Mapping the physico-chemical properties of mineral dust in western Africa: mineralogical composition

International audienceIn the last few years, several ground-based and air-borne field campaigns have allowed the exploration of theproperties and impacts of mineral dust in western Africa,one of the major emission and transport areas worldwide.In this paper, we explore the synthesis of these observationsto provide a large-scale quantitative view of the mineralogi-cal composition and its variability according to source regionand time after transport.This work reveals that mineral dust in western Africa is amixture of clays, quartz, iron and titanium oxides, represent-ing at least 92 % of the dust mass. Calcite ranged between0.3 and 8.4 % of the dust mass, depending on the origin. Ourdata do not show a systematic dependence of the dust min-eralogical composition on origin; this is to be the case as, inmost of the instances, the data represent the composition ofthe atmospheric burden after 1–2 days after emission, whenair masses mix and give rise to a more uniform dust load.This has implications for the representation of the mineraldust composition in regional and global circulation modelsand in satellite retrievals.Iron oxides account for 58±7 % of the mass of elementalFe and for between 2 and 5 % of the dust mass. Most of themare composed of goethite, representing between 52 and 78 %of the iron oxide mass. We estimate that titanium oxides ac-count for 1–2 % of the dust mass, depending on whether thedust is of Saharan or Sahelian origin.The mineralogical composition is a critical parameter forestimating the radiative and biogeochemical impact of min-eral dust. The results regarding dust composition have been used to estimate the optical properties as well as the iron frac-tional solubility of Saharan and Sahelian dust.Data presented in this paper are provided in numeri-cal form upon email request while they are being turnedinto a public database, the Dust-Mapped Archived Proper-ties (DUST-MAP), which is an open repository for compo-sitional data from other source regions in Africa and world-wide

Fractional solubility of iron in mineral dust aerosols over coastal Namibia: a link to marine biogenic emissions?

This paper presents the first investigation of the solubility of iron in mineral dust aerosols collected at the Henties Bay Aerosol Observatory (HBAO), in Namibia, from April to December 2017. During the study period, 10 intense dust events occurred. Elemental iron reached peak concentrations as high as 1.5 µg m−3, significantly higher than background levels. These events are attributed to wind erosion of natural soils from the surrounding gravel plains of the Namib desert. The composition of the sampled dust is found to be overall similar to that of aerosols from northern Africa but is characterized by persistent and high concentrations of fluorine which are attributed to local fugitive dust. The fractional solubility of Fe (%SFe) for both the identified dust episodes and background conditions ranged between 1.3 % and 20 % and averaged at 7.9 % (±4.1 %) and 6.8 (±3.3 %), respectively. Even under background conditions, the %SFe was correlated with that of Al and Si. The solubility was lower between June and August and increased from September onwards during the austral spring. The relation to measured concentrations of particulate MSA (methane sulfonic acid), solar irradiance, and wind speed suggests a possible two-way interaction whereby marine biogenic emissions from the coastal Benguela upwelling to the atmosphere would increase the solubility of iron-bearing dust according to the photo-reduction processes. This first investigation points to the western coast of southern Africa as a complex environment with multiple processes and active exchanges between the atmosphere and the Atlantic Ocean, requiring further research

Compositional variability of the aerosols collected in Sfax (Central Tunisia)

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A new generator for mineral dust aerosol production from soil samples in the laboratory : GAMEL

International audienceA generator has been developed for producing mineral dust from small samples of desert soils. The objective is to perform a thorough characterization of this new tool and show that it is adapted to the future laboratory studies of the relationship between aerosols and their parent soils. This work describes the principles and operating protocol of the so-called GAMEL generator. A first series of detailed measurements was performed with a Niger soil. During these tests the aerosol size-distribution was monitored in real time with an optical counter and the particles collected on filters submitted to XRF analysis. This allowed characterizing the emission in terms of time evolution of the aerosol production, repeatability of the experiment, and assessing the influence of such generation parameters as the mass of soil and the frequency and duration of the shaking. For this sandy Niger soil, the optimal generation parameters were found to be 1 g of soil agitated 9 min at the frequency of 500 cycles/min, but the effect of modifications of these recommended values have also been quantified. In terms of size-distribution as well as of elemental composition, the generated aerosol is found to compare well to the one collected in natural conditions during local events. For testing the capability of the GAMEL to produce aerosols from different soils, tests were also performed with 3 other soils from arid and semi-arid areas. Results showed that the GAMEL is able to produce aerosols whose characteristics encompass the regional variability of naturally produced mineral aerosols

Fluxes and sources of nutrient and trace metal atmospheric deposition in the northwestern Mediterranean

International audienceTotal atmospheric deposition was collected on a weekly basis over 3.5 years (March 2008-October 2011) at a remote coastal site on the west coast of Corsica. Deposition time series of macro-and micronutrients (N, P, Si, Fe) and trace metals (As, Cr, Cu, Mn, Ni, V, Zn) are investigated in terms of variability and source apportionment (from fluxes of proxies for aerosol sources (Al, Ti, Ca, Na, Mg, S, Sr, K, Pb)). The highest fluxes are recorded for Si, P, and Fe for nutrients and Zn and Mn for trace metals. For the majority of elements, data show some weeks with high episodic fluxes, except for N, Cr, and V, which present the lowest variability. A total of 12 intense mineral dust deposition events are identified during the sampling period. The contribution of these events to the fluxes of Fe and Si represents 52 % and 57 % of their total fluxes, respectively, confirming the important role of these sporadic dust events in the inputs of these elements in the Mediterranean. For N and P, the contribution of these intense dust deposition events is lower and reaches 10 % and 15 %, respectively. Out of these most intense events, positive matrix factorization (PMF) was applied to our total de-position database in order to identify the main sources of nutrients and trace metals deposited. Results show that P de-position is mainly associated with anthropogenic biomass burning inputs. For N deposition, inputs associated with marine sources (maybe associated with the reaction of anthro-pogenic N on NaCl particles) and anthropogenic sources are quasi-similar. A good correlation is obtained between N and S fluxes, supporting a common origin associated with inorganic secondary aerosol, i.e., ammonium sulfate. For trace metals, their origin is very variable: with a large contribution of natural dust sources for Ni or Mn and conversely of anthropogenic sources for V and Zn

Variability of the elemental composition of airborne mineral dust along the coast of Central Tunisia

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Experimental determination of the refractive index and single scattering albedo of African mineral dust as a function of source region

International audienceBy scattering and absorbing both the solar and terrestrial radiations, gaseous compounds and airborne particles, have an impact on climate. If the radiative impact of some of these compounds are now relatively well known, many uncertainties remain about mineral dust emitted from aeolian erosion in arid and semi-arid regions (IPCC 2007). These uncertainties are linked to our capacity in representing the spatial and temporal variability of their atmospheric concentration and their optical and radiative properties. Those latter depend on the physico-chemical properties of dust (chemical and mineralogical composition, shape and size) which, to date, are also poorly known. In this work we present measurements of the elemental and mineralogical composition, size distribution and shape of mineral dust observed during the African Monsoon Multidisciplinary Analyses (AMMA) field campaigns, conducted in western Africa during both the dry and wet seasons of 2006. As a matter of fact, in terms of emitted mass, the arid and semi-arid regions of western Africa are the most important dust sources at a global scale. Measurements have been carried out at the ground supersite of Banizoumbou in Niger, which is both a transport region for Saharan dust and a source region for Sahelian dust. We will focus our attention on the differences of the mineralogical composition of dust emitted from these two major source areas, in particular regarding minerals ruling its radiative effect (calcite, clay, iron oxides...), and on the variability of the size distribution and shape factor. All these results are used to estimate the dust complex refractive index and single scattering albedo in the visible and infrared spectra as a function of source region. These are key input parameters in order to evaluate the direct radiative impact of African mineral dust at the regional scale

Intra-event evolution of elemental and ionic concentrations in wet deposition in an urban environment

Abstract. A measurement campaign was conducted in the Paris region, focusing on the evolution of chemical composition of wet deposition during rainfall events from sequential sampling. A total of eight rain events were documented and characterized by varying meteorological conditions, atmospheric dynamics, and aerosol particle concentrations representative of urban conditions and influenced by long-range mineral dust transport. The intra-event evolution of the chemical composition of wet deposition revealed the predominant role of meteorological parameters and local sources in the observed mass concentration variability. From selected case studies, the washout ratios (WRs) and scavenging coefficients were quantified by conducting simultaneous measurements of aerosol particle composition and wet deposition. The results highlighted a variability of the WR and scavenging coefficients depending on the rainfall rate and on the chemical species. Scavenging coefficients estimated from WR ranged from 5.4×10-8 to 1.1×10-5 s−1 for chemical elements, and they are within the range of values reported in the literature for 0.2–2 µm particle diameters. Our results pointed out that the scavenging coefficient increases with rainfall rate according to a power law, as previously shown in the literature, indicating a stronger removal of particles from the atmosphere with greater precipitation intensity. Quantitative analysis of the data allowed us to estimate the relative contributions of in-cloud scavenging (ICS) for selected rain events. The ICS relative contributions ranged on average from 23 % to 62 % depending on the rain events, and they varied according to the chemical species within the same rain event. This highlights the variability and complexity of the wet deposition process and the influence of specific factors on the contribution of ICS, such as aerosol particle size and hygroscopicity. Overall, this study highlights the variability of wet deposition and its chemical composition and the need to consider the specificities of each event to fully understand the underlying mechanisms