AEROSOL SOIL FRACTIONATION FOR NAMIB DESERT SAMPLES

Four soil samples, collected in the central Namib Desert, were fractionated by dry sieving and aerosol generation into 16 size fractions in the range 0.15-300 mum diameter. The mass-size function of each soil and the dust (mineral aerosol) generated from the soil were studied. Due to the preferential lifting of smaller soil particles by the air stream, the soil underwent strong physical fractionation resulting in the bulk of the dust being found in the range of 1.3-10.3 mum, whereas the bulk of the soil was found in the range 63-300 mum. The concentrations of 11 elements in eight soil size fractions (from < 45 to > 300 mum) obtained by dry sieving were measured by X-ray fluorescence analysis while the concentration for these elements in eight size ranges (from <0.15 to > 10.3 mum) obtained by aerosol generation were determined by particle-induced X-ray emission. The concentrations of the elements Al, Si, K, Rb and Sr were found to vary by less than a factor of two throughout the particle size range studied. However, the concentration of the elements Ca, Ti, Mn, Fe, Y and Zr increased when the particle size decreased below 150 mum to reach a maximum around 63-45 mum and then to decrease. The concentrations of the elements in the generated aerosol particles were found to be more similar to those in the bulk soil than any particular size fraction. For the aerosol size fraction, elemental enrichment factors were calculated with respect to the composition of average crustal rock, average soil, the bulk Namib soil and the small size fraction of the Namib soil. For several elements, the enrichment factors varied quite significantly, depending on the choice of the reference material. The elemental ratios in the mineral aerosol were also compared to those in the atmospheric aerosol from the Namib Desert. It was confirmed that there is a marine contribution for S, Cl and Sr in the Namib natural aerosol. The composition of the mineral aerosol generated in this study should be useful in source apportionment studies for the Namib Desert and surrounding regions.