Regional variability of the composition of mineral dust from western Africa: Results from the AMMA SOP0/DABEX and DODO field campaigns

Abstract

This paper presents data on elemental and mineralogical composition of mineral dust from various source regions of Africa collected during the African Monsoon Multidisciplinary Analyses (AMMA) SOP0/DABEX and Dust Ouflow and Deposition to the Ocean (DODO) DODO1 experiments (January–February 2006), and the DODO2 campaign (August 2006). Bulk filter samples were collected at the AMMA supersite of Banizoumbou, Niger, as well as on board the Facility for Airborne Atmospheric Measurements (FAAM) BAe‐146 aircraft. Both mineral dust and biomass burning in external mixing occurred in surface and elevated layers during the winter field phase of the campaign. However, mineral dust was overwhelming, accounting for 72% of the estimated aerosol mass in aged elevated biomass burning layers and up to 93% in plumes of mineral dust, which generally occurred in the boundary layer. A number of well‐defined episodes of advection of mineral dust could be identified both at the ground and on the aircraft. The elemental and mineralogical composition varied depending on source region. This variability could be well traced by the calcium content, which is enhanced in dust from North Africa but depleted in dust from the Sahel. Iron oxides in the form of hematite and goethite are enriched in dust emitted within Sahel and in Mauritania, whereas dust from the Bodélé depression is iron‐oxide depleted. Iron oxides represented between 2.4% and 4.5% of the total estimated dust oxide mass. This regional variability will have to be taken into account in estimating the optical properties of absorption of mineral dust from western Africa.

Size distribution, shape, and composition of mineral dust aerosols collected during the African Monsoon Multidisciplinary Analysis Special Observation Period 0: Dust and Biomass‐Burning Experiment field campaign in Niger, January 2006

“…This is consistent with findings on the bulk elemental composition by particle induced X-ray emission (PIXE) analysis of the same samples [Formenti et al, 2008].…”

“…[5] This paper completes the body of observations on dust composition and optical properties presented by Formenti et al [2008] and Osborne et al [2008].…”

“…Back in the laboratory, samples were cut in subportions of various surface areas (from one eighth up to one half of the filter) which were then analyzed with various techniques. Further details are given by Formenti et al [2008]. One eighth of the filter was dedicated to electron microscopy analysis.…”

“…During this long run, the aircraft altitude was adjusted twice between 600 and 1500 m above ground level (agl) in order to maximize the filter loading. Dust concentration was not measured directly, but as a first approximation, it can be estimated from the elemental Al concentration measured by particle induced X-ray emission (PIXE) analysis of one of the filter subportions [Formenti et al, 2008], knowing that Al is about 8%. When doing so, the dust concentration was 360 mg m À3 .…”

Motivation, rationale and key results from the GERBILS Saharan dust measurement campaign

“…majority of absorption of solar radiation, were derived by chemical extraction methods to be 1-3% of the total aerosol mass. This is in the range of values also found by Formenti et al (2008) and Kandler et al (2009) for near-emission and transported dust during DABEX, Dust Outflow and Deposition to the Ocean (DODO) and Saharan Mineral Dust Experiment (SAMUM) measurement campaigns. Klaver et al (2011) also derived the median aspect ratios for mineral dust particles of around 1.5-1.6, similar to those derived by Chou et al (2008) and Kandler et al (2009) for the AMMA/DABEX and SAMUM measurement campaigns.…”