Optical properties of Saharan dust aerosol and contribution from the coarse mode as measured during the Fennec 2011 aircraft campaign
1Department of Meteorology, University of Reading, RG6 6BB, Reading UK
2School of Earth and Environment, University of Leeds, LS2 9JT, Leeds, UK
3Facility for Airborne Atmospheric Measurements, Cranfield, UK
4School of Earth, Atmospheric and Environmental Sciences, University of Manchester, Manchester, UK
5Space and Atmospheric Physics, Department of Physics, Imperial College, London, UK
6National Centre for Atmospheric Science, University of Leeds, Leeds, UK
7Institute for Atmospheric and Climate Science, ETH Zürich, Switzerland
8Department of Geography, University of Oxford, Oxford, UK
*now at: Aeroqual Ltd, 109 Valley Road, Auckland, New Zealand
Abstract. New in-situ aircraft measurements of Saharan dust originating from Mali, Mauritania and Algeria taken during the Fennec 2011 aircraft campaign over a remote part of the Sahara Desert are presented. Size distributions extending to 300 μm are shown, representing measurements extending further into the coarse mode than previously published for airborne Saharan dust. A significant coarse mode was present in the size distribution measurements with effective diameter (deff) from 2.3 to 19.4 μm and coarse mode volume median diameter (dvc) from 5.8 to 45.3 μm. The mean size distribution had a larger relative proportion of coarse mode particles than previous aircraft measurements. The largest particles (with deff > 12 μm, or dvc > 25 μm) were only encountered within 1 km of the ground. Number concentration, mass loading and extinction coefficient showed inverse relationships to dust age since uplift. Dust particle size showed a weak exponential relationship to dust age. Two cases of freshly uplifted dust showed quite different characteristics of size distribution and number concentration.
Single Scattering Albed (SSA) values at 550 nm calculated from the measured size distributions revealed high absorption ranging from 0.70 to 0.97 depending on the refractive index. SSA was found to be strongly related to deff. New instrumentation revealed that direct measurements, behind Rosemount inlets, overestimate SSA by up to 0.11 when deff is greater than 2 μm. This is caused by aircraft inlet inefficiencies and sampling losses. Previous measurements of SSA from aircraft measurements may also have been overestimates for this reason. Radiative transfer calculations indicate that the range of SSAs during Fennec 2011 can lead to underestimates in shortwave atmospheric heating rates by 2.0 to 3.0 times if the coarse mode is neglected. This will have an impact on Saharan atmospheric dynamics and circulation, which should be taken into account by numerical weather prediction and climate models.