1. Measurement of free iron content in desert dust : effect on light absorption, size dependence and soil influence S. Lafon, J.-L. Rajot, S. C. Alfaro, A. Gaudichet

2. Free iron Total iron = Free iron (oxides or hydroxides), as discrete particles, isolated or linked to surface of other particles (clays) + Structural iron, trapped in the silicates crystal lattice Iron oxides (hematite) or iron hydroxides (geothite) red and yellow colours in soils Structural iron Iron oxide “coating” * * * * * Isolated iron oxide particle Free iron oxides

3. Iron in aerosol

4. Adaptation to aerosols (Lafon et al., submitted) of the chemical extraction method (Mehra and Jackson, 1960) used in soil science (selective dissolution of free-iron with citrate, bicarbonate and dithionite (CBD) + XRF elemental analysis) Quantification of free iron in dust Natural dust sample collected on polycarbonate filter before and after CBD treatment

5. Experimental strategy Wind tunnel Dust generation in wind tunnelIsokinetic collection on bulk filter and cascade impactor Bulk filter Cascade impactor PM20 Inlet Isokinetic particle collector

6. Analytical methods AerosolParent soil Attenuation of bulk filter Transfer of dust on quartz fibre filters Measurement : white light aethalometer (Hansen et al., 1984) ATTN = 100 ln I o /I Free iron content of bulk filter CBD directly on filter Mass size distribution Cascade impactor (13 stages, vaseline greased) elemental mass measured by XRF Free iron content in each size population CBD on stage filters Bulk soils (< 1mm) classical analyses texture mineralogy of < 20µm fraction free iron content Free iron content of fine fractions Separation by sedimentation (Robinson method) Fine silts (2-20 µm) Clays (< 2 µm) CBD on filters Filters

7. Sample Quartz Calcite Clay NIGER +++ - K TUNISIA +++ + K, I CHINA +++ - I, C Mineralogy of < 20µm fraction Soil texture Soil sample characteristics Coarse silt + fine sand (20-200 µm) Clay + fine silt (<20 µm) Coarse sand (>200 µm) Niger Tunisia China

8. Free iron measurement in bulk aerosol filters 0 2 4 6 8 10 NigerTunisiaChina % Fe / total oxide mass Free iron Structural iron Variable iron content (45 to 75 % of total iron) No relationship between free iron and total iron

9. Free iron is responsible for light attenuation by dust

10. What controls the free-iron content in dust ?

11. Bulk soil free-iron content No relationship between aerosol and bulk soil Strong enrichment in the aerosol relative to soil (size fractionation during emission) 0 1 2 3 4 5 6 7 NigerTunisiaChina % free iron / total mass aerosol soil 23.3 10.7 8.4

12. Mass size distribution Consistent with DPM (Alfaro and Gomes, 2000) Lognormal fit to measured mass distribution : two populations Aerosol Soil Clays (< 2 µm) ? Fine silts (2-20 µm)

13. Size-resolved free-iron content in dust 0 1 2 3 4 5 6 7 3 µm12 µm % / total oxides mass Niger Tunisia China Higher iron content in coarse mode (Tunisia and China) Differences between Niger aerosol and others

14. Free-to-total Fe (x100) y = 0.971x R 2 = 0.9306 0 20 40 60 80 100 020406080100 soil aerosol coarse fine Heavy minerals Relationship between soil and aerosol for corresponding size classes Free-to-total iron ratio

15. Dust attenuation (white light) linearly correlates to the free- iron mass of the aerosol. It is possible to assess light absorption by dust directly from free-iron content. For aerosols, free-iron content relative to the total mass of dust can vary : as a function of parent soils as a function of size. For corresponding size classes there seems to be an unique relationship between the free-to-total iron ratio in the aerosol and in the parent soil. It is possible to assess the free-iron content of dust by knowing the free-to-total iron in the corresponding soil fraction Conclusions

16. Soil Aerosol Clays Free Fe Total Fe R c = Fine silts Free Fe Total Fe R s = RcRc RsRs DPM Size-resolved total iron content Free-iron content Attenuation (on filter) Absorption coefficient