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

LISA

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 ironIron 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)

To

tal-

Fe

(%)

Hapex-Sahel 92Hapex-Sahel 92Tillabéry 89Mekkerane 89El Abiod 85El Abiod 87Sahara 85GEO 86

Banizoumbou 93Yulin 02

Corsica Island 85Cap-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

Wind tunnel

Dust generation in wind tunnel Isokinetic collection on bulk

filter and cascade impactor

Bulk filterCascade impactor

PM20

Inlet

Isokinetic particle collector

Analytical methodsAerosol Parent soil

• Attenuation of bulk filterTransfer of dust on quartz fibre filters

Measurement : white light aethalometer

(Hansen et al., 1984)

ATTN = 100 ln Io/I

• Free iron content of bulk filterCBD directly on filter

• Mass size distributionCascade impactor

(13 stages, vaseline greased)elemental mass measured by XRF

• Free iron content in each size

populationCBD on stage filters

• Bulk soils (< 1mm) classical analyses

texture

mineralogy of < 20µm fraction

free iron content

• Free iron content of fine fractionsSeparation by sedimentation

(Robinson method)

Fine silts (2-20 µm)

Clays (< 2 µm)

CBD on filters

Filters

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

Free iron measurement in bulk aerosol filters

0

2

4

6

8

10

Niger Tunisia China

% F

e /

tota

l o

xid

e m

ass

Free iron

Structural iron

• Variable iron content (45 to 75 % of total iron)

• No relationship between free iron and total iron

Free iron y = 2.197x - 1.638

R2 = 0.935

0

5

10

15

20

25

30

0 5 10 15

mass (µg/filter)

AT

TN Niger

Tunisia

China

Total iron y = 1.322x - 1.407

R2 = 0.698

0

5

10

15

20

25

30

0 10 20 30

mass (µg/filter)

AT

TN Niger

Tunisia

China

Total mass y = 0.069x + 2.760

R2 = 0.499

0

5

10

15

20

25

30

0 100 200 300 400

mass (µg/filter)

AT

TN Niger

Tunisia

China

Free iron is responsible for light attenuation by dust

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

% f

ree

iro

n /

tota

l mas

s

aerosol

soil

23.3

10.78.4

Mass size distribution

Consistent with DPM (Alfaro and Gomes, 2000)

Lognormal fit to measured mass distribution : two populations

0

0.2

0.4

0.6

0.8

1

1.2

0 1 10 100

Particle diameter (µm)

dM

/dlo

gd

(ar

bit

rary

un

its)

Niger 1

Niger 2

Tunisia

China 1

China 2

12 (+/- 1) µm

3.2 (+/- 0.3) µmAerosol

SoilClays

(< 2 µm)

?

Fine silts

(2-20 µm)

Size-resolved free-iron content in dust

0

1

2

3

4

5

6

7

3 µm 12 µm

% /

to

tal o

xid

es m

ass

Niger

Tunisia

China

• Higher iron content in coarse mode (Tunisia and China)

• Differences between Niger aerosol and others

Free-to-total Fe (x100) y = 0.971x

R2 = 0.9306

0

20

40

60

80

100

0 20 40 60 80 100

soil

aero

sol

coarse

fine

Heavy minerals

Relationship between soil and aerosol for corresponding size classes

Free-to-total iron ratio

• Dust attenuation (white light) linearly correlates to the free-iron mass of the aerosol.

It is possible to assess light absorption bydust 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

Soil

Aerosol

ClaysFree Fe

Total FeRc =

Fine siltsFree Fe

Total FeRs =

Rc

RsDPM

Size-resolvedtotal iron content

Free-iron content

Attenuation (on filter) Absorptioncoefficient