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 (formenti@lisa.univ-paris12.fr) 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 1991-1994 Cape Verde Chiapello, 1996 1982-1983 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 ! formenti@lisa.univ-paris12.fr Asia, source and transport regions Lafon et al., Atmos Env., 38, 1211-1218, 2004 Sokolik and Toon, JGR, 104, 9423-9444, 1999