Abstract Recently atmospheric brown carbon (BrC) is recognized as an important contributor to light absorption and positive climate forcing. In this work, daily fine particulate matter (PM2.5) samples were collected over a full year (May 2015–May 2016) in Nanjing, and seasonal light absorption properties of water-soluble BrC were investigated. We found that winter samples had the strongest light absorption among four seasons. The light absorption at 365 nm (Abs365) for all seasons linked closely with secondary organic carbon (SOC), indicating a dominant contribution from secondary sources to BrC. However primary biomass burning might also contribute to BrC as revealed by the good correlations of Abs365 versus levoglucosan fragments and/or K+, and such influence was more evident during summer. Furthermore, an Aerodyne soot-particle aerosol mass spectrometer (SP-AMS) was employed to determine the elemental ratios of BrC. We found that except in winter, the Abs365 in general positively correlated with the average oxidation states (OSc) of BrC, suggesting more BrC were produced at higher OSc. The mass absorption efficiency at 365 nm (MAE365) showed no clear dependences on OSc during spring, summer and fall, but decreased against OSc during winter, indicating chemical aging may lead to photo-bleaching of BrC in winter. Moreover, positive responses of Abs365 and MAE365 to N/C ratios were found during all seasons, indicating nitrogen-containing organics can be important BrC chromophores. Potential source areas of BrC were further discussed to improve our understanding of BrC sources in this region.

中文翻译：

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南京大气细颗粒物中水溶性棕碳的季节性吸光特性

摘要 最近，大气棕碳 (BrC) 被认为是光吸收和正气候强迫的重要贡献者。在这项工作中，在南京整年（2015 年 5 月至 2016 年 5 月）收集了每日细颗粒物 (PM2.5) 样本，并研究了水溶性 BrC 的季节性光吸收特性。我们发现冬季样品在四个季节中具有最强的光吸收。所有季节的 365 nm (Abs365) 光吸收与二次有机碳 (SOC) 密切相关，表明二次来源对 BrC 的主要贡献。然而，正如 Abs365 与左旋葡聚糖碎片和/或 K+ 的良好相关性所揭示的那样，初级生物质燃烧也可能对 BrC 有贡献，并且这种影响在夏季更为明显。此外，Aerodyne 烟尘颗粒气溶胶质谱仪 (SP-AMS) 用于确定 BrC 的元素比率。我们发现，除了在冬季，Abs365 通常与 BrC 的平均氧化态 (OSc) 呈正相关，表明在较高的 OSc 下会产生更多的 BrC。365 nm 处的质量吸收效率（MAE365）在春季、夏季和秋季对 OSc 没有明显的依赖性，但在冬季对 OSc 降低，表明化学老化可能导致冬季 BrC 光漂白。此外，在所有季节都发现 Abs365 和 MAE365 对 N/C 比率的积极响应，表明含氮有机物可能是重要的 BrC 生色团。进一步讨论了 BrC 的潜在来源区域，以提高我们对该地区 BrC 来源的了解。