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

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

Iron in aerosol 0 2 4 6 8 10 12 1 10 100 1000 10000 Estimated dust mass (µg.m -3 ) Total-Fe (%) Hapex-Sahel 92 Hapex-Sahel 92 Tillabéry 89 Mekkerane 89 El Abiod 85 El Abiod 87 Sahara 85 GEO 86 Banizoumbou 93 Yulin 02 Corsica Island 85 Cap-Vert 92-94

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

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

Analytical methods Aerosol Parent 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

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

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

Free iron measurement in bulk aerosol filters Variable iron content (45 to 75 % of total iron) No relationship between free iron and total iron 0 2 4 6 8 10 Niger Tunisia China % 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

Free iron is responsible for light attenuation by dust Free iron y = 2.197x - 1.638 R 2 = 0.935 0 5 10 15 20 25 30 0 5 10 15 mass (µg/filter) ATTN Niger Tunisia China Total iron y = 1.322x - 1.407 R 2 = 0. 698 0 5 10 15 20 25 30 0 10 20 30 mass (µg/filter) ATTN Niger Tunisia China Total mass y = 0.069x + 2.760 R 2 = 0.499 0 5 10 15 20 25 30 0 100 200 300 400 mass (µg/filter) ATTN Niger Tunisia China

What controls the free-iron content in dust ?

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 Niger Tunisia China % free iron / total mass aerosol soil 23.3 10.7 8.4

No relationship between aerosol and bulk soil

Strong enrichment in the aerosol relative to soil

(size fractionation during emission)

Mass size distribution Consistent with DPM (Alfaro and Gomes, 2000) Lognormal fit to measured mass distribution : t wo populations Aerosol 0 0.2 0.4 0.6 0.8 1 1.2 0 1 10 100 Particle diameter (µm) dM/dlogd (arbitrary units) Niger 1 Niger 2 Tunisia China 1 China 2 12 (+/- 1) µm 3.2 (+/- 0.3) µm Soil Clays (< 2 µm) ? Fine silts (2-20 µm)

Size-resolved free-iron content in dust Higher iron content in coarse mode (Tunisia and China) Differences between Niger aerosol and others 0 1 2 3 4 5 6 7 3 µm 12 µm % / total oxides mass Niger Tunisia China

Higher iron content in coarse mode (Tunisia and China)

Differences between Niger aerosol and others

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

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

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

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