Biomass burning in southwestern Africa produces smoke plumes that are transported over the Atlantic Ocean and overlie vast regions of stratocumulus clouds. This aerosol layer contributes to direct and indirect radiative forcing of the atmosphere in this region particularly during the months of August, September, and October. There was a multi-year international campaign to study this aerosol and its interactions with clouds. Here, we report on the evolution of aerosol distributions and properties as measured by the airborne high spectral resolution lidar (HSRL-2) during the ORACLES (Observations of Aerosols above Clouds and their intEractionS) campaign in September 2016. The NASA Langley HSRL-2 instrument was flown on the NASA ER-2 aircraft for several days in September 2016. Data were aggregated at two pairs of 2^∘ × 2^∘ grid boxes to examine the evolution of the vertical profile of aerosol properties during transport over the ocean. Results showed that the structure of the profile of aerosol extinction and microphysical properties is maintained over a 1 to 2 d timescale. In the 3–5 km altitude range, 95 % of the aerosol extinction was contributed by particles in the 0.05–0.50 µm radius size range with the aerosol in this size range having an average effective radius of 0.16 µm. This indicates that there is essentially no scavenging or dry deposition at these altitudes. Moreover, there is very little day-to-day variation in these properties, such that time sampling as happens in such campaigns may be representative of longer periods such as monthly means. Below 3 km, there is considerable mixing with larger aerosol, most likely continental source near land. Furthermore, these measurements indicated that there was often a distinct gap between the bottom of the aerosol layer and cloud tops at the selected locations as evidenced by a layer of several hundred meters that contained relatively low aerosol extinction values above the clouds.

中文翻译：

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东南大西洋上空输送的生物质燃烧气溶胶垂直结构

非洲西南部的生物质燃烧产生的烟羽流经大西洋，覆盖大片层积云。该气溶胶层对该地区大气的直接和间接辐射强迫作出贡献，特别是在 8 月、9 月和 10 月期间。有一个多年的国际运动来研究这种气溶胶及其与云的相互作用。在这里，我们报告了 2016 年 9 月 ORACLES（云层上方气溶胶观测及其相互作用）活动期间通过机载高光谱分辨率激光雷达 (HSRL-2) 测量的气溶胶分布和性质的演变。美国宇航局兰利 HSRL-2仪器于 2016 年 9 月在 NASA ER-2 飞机上飞行了数天。数据汇总为两对 2 ^∘ ×  2 ^∘网格框，用于检查海洋运输过程中气溶胶特性垂直剖面的演变。结果表明，气溶胶消光和微物理特性的剖面结构在 1 到 2 天的时间尺度上保持不变。在 3-5 km 高度范围内，95% 的气溶胶消光是由 0.05-0.50  µm半径尺寸范围内的颗粒造成的，该尺寸范围内的气溶胶平均有效半径为 0.16  µm. 这表明在这些高度基本上没有清除或干沉积。此外，这些属性的日常变化非常小，因此此类活动中发生的时间采样可能代表更长的时期，例如每月平均值。在 3 公里以下，有相当大的气溶胶与较大的气溶胶混合，很可能是陆地附近的大陆源。此外，这些测量表明，在所选位置的气溶胶层底部和云顶之间通常存在明显的间隙，这可以从云层上方数百米的气溶胶消光值相对较低的层中得到证明。