Laskin Alexander, West Christopher P, Hettiyadura Anusha P S
Department of Chemistry, Purdue University, West Lafayette, Indiana, 47906, USA.
Department of Earth, Atmospheric & Planetary Sciences, Purdue University, West Lafayette, Indiana, 47906, USA.
Chem Soc Rev. 2025 Feb 3;54(3):1583-1612. doi: 10.1039/d3cs00609c.
The light-absorbing chemical components of atmospheric organic aerosols are commonly referred to as Brown Carbon (BrC), reflecting the characteristic yellowish to brown appearance of aerosol. BrC is a highly complex mixture of organic compounds with diverse compositions and variable optical properties of its individual chromophores. BrC significantly influences the radiative budget of the climate and contributes to adverse air pollution effects such as reduced visibility and the presence of inhalable pollutants and irritants. However, a fundamental understanding of the sources, formation, and transformation (aging effects) of BrC remains incomplete. This gap in knowledge necessitates advanced chemical characterization of individual aerosol components and the correlation of their composition with optical properties. Over the past decade, a multi-modal analytical platform composed of high-performance liquid chromatography with a photodiode array UV-vis detector and high-resolution mass spectrometry has been extensively used for the untargeted analysis of BrC components in complex mixtures of atmospheric organic aerosols and their laboratory proxies. This method separates solvent-extractable BrC compounds into distinct fractions, each characterized by specific retention times, UV-vis absorption spectra, and elemental compositions, offering comprehensive molecular insights into BrC components. In this review, we highlight the application of this platform in analyzing both real-world aerosol samples and laboratory-generated proxies. These studies have identified composition-specific sources and transformations of BrC, advancing our understanding of these complex atmospheric mixtures. Atmospheric humic-like substances (HULIS), formed through cloud processing of wildfire smoke and the oligomerization of water-soluble organics, are key contributors to BrC. Additional HULIS originate from fossil fuel combustion, biogenic, and marine emissions. Key BrC chromophores include nitroaromatics, imidazoles, N-heterocycles, polyaromatic hydrocarbons, quinones, and others. Aging processes, including photolysis and multiphase reactions, can significantly alter BrC optical properties by generating new chromophores or degrading existing ones. The fundamental knowledge gained from these investigations is essential for assessing BrC optical properties. Additionally, it provides practical composition metrics necessary to inform and improve future atmospheric models, enabling more accurate predictions of BrC behavior and its impact on climate and air quality.
大气有机气溶胶的吸光化学成分通常被称为棕碳（BrC），这反映了气溶胶特有的黄至棕色外观。BrC是一种高度复杂的有机化合物混合物，其组成多样，各个发色团的光学性质也各不相同。BrC对气候的辐射收支有显著影响，并导致诸如能见度降低以及可吸入污染物和刺激性物质存在等不良空气污染效应。然而，对BrC的来源、形成和转化（老化效应）的基本理解仍不完整。这种知识上的差距需要对单个气溶胶成分进行先进的化学表征，并将其组成与光学性质相关联。在过去十年中，一个由配备光电二极管阵列紫外可见检测器的高效液相色谱和高分辨率质谱组成的多模态分析平台已被广泛用于对大气有机气溶胶及其实验室替代物的复杂混合物中的BrC成分进行非靶向分析。该方法将可溶剂萃取的BrC化合物分离成不同的馏分，每个馏分具有特定的保留时间、紫外可见吸收光谱和元素组成，从而提供对BrC成分的全面分子见解。在本综述中，我们重点介绍了该平台在分析实际气溶胶样品和实验室生成的替代物方面的应用。这些研究已经确定了BrC的特定组成来源和转化，增进了我们对这些复杂大气混合物的理解。通过野火烟雾的云处理和水溶性有机物的低聚作用形成的大气类腐殖质（HULIS）是BrC的关键贡献者。其他HULIS源自化石燃料燃烧、生物源和海洋排放。关键的BrC发色团包括硝基芳烃、咪唑、N-杂环、多环芳烃、醌等。老化过程，包括光解和多相反应，可通过产生新的发色团或降解现有发色团来显著改变BrC的光学性质。从这些研究中获得的基础知识对于评估BrC的光学性质至关重要。此外，它提供了必要的实际组成指标，以指导和改进未来的大气模型，从而能够更准确地预测BrC的行为及其对气候和空气质量的影响。
