Open cells exhibit weaker entrainment of free-tropospheric biomass burning aerosol into the south-east Atlantic boundary layer
Open cells exhibit weaker entrainment of free-tropospheric biomass burning aerosol into the south-east Atlantic boundary layer
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DOI：
10.5194/acp-20-4059-2020
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发表时间：
2020-04-06
影响因子：
6.3
通讯作者：
Flynn, Michael
中科院分区：
文献类型：
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作者：
Abel, Steven J.;Barrett, Paul A.;Flynn, Michael
这项工作展示了对遥远的东南大西洋开放环流(POC)的卫星、航空和地面协同观测。这些观测是在云和气溶胶辐射碰撞和强迫(CRILAY)和层状烟雾与云相互作用(LASIC)野外实验期间在阿森松岛的上风向获得的。这个案例研究的一个新的方面是，观察到从非洲大陆输送的广泛的自由对流层生物质燃烧气溶胶羽流与POC和周围封闭的蜂窝状云区的边界层逆温相接触。现场测量结果表明，开放环流与周围阴云场在边界层热力学结构、云性质、降水和气溶胶条件等方面存在明显的反差。结果表明，在POC的下风向层积云中，上层生物质燃烧气溶胶混合进入边界层，在逆风逆温下测得一氧化碳、黑碳质量负荷和积累型气溶胶浓度升高。该地区的层积云为中度污染，降水极少落在云底以下。观测到了一个迅速转变为活跃的积云和以薄而静止的面纱云形式存在的层状残留物的快速转变，这些残留物越过边界进入和深入POC。POC中的次云层比层积云区的次云层干净得多。POC中的云形成在边界层上部的超清洁层(积累式气溶胶浓度约为几厘米(-3))内，很可能是通过有效的碰撞合并和沉积过程形成的。在这个超清洁层中还间歇性地观察到艾特肯模式气溶胶浓度的增加，这表明正在发生新的粒子形成。在边界层逆温和超清洁层上方，积累型气溶胶浓度接近1000 cm(-3)。重要的是，POC中的空气质量没有证据表明，在一年中的这个位置和时间，一氧化碳含量超过了典型的本底条件。由于一氧化碳是生物质燃烧气溶胶的良好示踪剂，不容易被云处理和降水去除，这表明POC中的开放蜂窝对流不能携带大量直接位于边界层逆温顶部的自由对流层气溶胶。这表明，中尺度细胞对流的结构可能在调节气溶胶从自由对流层向下进入海洋边界层的过程中发挥重要作用。然后，我们从19年9月的中分辨率成像光谱仪(MODIS)TERRA图像发展了东南大西洋开放细胞云条件的气候学。这表明开放单元频率的极大值(>；0.25)出现在遥远的近海，并且在下沉的生物质燃烧的气溶胶羽流可能经常与下伏的边界层云接触的区域。如果观测案例研究的结果适用范围更广，那么开放单元对额外云凝结核的自由对流层侵入的明显低敏感性可能会对该地区的气溶胶-云相互作用产生一些重要的影响。
This work presents synergistic satellite, airborne and surface-based observations of a pocket of open cells (POC) in the remote south-east Atlantic. The observations were obtained over and upwind of Ascension Island during the CLouds and Aerosol Radiative Impacts and Forcing (CLARIFY) and the Layered Smoke Interacting with Clouds (LASIC) field experiments. A novel aspect of this case study is that an extensive free-tropospheric biomass burning aerosol plume that had been transported from the African continent was observed to be in contact with the boundary layer inversion over the POC and the surrounding closed cellular cloud regime. The in situ measurements show marked contrasts in the boundary layer thermodynamic structure, cloud properties, precipitation and aerosol conditions between the open cells and surrounding overcast cloud field.The data demonstrate that the overlying biomass burning aerosol was mixing down into the boundary layer in the stratocumulus cloud downwind of the POC, with elevated carbon monoxide, black carbon mass loadings and accumulation-mode aerosol concentrations measured beneath the trade-wind inversion. The stratocumulus cloud in this region was moderately polluted and exhibited very little precipitation falling below cloud base. A rapid transition to actively precipitating cumulus clouds and detrained stratiform remnants in the form of thin quiescent veil clouds was observed across the boundary into and deep within the POC. The subcloud layer in the POC was much cleaner than that in the stratocumulus region. The clouds in the POC formed within an ultra-clean layer (accumulation-mode aerosol concentrations of approximately a few cm(-3)) in the upper region of the boundary layer, which was likely to have been formed via efficient collision-coalescence and sedimentation processes. Enhanced Aitken-mode aerosol concentrations were also observed intermittently in this ultra-clean layer, suggesting that new particle formation was taking place. Across the boundary layer inversion and immediately above the ultra-clean layer, accumulation-mode aerosol concentrations were similar to 1000 cm(-3). Importantly, the air mass in the POC showed no evidence of elevated carbon monoxide over and above typical background conditions at this location and time of year. As carbon monoxide is a good tracer for biomass burning aerosol that is not readily removed by cloud processing and precipitation, it demonstrates that the open cellular convection in the POC is not able to entrain large quantities of the free-tropospheric aerosol that was sitting directly on top of the boundary layer inversion. This suggests that the structure of the mesoscale cellular convection may play an important role in regulating the transport of aerosol from the free troposphere down into the marine boundary layer.We then develop a climatology of open cellular cloud conditions in the south-east Atlantic from 19 years of September Moderate Resolution Imaging Spectroradiometer (MODIS) Terra imagery. This shows that the maxima in open cell frequency (> 0.25) occurs far offshore and in a region where subsiding biomass burning aerosol plumes may often come into contact with the underlying boundary layer cloud. If the results from the observational case study applied more broadly, then the apparent low susceptibility of open cells to free-tropospheric intrusions of additional cloud condensation nuclei could have some important consequences for aerosol-cloud interactions in the region.