Abstract/Summary

The Saharan Air Layer (SAL) in the summertime Eastern Atlantic is typically well-mixed and 3–4km 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 AER-D airborne campaign in August 2015, the typical structure described above was observed most of the times, and was associated with a moderate dust content yielding an Aerosol Optical Depth (AOD) of 0.3–0.4 at 355nm. 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 (0.3–2km). Estimated dust concentrations and column loadings spanned 300–5500μgm−3 and 0.8–7.5gm−2, respectively. The shortwave direct radiative impact of the intense dust event has been evaluated to be as large as −260±30 and −120±15Wm−2 at the surface and top of atmosphere, respectively. This event was also correlated 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 ~80μm (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 10–25μm. 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 should 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.