Carbonaceous aerosols affect the global radiative balance by absorbing and scattering radiation, which leads to warming or cooling of the atmosphere, respectively. Black carbon is the main light-absorbing component. A portion of the organic aerosol known as brown carbon also absorbs light. The climate sensitivity to absorbing aerosols rapidly increases with altitude, but brown carbon measurements are limited in the upper troposphere. Here we present aircraft observations of vertical aerosol distributions over the continental United States in May and June 2012 to show that light-absorbing brown carbon is prevalent in the troposphere, and absorbs more short-wavelength radiation than black carbon at altitudes between 5 and 12 km. We find that brown carbon is transported to these altitudes by deep convection, and that in-cloud heterogeneous processing may produce brown carbon. Radiative transfer calculations suggest that brown carbon accounts for about 24% of combined black and brown carbon warming effect at the tropopause. Roughly two-thirds of the estimated brown carbon forcing occurs above 5 km, although most brown carbon is found below 5 km. The highest radiative absorption occurred during an event that ingested a wildfire plume. We conclude that high-altitude brown carbon from biomass burning is an unappreciated component of climate forcing.

中文翻译：

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对流层上层褐碳对大气顶辐射强迫的影响

碳质气溶胶通过吸收和散射辐射来影响全球辐射平衡，分别导致大气的变暖或变冷。黑炭是主要的吸光成分。一部分有机气溶胶（称为棕碳）也吸收光。随着高度的增加，对吸收气溶胶的气候敏感性迅速增加，但对流层高层的棕色碳测量受到限制。在这里，我们展示了飞机在2012年5月和2012年6月在美国大陆上垂直气溶胶分布的观测结果，以表明在对流层中吸收光的棕色碳普遍存在，并且在5到12之间的高度比黑碳吸收更多的短波长辐射 公里 我们发现棕色碳通过深对流被运输到这些高度，并且云中的异质处理可能会产生棕色碳。辐射转移计算表明，在对流层顶，褐碳约占黑碳和褐碳综合升温效应的24％。估计的约三分之二的棕色碳强迫发生在5  km以上，尽管发现大多数棕色碳在5  km以下。最高的辐射吸收发生在摄入野火烟羽的事件中。我们得出的结论是，来自生物质燃烧的高海拔棕色碳是气候强迫中不可理解的组成部分。