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Discussion papers
https://doi.org/10.5194/acp-2019-421
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2019-421
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 27 Jun 2019

Submitted as: research article | 27 Jun 2019

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This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Chemistry and Physics (ACP).

Coarse and Giant Particles are Ubiquitous in Saharan Dust Export Regions and are Radiatively Significant over the Sahara

Claire L. Ryder1, Eleanor J. Highwood1, Adrian Walser2, Petra Seibert3, Anne Philipp2, and Bernadett Weinzierl2 Claire L. Ryder et al.
  • 1Department of Meteorology, University of Reading, Whiteknights, Reading, RG6 6BB, UK
  • 2University of Vienna, Faculty of Physics, Aerosol Physics and Environmental Physics, Vienna, Austria
  • 3University of Natural Resources and Life Sciences, Institute of Meteorology, Vienna, Austria

Abstract. Mineral dust is an important component of the climate system, interacting with radiation, clouds and biogeochemical systems, and impacting atmospheric circulation, air quality, aviation and solar energy generation. These impacts are sensitive to dust particle size distribution (PSD), yet models struggle or even fail to represent coarse (diameter (d) > 2.5 µm) and giant (d>20 µm) dust particles and the evolution of the PSD with transport. Here we examine three state-of-the-art airborne observational datasets, all of which measured the full size range of dust (d = 0.1 to > 100 µm) at different stages during transport, with consistent instrumentation. We quantify the presence and evolution of coarse and giant particles and their contribution to optical properties. Observations are taken from the Fennec fieldwork over the Sahara and in the Saharan Air Layer (SAL) near the Canary Islands, and from the AER-D fieldwork in the vicinity of the Cape Verde Islands in the SAL.

Observations show significantly more abundant coarse and giant dust particles over the Sahara compared to the SAL: effective diameters of up to 20 µm were observed over the Sahara, compared to 4 µm in the SAL. Mass profiles show that over the Sahara 40 % of dust mass was found in the giant mode, contrasting to 2 to 12 % in the SAL. Size-resolved optical property calculations show that in the shortwave (longwave) spectrum excluding the giant mode omits 18 % (26 %) of extinction over the Sahara, compared to 1–4 % (2–6 %) in the SAL. Excluding giant particles results in significant underestimation of both shortwave and longwave extinction over the Sahara, as well as of mass concentration, while the effects in the SAL are smaller but non-negligible. Omitting the giant mode results in a greater omission of dust longwave radiative effects compared to the shortwave, suggesting a bias towards a radiative cooling effect of dust when the giant mode is excluded and/or the coarse mode is underestimated. This will be important in dust models, which typically exclude giant particles and underestimate coarse mode concentrations.

A compilation of effective diameters against dust age since uplift time suggests that two regimes of dust transport exist. During the initial 1.5 days, both coarse and giant particles are rapidly deposited. During the subsequent 1.5 to 10 days, PSD barely changes with transport, and the coarse mode is retained to a much greater degree than expected from estimates of gravitational sedimentation alone. The reasons for this are unclear, and warrant further investigation in order to improve dust transport schemes, and the associated radiative effects of coarse and giant particles in models.

Claire L. Ryder et al.
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Short summary
Mineral dust is lifted into the atmosphere from desert regions, where it can be transported over thousands of kilometres around the world. Dust impacts weather, climate, aviation and air quality. We evaluate new aircraft observations of dust size. We find that the largest particles, typically omitted by models, have a significant impact on the interactions of dust with radiation and therefore climate. We also find that large dust particles are retained in the atmosphere longer than expected.
Mineral dust is lifted into the atmosphere from desert regions, where it can be transported over...
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