Unexpected vertical structure of the Saharan Air Layer and giant dust particles during AER-D
Unexpected vertical structure of the Saharan Air Layer and giant dust particles during AER-D
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DOI：
10.5194/acp-18-17655-2018
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发表时间：
2018-12
影响因子：
6.3
通讯作者：
F. Marenco;C. Ryder;V. Estellés;D. O’Sullivan;J. Brooke;Luke Orgill;G. Lloyd;M. Gallagher
中科院分区：
文献类型：
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作者：
F. Marenco;C. Ryder;V. Estellés;D. O’Sullivan;J. Brooke;Luke Orgill;G. Lloyd;M. Gallagher
抽象的。夏季东大西洋的撒哈拉空气层（SAL）通常混合良好，3-4公里深，覆盖在海洋边界层（MBL）上。在本文中，我们展示了实验证据，有时可以观察到一个非常不同的结构。在2015年8月的气溶胶特性-尘埃（AER-D）空中活动期间，大部分时间都观察到了上述典型结构，并且与中等尘埃含量相关，在355 nm处产生0.3-0.4的气溶胶光学厚度（AOD）。然而，在一个强烈的事件，一个前所未有的垂直结构被观察到靠近盆地的东部边界，显示出不均匀的垂直分布和一个非常大的AOD（1.5-2），与大多数的灰尘在一个比平常低得多的水平（0.3-2公里）。在活动期间，所有飞行的估计粉尘浓度和柱负载分别为300-5500和0.8-7.5 g m-2。强沙尘事件的短波直接辐射影响在地表和大气层顶（TOA）分别高达-260 30和-120 15 W m−2。我们还报告了这一事件与加那利群岛异常闪电活动的相关性。在所有情况下，我们的测量都检测到了广泛的气溶胶尺寸分布，范围从0.1到0.80 µm（直径），从而突出了巨型粒子的存在。在MBL中还发现了巨大的尘埃颗粒。我们注意到，大多数气溶胶模型可能会错过巨大的颗粒，因为它们使用的尺寸箱高达10-25 µm。这种不寻常的垂直结构和巨大的粒子可能对强烈事件期间大西洋上空的尘埃输送产生影响，并可能影响对沉积到海洋中的尘埃的估计。我们认为，未来的活动可以更多地关注高气溶胶负载的事件，能够探测巨型粒子的仪器将是世界这一地区尘埃观测的关键。
Abstract. The Saharan Air Layer (SAL) in the summertime eastern Atlantic is typically well mixed and 3–4 km deep, overlying the marine boundary layer (MBL). In this paper, we show experimental evidence that at times a very different structure can be observed. During the AERosol properties – Dust (AER-D) airborne campaign in August 2015, the typical structure described above was observed most of the time, and was associated with a moderate dust content yielding an aerosol optical depth (AOD) of 0.3–0.4 at 355 nm. In an intense event, however, an unprecedented vertical structure was observed close to the eastern boundary of the basin, displaying an uneven vertical distribution and a very large AOD (1.5–2), with most of the dust in a much lower level than usual (0.3–2 km). Estimated dust concentrations and column loadings for all flights during the campaign spanned 300–5500 and 0.8–7.5 g m−2, respectively. The shortwave direct radiative impact of the intense dust event has been evaluated to be as large as -260±30 and -120±15 W m−2 at the surface and top of atmosphere (TOA), respectively. We also report the correlation of this event with anomalous lightning activity in the Canary Islands. In all cases, our measurements detected a broad distribution of aerosol sizes, ranging from ∼0.1 to ∼80 µm (diameter), thus highlighting the presence of giant particles. Giant dust particles were also found in the MBL. We note that most aerosol models may miss the giant particles due to the fact that they use size bins up to 10–25 µm. The unusual vertical structure and the giant particles may have implications for dust transport over the Atlantic during intense events and may affect the estimate of dust deposited to the ocean. We believe that future campaigns could focus more on events with high aerosol load and that instrumentation capable of detecting giant particles will be key to dust observations in this part of the world.