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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