ACPD Atmospheric Chemistry and Physics Discussions ACPD 1680-7375 Copernicus GmbH Göttingen, Germany 10.5194/acpd-10-13445-2010 Observations of Saharan dust microphysical and optical properties from the Eastern Atlantic during NAMMA airborne field campaign Chen G. 1 Ziemba L. D. 1 2 Chu D. A. 3 Thornhill K. L. 1 4 Schuster G. L. 1 Winstead E. L. 1 4 Diskin G. S. 1 Ferrare R. A. 1 Burton S. P. 1 Ismail S. 1 Kooi S. A. 1 4 Omar A. H. 1 Slusher D. L. 5 Kleb M. M. 1 Reid J. S. 6 Twohy C. H. 7 Zhang H. 8 Anderson B. E. 1 NASA Langley Research Center, Hampton, VA 23681, USA Oak Ridge Associated Universities, Oak Ridge, TN 37831, USA University of Maryland Baltimore County/NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA Science Systems and Applications, Inc., Hampton, VA 23666, USA Coastal Carolina University, Conway, SC 29528, USA Naval Research Laboratory, Monterey, CA 93943, USA Oregon State University, Corvallis, OR 97331, USA Georgia Institute of Technology, Atlanta, GA 30332, USA 26 05 2010 10 5 13445 13493 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.atmos-chem-phys-discuss.net/10/13445/2010/acpd-10-13445-2010.html The full text article is available as a PDF file from http://www.atmos-chem-phys-discuss.net/10/13445/2010/acpd-10-13445-2010.pdf

As part of the international project entitled "African Monsoon Multidisciplinary Analysis (AMMA)", NAMMA (NASA AMMA) aimed to gain a better understanding of the relationship between the African Easterly Waves (AEWs), the Sahara Air Layer (SAL), and tropical cyclogenesis. The NAMMA airborne field campaign was based out of the Cape Verde Islands during the peak of the hurricane season, i.e., August and September 2006. Multiple Sahara dust layers were sampled during 62 encounters in the eastern portion of the hurricane main development region, covering both the eastern North Atlantic Ocean and the western Saharan desert (i.e., 5–22° N and 10–35° W). The centers of these layers were located at altitudes between 1.5 and 3.3 km and the layer thickness ranged from 0.5 to 3 km. Detailed dust microphysical and optical properties were characterized using a suite of in situ instruments aboard the NASA DC-8 that included a particle counter, an Ultra-High Sensitivity Aerosol Spectrometer, an Aerodynamic Particle Sizer, nephelometer, and Particle Soot Absorption Photometer. The NAMMA dust observations showed relatively low particle number densities, ranging from 268 to 461 cm<sup>−3</sup>, but highly elevated volume density with an average at 45 &mu;m<sup>3</sup> cm<sup>−3</sup>. NAMMA dust particle size distributions were well represented by tri-modal lognormal regressions. The estimated volume median diameter (VMD) is averaged at 2.1 &mu;m with a small range of variation regardless of the vertical and geographical sampling locations. The absorption coefficient measurements exhibited a strong wavelength dependence for absorption but a weak one for scattering. The single scattering albedo was estimated at 0.97±0.02. Closure analyses showed that observed scattering and absorption coefficients are highly correlated with those calculated from spherical Mie-Theory and observed dust particle size distributions. The imaginary part of the refractive index for Sahara dust was estimated at 0.0022, with a range from 0.0015 to 0.0044. These values are generally consistent with literature values.

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