1. How to predict dust absorption in the atmosphere
An experimental dataset on elemental iron concentrations in mineral dust from arid and semi-arid areas: regional features and relevance to light absorption P. Formenti, G. Bergametti, S. Lafon, S. Alfaro and B. Chatenet LISA, CNRS/Univ. Paris7/Paris12, Créteil, France
Rationale
How to predict dust absorption in the atmosphere
at the global scale?
In the aerosol fraction, as a function of size
Free-Feaer = total-Feaer * [Free-Fe / total-Fe]soil
The free-to-total Fe ratio
depends on soil type
 it can be mapped! 
R seems to be conserved from soil to aerosol
Size distribution
Dust Production Model
This work!
In the picture, two aerosol samples collected on filter media are shown.
The reddish-color on the left sample is due to iron-oxides (mostly hematite and goethite), which determine the absorbing character of particulate dust at near-UV and visible wavelengths.
The variability of the daily mean net direct radiative forcing induced by changing the dust mineralogy is predicted by Sokolik and Toon [1999].
A novel method (adapted CBD) for measuring the iron oxide (= free iron) content in dust aerosol has been recently developed and applied to aerosol samples issued from various desert areas of the globe [Lafon et al., 2004].
The few samples analyzed show that free-to-total Fe ratio varies depending on the source region.
Presentation of the data set
This data set presents the total (elemental) iron concentrations from various source and transport regions, mainly located in North Africa and Asia.
Data are issued from several ground-based, airborne and ship-borne campaigns, dedicated or of opportunity, that were conducted by the LISA or other laboratories in the last two decades.
Values largely refer to the TSP (total suspended particulate) fraction, no size-segregation
Bulk concentrations obtained by X-Ray Fluorescence (XRF) or equivalent (e.g., PIXE)
However, it is not the total iron, but the free-iron content, that controls light attenuation by dust aerosol in the visible, independently on source region!
Some features
Data were collected mainly in Africa and Asia, over source or transport regions. When plotting the total iron concentrations against those of aluminum, a major and almost unique tracer for mineral dust, significant differences are observed in their ratios in Africa and Asia (left plots). The Asian samples have the highest Fe-to-Al ratios (~0.74). Within the Africa source regions, differences are seen between Sahelian and Saharan samples (the former have a lower Fe-to-Al ratios than the latter). Data collected in African transport regions are mostly downwind the Sahara desert, therefore their characteristic ratio is similar. Outliers likely due to contamination by anthropogenic sources are evident.
On the right, the total Fe percent content with respect to the estimated dust mass EDM (EDM = sum of Al, Si, Ca, Fe oxides) is shown. Only two data series are retained in Asia, due to the scarcity of Si and Ca data. From this very limited dataset, we can observe that the Fe content is almost equivalent in Asia and Africa. The apparent enrichment on the samples of the African transport regions needs to be investigated further.
These figures are consistent with those of Lafon et al. [2004].
Africa, source regions
Region
Period
Sites
Authors
Africa (source)
Aug–Sep 1992
Sahel
unpublished (LISA)
Nov 1989
Tillabery
Jan 1989
Mekkerane
Mar 1985 , Dec 1987
El Abiod
Dec 1985
Saharan sites
Feb 1986
Europe (source)
Spain
Africa (source)
Jun 1993
Niger
Asia (source)
Mar 2002
China
Alfaro et al., 2004
Africa (transport)
Mar-Apr 1984, Jul 1985
Canary Isl.
Feb-Dec 1985
Corsica
Cape Verde
Chiapello, 1996
Med. cruise
Corregiari et al.
Africa (source + transport)
Israel
Foner and Ganor, 1992
Sardinia
Molinari et al., 1993
Asia (transport)
June 1980
Pacific cruise
Tsunogai and Kondo, 1982
April-May 1989
Gao et al., 1992
Mar 1998
Brazil
Formenti et al., 2001
Mar-Apr 2002
Guo et al., 2004
Africa, transport regions
Finally…..
Promising approach
The quantification of free-Fe by adapted CBD is time consuming, only for loaded dust samples (total dust mass > 50 µg)
But
Data are sparse and scarce, specially over Asia
No size-segregation
SEND US YOUR DATA !
Asia, source and transport regions
Lafon et al., Atmos Env., 38, , 2004
Sokolik and Toon, JGR, 104, , 1999