Physical and optical properties of mineral dust aerosol during the Dust and Biomass-burning Experiment

[1] We present aircraft measurements of dust aerosol during the Dust and Biomassburning Experiment ( DABEX), a project affiliated with the African Monsoon Multidisciplinary Analysis. DABEX took place between 13 January and 3 February 2006 in Sahelian west Africa, with the aircraft based at Niamey, Niger. The data set is augmented with Aerosol Robotic Network ( AERONET) data. A mineral dust layer below 1 - 2 km ( sourced from the north) and an overlying biomass burning ( BB) layer ( sourced from anthropogenic fires to the south) was observed on all days, although variability was observed in both layers. There is evidence of ozone loss within the dust, but with CO levels between 140 and 170 ppbv some history of combustion has occurred. Size distribution of the dust is compared with that of the BB aerosol and with dust measured near Senegal, during the Dust Outflow and Deposition to the Ocean ( DODO- 1) experiment. For accurate representation of the optical properties, five log- normals to the size distribution across sizes 0.05 - 5 mm are required, although two log- normals are adequate. The single scattering albedo was almost purely scattering, with values of 0.99 +/- 0.01. During the strongest dust events the dust contribution to the column optical depth was 75 - 80%, compared to a DABEX mean of 50%. The aircraft- derived optical depth varied between 0.19 and 1.07, with the dust- only contribution between 0.07 and 0.81. AERONET optical depth trends are in good agreement with aircraft during DABEX, albeit with a bias to higher aircraft values. Retrieved AERONET aerosol size distributions show variable agreement with the aircraft. Differences between Versions 1 and 2 of the AERONET algorithm are highlighted.