composition). The light absorption properties of ADOM were positively correlated with the levels of unsaturated and aromatic structures. Particularly, 17 nitrogen-containing species, which are identified by a random forest, characterized the variation of BrC absorption well. Aggregated boosted tree model and nonmetric multidimensional scaling analysis show that the BrC composition was largely driven by meteorological conditions and anthropogenic activities, among which biomass burning (BB) and OH radical were the two important factors. BrC compounds often accumulate with elevated BB emissions and related secondary processes, whereas the photolysis/photooxidation of BrC usually occurs under high solar radiance/^•OH concentration. This study first illuminated how environmental factors influence BrC at the molecular level and provided clues for the molecular-level research of BrC in the future.

中文翻译：

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大气溶解有机物的分子动力学和吸光特性

称为褐碳 (BrC) 的吸光有机气溶胶会影响全球辐射平衡。其分子组成与光吸收特性之间的联系以及环境因素如何影响 BrC 组成尚不清楚。在这项研究中，使用傅里叶变换离子回旋共振质谱和光吸收测量对来自广州的 55 个气溶胶样品中的大气溶解有机物 (ADOM) 进行了表征。ADOM 中的丰富成分是脂肪族和肽类（在结构上），或含氮和硫的化合物（在元素组成上）。ADOM 的光吸收特性与不饱和和芳香结构的水平呈正相关。特别是随机森林鉴定的17种含氮物种，很好地表征了 BrC 吸收的变化。聚合增强树模型和非度量多维尺度分析表明，BrC 组成在很大程度上受气象条件和人为活动的驱动，其中生物质燃烧（BB）和 OH 自由基是两个重要因素。BrC 化合物通常随着 BB 排放和相关二次过程的增加而积累，而 BrC 的光解/光氧化通常发生在高太阳辐射下/^• OH 浓度。本研究首先从分子水平阐明了环境因素对BrC的影响，为今后BrC分子水平的研究提供了线索。