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Poster De Conférence Année : 2018

Size-resolved deposition fluxes and deposition velocities over a sandy surface: In-situ experimental determination and comparison with existing parameterizations

Résumé

Key words dust dry deposition, measurements, test of parameterizations An intense dust deposition event occurred in June 2006 in Niger. It was the consequence of the turn-back of a dust cloud resulting from a large wind erosion event that occurred the day before due to the passage of a mesoscale convective system. Bulk and size resolved particle concentrations were measured at 2.1 and 6.5 m over an agricultural field. At the date of the experiment (mid-June), almost all the vegetative residues from the previous year were decomposed or grazed and the sandy surface was almost bare, with only few percents of residues remaining. In the wind direction, the fetch was greater than 500 m. Bulk concentrations measurements were performed using two TEOM instruments and size resolved particle concentrations in 15 channels (from 0.3 µm to 20 µm) using two Optical Particle Counters (OPC, GRIMM instruments). These instruments have been carefully cross-calibrated before the campaign. At each level, the particle concentrations from TEOM and the particle volume from OPC were highly correlated with a slope 2.265 (+/-0.02) which is the apparent mass density of the particles. In complement wind and temperature profiles were measured at 5 and 4 different heights, respectively. The stability conditions remained near-neutral (-0.05<1/L<0.01) for the 10h duration of the deposition period. The dust deposition event was so huge that the difference in particles concentrations over a H = 4.4 m reached 100 µg m-3. All these observations allowed to compute size-resolved dust deposition fluxes and dry deposition velocities with a good confidence level. The deposition fluxes and velocities were computed using a 20-min sliding average and a 5-min time step. Size-resolved dry deposition velocities were computed only if the difference in concentrations for the considered channel between the two levels was greater than 5%. Deposition fluxes derived from bulk particle concentrations measured by TEOM instruments and those derived from size-resolved concentration measurements performed by OPC are in very good agreement. The evolution of the dust deposition fluxes during the day follows the dust concentration: higher deposition fluxes being recorded when the concentrations were maximal. This is not the case of the dry deposition velocities that are mainly controlled by the sensible heat flux in the early morning and by the wind friction velocity the rest of the day. Dry deposition velocities exhibit a maximum around noon as frequently reported in the literature. Size-resolved dry deposition velocities were compared with the rare existing data of dust deposition over bare sandy soils. A very good agreement was found with the data obtained by Lamaud et al. (1994) in Niger for submicron particles and with those obtained in wind tunnel for deposition of dust particles (1 to 40 µm) over sand and sandy loam by Zhang et al. (2014). Then, our in situ measurements were confronted to exiting parameterizations of deposition velocity (Slinn, 1982; Zhang et al., 2001; Zhang and Shao, 2014). The parameterization of Zhang and Shao (2014) is the only one that reproduces with a good confidence level the dry deposition velocities on sandy soils for particles in the size range 1-10 µm. This good agreement is mainly because, unlike the others, this model considers the bare sandy surfaces as rough surfaces allowing the interception of dust particles by the sand grains and small roughness elements present at the surface.
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Dates et versions

hal-02329936 , version 1 (23-10-2019)

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  • HAL Id : hal-02329936 , version 1

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G Bergametti, Beatrice Marticorena, J-L. Rajot, Gilles Foret, Stephane C. Alfaro, et al.. Size-resolved deposition fluxes and deposition velocities over a sandy surface: In-situ experimental determination and comparison with existing parameterizations. INTERNATIONAL CONFERENCE ON AEOLIAN RESEARCH 2018, Jun 2018, Bordeaux, France. ⟨hal-02329936⟩
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