Physical and optical properties of mineral dust aerosol measured by aircraft during the GERBILS campaign

This paper presents aircraft measurements of the physical and optical properties of mineral dust from the GERBILS campaign. The campaign involved ten flights of the UK Facility for Airborne Atmospheric Measurements (FAAM) BAe‐146 aircraft over the western region of the Sahara desert. Vertical profiles showed dust layers at varying altitudes extending as high as 6.5 km. Dust layers were typically associated with a deep well‐mixed boundary layer or a residual boundary layer above (the Saharan air layer). Aerosol optical depths (AODs), measured by integrating vertical profiles of extinction coefficient, ranged from 0.3 to 2.4 (at 0.55 μm). Aircraft AODs were generally within 20% of AERONET and Microtops sun‐photometer measurements. Single‐scattering albedos at 0.55 μm were measured in the range 0.92–0.99 with a campaign mean of 0.97. The in situ size distribution compared well with AERONET retrievals made at Banizoumbou (Niger) and Dakar (Senegal). The proportion of aerosol volume associated with particles of radii >1.5 μm was highly variable and also more difficult to measure. Models of dust as spheres, spheroids and more complex irregular‐shaped particles were used to calculate single‐scattering optical properties. The single‐scattering albedo showed a low sensitivity to particle shape. The asymmetry parameter and specific extinction coefficient showed greater sensitivity to particle shape.