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Atmos. Chem. Phys., 18, 1023–1043, 2018 https://doi.org/10.5194/acp-18-1023-2018 © Author(s) 2018. This work is distributed under the Creative Commons Attribution 3.0 License. A detailed characterization of the Saharan dust collected during the Fennec campaign in 2011: in situ ground-based and laboratory measurements Adriana Rocha-Lima1,2, J. Vanderlei Martins1,3, Lorraine A. Remer1, Martin Todd4, John H. Marsham5,6, Sebastian Engelstaedter7, Claire L. Ryder8, Carolina Cavazos-Guerra9, Paulo Artaxo10, Peter Colarco2, and Richard Washington7 1University of Maryland, Baltimore County, Baltimore, MD, USA 2Atmospheric Chemistry and Dynamic Laboratory, NASA Goddard Space Flight Center, Greenbelt, MD, USA 3Climate and Radiation Laboratory, NASA Goddard Space Flight Center, Greenbelt, MD, USA 4Department of Geography, University of Sussex, Sussex, UK 5School of Earth and Environment, University of Leeds, Leeds, UK 6National Center for Atmospheric Science, Leeds, UK 7Climate Research Lab, Oxford University Center for the Environment, Oxford, UK 8Department of Meteorology, University of Reading, Reading, UK 9Institute for Advanced Sustainability Studies, Potsdam, Germany 10Instituto de Física, Universidade de São Paulo, São Paulo, Brazil Correspondence: Adriana Rocha-Lima ([email protected]) Received: 25 March 2017 – Discussion started: 20 June 2017 Revised: 14 November 2017 – Accepted: 7 December 2017 – Published: 26 January 2018 Abstract. Millions of tons of mineral dust are lifted by the atively high mean SSA of 0.995 at 670 nm was observed at wind from arid surfaces and transported around the globe ev- this site. The laboratory results show for the fine particle size ery year. The physical and chemical properties of the min- distributions (particles diameter < 5µm and mode diameter at eral dust are needed to better constrain remote sensing ob- 2–3 µm) in both sites a spectral dependence of the imaginary servations and are of fundamental importance for the under- part of the refractive index Im.m/ with a bow-like shape, standing of dust atmospheric processes. Ground-based in situ with increased absorption in UV as well as in the short- measurements and in situ filter collection of Saharan dust wave infrared. The same signature was not observed, how- were obtained during the Fennec campaign in the central ever, in the mixed particle size distribution (particle diame- Sahara in 2011. This paper presents results of the absorp- ter < 10 µm and mode diameter at 4 µm) in Algeria. Im.m/ tion and scattering coefficients, and hence single scattering was found to range from 0.011 to 0.001i for dust collected in albedo (SSA), of the Saharan dust measured in real time dur- Algeria and 0.008 to 0.002i for dust collected in Mauritania ing the last period of the campaign and subsequent labora- over the wavelength range of 350–2500 nm. Differences in tory analysis of the dust samples collected in two supersites, the mean elemental composition of the dust collected in the SS1 and SS2, in Algeria and in Mauritania, respectively. The supersites in Algeria and in Mauritania and between fine and samples were taken to the laboratory, where their size and as- mixed particle size distributions were observed from EDXRF pect ratio distributions, mean chemical composition, spectral measurements, although those differences cannot be used to mass absorption efficiency, and spectral imaginary refractive explain the optical properties variability between the sam- index were obtained from the ultraviolet (UV) to the near- ples. Finally, particles with low-density typically larger than infrared (NIR) wavelengths. At SS1 in Algeria, the time se- 10 µm in diameter were found in some of the samples col- ries of the scattering coefficients during the period of the lected at the supersite in Mauritania, but these low-density campaign show dust events exceeding 3500 Mm−1, and a rel- particles were not observed in Algeria. Published by Copernicus Publications on behalf of the European Geosciences Union. 1024 A. Rocha-Lima et al.: A detailed characterization of the Saharan dust collected during the Fennec campaign 1 Introduction aerosol is considered. For instance, for dust aerosols, models show a range in simulated atmospheric loading by a factor Mineral dust originating from deserts and other arid surfaces of 4 and a range of simulated emissions of nearly a factor is one of the most abundant aerosols in the atmosphere. Ac- of 10 (Huneeus et al., 2011). A large part of this variabil- cording to Boucher et al.(2013), dust corresponds to 35 % ity among various models predictions is associated with dif- of the total continental aerosol mass of particles with diam- ferences in the parameters used to describe emission, trans- eter smaller than 10 µm. Roughly half of all aerosols above port, and optical and microphysical properties of the aerosols North America are dust particles that have been transported (Textor et al., 2006). Observational constraints on Saharan from other continents (Yu et al., 2012). Dust has a signif- dust are still too poor to bound estimates of the parame- icant direct radiative effect on the Earth’s energy balance, ters necessary for quantitative determination of dust climate which likely acts globally to cool the planet. Regionally, be- forcing and potential for fertilization of ecosystems. These cause aerosol forcing depends on the brightness of the un- parameters include dust emissions, lofting, transport, depo- derlying surface, over the Sahara itself dust imposes a pos- sition, composition, microphysical, and optical properties. itive radiative forcing primarily through longwave warm- Specifically, while models have been constrained over the ing (Miller et al., 2014). Depending on the fraction of the past 15 years by global measures of aerosol optical thickness dust contributed by anthropogenic sources, the direct ra- (AOT) made by a constellation of satellite sensors (Leno- diative forcing exerted on the climate system is estimated ble et al., 2013), translating from the observed optical loading at −0.1 (−0.3 to C0.1) W m−2 (Boucher et al., 2013). To put to a mass loading requires knowledge of the microphysical that in perspective, the total radiative forcing exerted by all and optical properties of each individual aerosol type, and aerosols is estimated to be −0.45 (−0.95 to C0.05) W m−2 satellite sensors are incapable of providing this information. (Boucher et al., 2013). Recently, Kok et al.(2017) noted that The project “Fennec – The Saharan Climate System” the global dust cooling effect would likely be smaller if the was conducted by a consortium of universities in France, coarse mode of dust particles were better represented in cli- UK, and USA in 2011 (Washington et al., 2012). This mate models. Dust also plays a role in cloud microphysics, project joined efforts to address open questions on atmo- acting as ice nuclei and thereby influencing cloud develop- spheric processes in central Sahara. Combining aircraft (Ry- ment and subsequently ice cloud radiative effects and pre- der et al., 2013a, 2015), ground-based (Marsham et al., 2013; cipitation characteristics (Atkinson et al., 2013; Prenni et al., Todd et al., 2013; Hobby et al., 2013; Allen et al., 2013), 2009). In addition to their effects on Earth’s energy balance modeling, and/or satellite observations (Banks et al., 2013; and water cycle, the transport of mineral dust particles is Chaboureau et al., 2016), the Fennec project successfully ob- known to be important for biological productivity in ocean tained a broad data set of meteorological conditions, atmo- regions (Mahowald et al., 2009). Dust particles contain iron spheric dynamics and structure, as well as dust emission and and phosphorous, and when these nutrients are bioavailable transport mechanisms for the central Saharan region (Wash- and dust is deposited into the ocean, phytoplankton use these ington et al., 2012). nutrients in photosynthetic activity (Jickells et al., 2005; Ma- The present study focuses on the ground-based measure- howald et al., 2008, 2009; Johnson and Meskhidze, 2013). ments of the dust optical properties obtained using a custom- Likewise, dust is known to bring important nutrients to the made inverse integrated nephelometer and optical reflec- Amazon (Swap et al., 1992; Bristow et al., 2010; Rizzo et al., tometer developed by the Laboratory of Aerosol, Clouds 2013; Yu et al., 2015). Long distance transport of dust con- and Optics (LACO) at University of Maryland, Baltimore tributes to air quality degradation (Yu et al., 2013; Pros- County (UMBC) and subsequent detailed laboratory anal- pero et al., 2014) and may be a means for intercontinen- yses of the samples collected by the LACO aerosol sam- tal transport of biological and disease agents (Smith et al., pling stations during the Fennec campaign. The in situ mea- 2012; Molesworth et al., 2002). The Sahara desert is the main surements were taken during the intensive observation pe- source of dust globally, contributing more than half of all riod, from the end of May through June 2011. The LACO– global emissions, with an estimated amount of 182 million t UMBC instruments were deployed in two locations: Super- of dust carried across the western edge of the Sahara each site 1 (SS1) in Bordj Badji Mokhtar (BBM), southern Alge- year (Chin et al., 2009; Yu et al., 2015). ria, and a small village called Bir Moghrein near the Fennec While we expect Saharan dust to affect Earth’s climate Supersite 2 (SS2) in Zouérat, Mauritania. The instruments system and biogeochemical cycles, quantifying the effect is in both locations were operated by the Office National de la still highly uncertain. Uncertainties are large because, lack- Meteorologie (ONM) of Algeria and Mauritania with remote ing strong observational constraints, diversity between model assistance from the Fennec team. estimates of key aerosol properties and processes is large. In situ measurements of Saharan dust were complemented For example, comparisons between different models show with laboratory analyses for the characterization of their op- high variability in the prediction of the most straightforward tical properties using the methods presented in Martins et al.

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