Airborne observations of mineral dust over western Africa in the summer Monsoon season: spatial and vertical variability of physico-chemical and optical properties

Formenti, Paola; Rajot, Jean-Louis; Desboeufs, Karine; Saïd, F.; Grand, N.; Chevaillier, Servanne; Schmechtig, Catherine

doi:10.5194/acp-11-6387-2011

Cited by 78 publications

(110 citation statements)

References 89 publications

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“…These measurements are comparable but on the lower edge of previous measurements performed at 550 nm during the SHADE, AMMA SOP0/DABEX and DODO field campaigns, also performed in the area under with the same experimental set-up and data treatment procedures Osborne et al, 2008;McConnell et al, 2008). The GERBILS values are close (although lower) to those of the summertime DODO campaign (DODO2 in McConnell et al, 2008) and to those of the AMMA wet season campaign conducted over Niger (Formenti et al, 2011). This could be imputed to the different mineralogy of the Sahelian soils, which are expected to be richer in iron oxides than Saharan soils (Claquin et al, 1999).…”

Section: Scattering and Absorption Coefficients And Single Scatteringsupporting

confidence: 77%

“…The dispersion modelling results (shown in Figure 1) indicate that the Nouakchott and Niamey areas might have been under the influence of air masses coming from different sources. In addition to transport from eastern sources in the Saharan Air Layer above 1.5 km, which is common at this time of the year (Formenti et al, 2011) and which is evident in Figure 1 for the Niamey area, the Nouakchott area was most of the time also in the outflow region of dust plumes from north of Mauritania/Western Sahara/Morocco. This is also suggested by comparison of the vertical profiles of the scattering coefficients.…”

Section: Apportionment Of the Aerosol Load And Origin Of Mineral Dustmentioning

confidence: 82%

“…The dispersion model maps in Figure 1 are accompanied by a figure of the correspondent ratio of elemental Fe to elemental Ca (Fe/Ca ratio in the following), which is generally considered a good atmospheric indicator of dust origin (Kandler et al, 2007;Rajot et al, 2008;Formenti et al, 2008Formenti et al, , 2011. Fe/Ca ratios below 2 are indicative of dust emitted from Saharan sources, whereas Fe/Ca ratios above 2 are associated with dust storms from the Sahelian part of West Africa.…”

Section: Apportionment Of the Aerosol Load And Origin Of Mineral Dustmentioning

confidence: 99%

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Physico‐chemical and optical properties of Sahelian and Saharan mineral dust: in situ measurements during the GERBILS campaign

Klaver

Formenti

Caquineau

et al. 2011

Quart J Royal Meteoro Soc

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Regardless of origin, clays (illite, kaolinite) dominated the total volume (79-90%); the remainder was composed of quartz, calcium-rich minerals (calcite, dolomite, gypsum) and alkali feldspars. Iron oxides, measured using a selective chemical extraction method, accounted for 1-3% of the total dust mass. The dependence of particle number size and shape distribution on the origin of dust seems minor too, although our results might be slightly misleading due to the fact that those kinds of data have been gathered on flights when dust had comparable origins and residence time.Mineral dust is only weakly absorbing in the mid-visible wavelengths (single scattering albedo ω 0 > 0.95 at 550 nm), and ω 0 measured values can be reproduced by measuring the bulk fractions of the major minerals, i.e. clays, quartz, calcite and iron oxides. At this wavelength, knowledge of the nature of clays and iron oxides, or the state of mixing of the minerals, does not induce significant differences in the results. A more precise description of the nature of clays and iron oxides is necessary at lower wavelengths owing to larger differences in their spectral optical properties. In particular, knowledge of the nature of the dominant clay is important for determining light scattering in the backward hemisphere.

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Section: Scattering and Absorption Coefficients And Single Scatteringsupporting

confidence: 77%

Section: Apportionment Of the Aerosol Load And Origin Of Mineral Dustmentioning

confidence: 82%

Section: Apportionment Of the Aerosol Load And Origin Of Mineral Dustmentioning

confidence: 99%

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Physico‐chemical and optical properties of Sahelian and Saharan mineral dust: in situ measurements during the GERBILS campaign

Klaver

Formenti

Caquineau

et al. 2011

Quart J Royal Meteoro Soc

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“…The values obtained during our experiments are reported in Table 3. There is a very good correspondence between the values obtained for the Mali, Libya, Algeria and (to a lesser extent) Morocco experiments to values found in environmental aerosol samples by Chiapello et al (1997) and Formenti et al (2011Formenti et al ( , 2014a. These authors indicate that dust from local erosion of Sahelian soils, such as from Mali, has Si / Al ratios in the range of 2-2.5 and Fe / Ca ratios in the range of 3-20, depending on the time proximity to the erosion event.…”

Section: Elemental Composition and Iron Oxide Contentsupporting

confidence: 75%

“…Here we use the Fe / Ca and Si / Al elemental ratios obtained from XRF analysis to discriminate the origin of dust samples. These ratios have been extensively used in the past to discriminate the origin of African dust samples collected in the field (Chiapello et al, 1997;Formenti et al, 2011Formenti et al, , 2014a. The values obtained during our experiments are reported in Table 3.…”

Section: Elemental Composition and Iron Oxide Contentmentioning

confidence: 99%

Spectral- and size-resolved mass absorption efficiency of mineral dust aerosols in the shortwave spectrum: a simulation chamber study

et al. 2017

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Abstract. This paper presents new laboratory measurements of the mass absorption efficiency (MAE) between 375 and 850 nm for 12 individual samples of mineral dust from different source areas worldwide and in two size classes: PM 10.6 (mass fraction of particles of aerodynamic diameter lower than 10.6 µm) and PM 2.5 (mass fraction of particles of aerodynamic diameter lower than 2.5 µm). The experiments were performed in the CESAM simulation chamber using mineral dust generated from natural parent soils and included optical and gravimetric analyses.The results show that the MAE values are lower for the PM 10.6 mass fraction (range 37-135 × 10 −3 m 2 g −1 at 375 nm) than for the PM 2.5 (range 95-711 × 10 −3 m 2 g −1 at 375 nm) and decrease with increasing wavelength as λ −AAE , where the Ångström absorption exponent (AAE) averages between 3.3 and 3.5, regardless of size. The size independence of AAE suggests that, for a given size distribution, the dust composition did not vary with size for this set of samples. Because of its high atmospheric concentration, light absorption by mineral dust can be competitive with black and brown carbon even during atmospheric transport over heavy polluted regions, when dust concentrations are significantly lower than at emission. The AAE values of mineral dust are higher than for black carbon (∼ 1) but in the same range as light-absorbing organic (brown) carbon. As a result, depending on the environment, there can be some ambiguity in apportioning the aerosol absorption optical depth (AAOD) based on spectral dependence, which is relevant to the development of remote sensing of light-absorbing aerosols and their assimilation in climate models. We suggest that the sample-to-sample variability in our dataset of MAE values is related to regional differences in the mineralogical composition of the parent soils. Particularly in the PM 2.5 fraction, we found a strong linear correlation between the dust lightPublished by Copernicus Publications on behalf of the European Geosciences Union. 7176 L. Caponi et al.: Spectral-and size-resolved mass absorption efficiency of mineral dust aerosols absorption properties and elemental iron rather than the iron oxide fraction, which could ease the application and the validation of climate models that now start to include the representation of the dust composition, as well as for remote sensing of dust absorption in the UV-vis spectral region.

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Dominance of goethite over hematite in iron oxides of mineral dust from Western Africa: Quantitative partitioning by X‐ray absorption spectroscopy

et al. 2014

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This paper reports on the X-ray absorption analysis of samples of mineral dust emitted from or transported to Western Africa. We found that iron oxides account, by mass, for 38% to 72% of the total elemental iron. They are composed of minerals in the Fe(III) oxidation state: goethite (FeO·OH) and hematite (Fe 2 O 3 ). The apparent fraction of iron oxide attributed to goethite is higher than hematite regardless of the source region from which the dust originated. The goethite percent content of iron oxides is in the range 52-78% (by mass), the highest values being measured for dust originating in the Sahel. The limited number of samples analyzed and the sample-to-sample variability prevent us from concluding firmly on the regional variability of the goethite-to-hematite ratio. Based on the experimental data on mineralogical composition and on concurrent measurements of the number size distribution, the optical properties of mineral dust have been calculated in a Mie approximation for homogeneous spherical particles. At 550 nm, the single-scattering albedo ω 0 ranges between 0.89 and 0.93, the asymmetry factor g ranges between 0.76 and 0.8 and the mass extinction efficiency k ext varies between 0.5 and 1.1 m 2 g À1 ; these values are all in the range of those from independent direct measurements. Neglecting the partitioning between hematite and goethite and the assimilation of iron oxides by hematite, as it is often done with models, lowers the single-scattering albedo and increases the asymmetry factor in the UV-visible spectral region below 550 nm. The mass extinction efficiency is insensitive to the nature of the iron oxides but rather responds to variations in the number size distribution. The mineralogy of iron oxides should therefore be taken into account when assessing the effect of mineral dust on climate and atmospheric chemistry, in particular via interactions involving photolysis.

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Airborne observations of mineral dust over western Africa in the summer Monsoon season: spatial and vertical variability of physico-chemical and optical properties

Cited by 78 publications

References 89 publications

Physico‐chemical and optical properties of Sahelian and Saharan mineral dust: in situ measurements during the GERBILS campaign

Physico‐chemical and optical properties of Sahelian and Saharan mineral dust: in situ measurements during the GERBILS campaign

Spectral- and size-resolved mass absorption efficiency of mineral dust aerosols in the shortwave spectrum: a simulation chamber study

Dominance of goethite over hematite in iron oxides of mineral dust from Western Africa: Quantitative partitioning by X‐ray absorption spectroscopy

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