Vertical variability of the properties of highly aged biomass burning aerosol transported over the southeast Atlantic during CLARIFY-2017
Vertical variability of the properties of highly aged biomass burning aerosol transported over the southeast Atlantic during CLARIFY-2017
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DOI：
10.5194/acp-2020-197
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发表时间：
2020-03
影响因子：
6.3
通讯作者：
Huihui Wu;Jonathan W. Taylor;K. Szpek;J. Langridge;P. Williams;M. Flynn;J. Allan;S. Abel;J. Pitt;M. Cotterell;C. Fox;Nicholas W. Davies;J. Haywood;H. Coe
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文献类型：
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作者：
Huihui Wu;Jonathan W. Taylor;K. Szpek;J. Langridge;P. Williams;M. Flynn;J. Allan;S. Abel;J. Pitt;M. Cotterell;C. Fox;Nicholas W. Davies;J. Haywood;H. Coe
抽象的。非洲中部和南部 6 月至 10 月的季节性生物质燃烧（BB）导致每年吸收气溶胶被输送到南大西洋，并对区域气候强迫产生重大影响。使用 2017 年云-气溶胶-辐射相互作用和强迫 (CLARIFY-2017) 活动期间进行的机载原位测量，描述了偏远东南大西洋上亚微米气溶胶的垂直分布及其特性。在阿森松岛周边地区、海洋边界层 (MBL) 和自由对流层 (FT) 长达 5 公里的区域，大量观测到由受控火焰释放的 BB 气溶胶。我们表明，如高度氧化的有机气溶胶所示，气溶胶在从源头运输的 > 7 d 期间经历了显着的老化过程。远场 CLARIFY 区域的高度老化的 BB 气溶胶也特别富含黑碳 (BC)，与其他 BB 传输区域的气溶胶相比，其单散射反照率 (SSA) 相对较低。 CLARIFY 期间的柱加权干 SSA 在 405、550 和 658 nm 处分别为 0.85、0.84 和 0.83。我们还发现干 SSA 存在显着的垂直变化，这是相对化学成分和尺寸的函数。柱中的最低SSA通常位于海拔2000 m左右的低FT层（平均值：405、550和658 nm处的0.82、0.81和0.79）。这一发现很重要，因为它意味着整个东南大西洋地区的 BB 气溶胶比目前气候模型中显示的更具吸收性，这意味着 BB 的辐射强迫可能比之前认为的更强烈。此外，在 FT 中，高度达到 5 公里时，405、550 和 658 nm 处的平均 SSA 增加到 0.87、0.86 和 0.85。这与无机硝酸盐质量分数和气溶胶尺寸的增加有关，可能是由于在较高海拔、较低温度和较高相对湿度下硝酸铵与现有颗粒的分配增加所致。夹带进入边界层 (BL) 后，气溶胶的干燥尺寸通常比 FT 中的更小，并且具有更大比例的散射材料，从而导致平均干燥 SSA 更高，这主要是由于海洋排放和毛毛雨去除气溶胶所致。在 BL 中，SSA 从表面到 BL 顶部逐渐减少，在表面附近观察到柱中的 SSA 最高。我们的结果为用于模拟这一重要区域的区域辐射相互作用的气溶胶参数化提供了独特的观测约束。我们建议未来的工作应考虑这种垂直变化对气候模型的影响。
Abstract. Seasonal biomass burning (BB) from June to October in central and southern Africa leads to absorbing aerosols being transported over the South Atlantic Ocean every year and contributes significantly to the regional climate forcing. The vertical distribution of submicron aerosols and their properties were characterized over the remote southeast Atlantic, using airborne in situ measurements made during the CLoud-Aerosol-Radiation Interactions and Forcing for Year 2017 (CLARIFY-2017) campaign. BB aerosols emitted from flaming-controlled fires were intensively observed in the region surrounding Ascension Island, in the marine boundary layer (MBL) and free troposphere (FT) up to 5 km. We show that the aerosols had undergone a significant ageing process during > 7 d transit from source, as indicated by the highly oxidized organic aerosol. The highly aged BB aerosols in the far-field CLARIFY region were also especially rich in black carbon (BC), with relatively low single-scattering albedos (SSAs), compared with those from other BB transported regions. The column-weighted dry SSAs during CLARIFY were observed to be 0.85, 0.84 and 0.83 at 405, 550 and 658 nm respectively. We also found significant vertical variation in the dry SSA, as a function of relative chemical composition and size. The lowest SSA in the column was generally in the low FT layer around 2000 m altitude (averages: 0.82, 0.81 and 0.79 at 405, 550 and 658 nm). This finding is important since it means that BB aerosols across the southeast Atlantic region are more absorbing than currently represented in climate models, implying that the radiative forcing from BB may be more strongly positive than previously thought. Furthermore, in the FT, average SSAs at 405, 550 and 658 nm increased to 0.87, 0.86 and 0.85 with altitude up to 5 km. This was associated with an enhanced inorganic nitrate mass fraction and aerosol size, likely resulting from increased partitioning of ammonium nitrate to the existing particles at higher altitude with lower temperature and higher relative humidity. After entrainment into the boundary layer (BL), aerosols were generally smaller in dry size than in the FT and had a larger fraction of scattering material with resultant higher average dry SSA, mostly due to marine emissions and aerosol removal by drizzle. In the BL, the SSA decreased from the surface to the BL top, with the highest SSA in the column observed near the surface. Our results provide unique observational constraints on aerosol parameterizations used in modelling regional radiation interactions over this important region. We recommend that future work should consider the impact of this vertical variability on climate models.