Research ArticleATMOSPHERIC SCIENCE

The mysterious long-range transport of giant mineral dust particles

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Science Advances  12 Dec 2018:
Vol. 4, no. 12, eaau2768
DOI: 10.1126/sciadv.aau2768
  • Fig. 1 Giant mineral dust particles sampled by the MWAC samplers at M3 (12°N, 38°W) and M4 (12°N, 49°W) in 2014 and 2015, with their approximate diameters.

    (A to C) 2014-M3; (D to F) 2014-M4; (G to I) 2015-M3; (J to L) 2015-M4.

  • Fig. 2 Seasonality of atmospheric transport from Africa to the buoy sites.

    Distribution of travel times for backward trajectories from buoys M3 (red) and M4 (blue) to the target area (C), for February (A) and August (B). (D) Frequency of the minimum number of deep convective uplift cycles needed for a 100-μm particle to travel from the target area (C) to the sampling buoys M3 (red) and M4 (blue), assuming a constant sedimentation velocity of 400 mm s−1 (Table 2), for June to October. All computations are based on ERA-Interim data during the 10-year period 2006 to 2015.

  • Fig. 3 Influence of charge and electric field on the net force on a particle.

    (A) Combination of particle charge and local electric field required for the magnitude of the electric force experienced by a particle to equal the particle’s weight, for particles having diameters of 0.1, 1, 10, and 100 μm. (B) Fall speed for a 100-μm quartz particle for increasing electric field and particle charge (density of quartz = 2648 kg m−3, drag coefficient CD = 1.5).

  • Fig. 4 Grain-size distributions of MWAC samples collected in 2014 (solid lines) and 2015 (dashed lines).
  • Table 1 Sampling duration of MWAC samplers on the dust-collecting buoys at M3 and M4 (Fig. 2), together with statistics on the colocated wind measurements.
    Sampling
    start
    Sampling
    end
    Days
    sampled
    Minimum wind
    velocity (m s−1)
    Maximum wind
    velocity (m s−1)
    Average wind
    velocity (m s−1)
    2014-M324 November 201301 September 20142811.913.68.7 ± 2.0
    2014-M428 November 201327 January 20154254.514.29.1 ± 1.8
    2015-M322 November 201529 March 20164320.912.76.7 ± 1.7
  • Table 2 Settling velocities after Bagnold (19) and estimates of traveled distance based on favorable summer (strong winds and elevated dust) and winter (lower wind speeds and elevation) conditions.
    Particle
    size (μm)
    Settling
    velocity (mm s−1)
    Summer: Traveled distance at
    25 m s−1 winds from 7-km altitude
    Winter: Traveled distance at
    10 m s−1 winds from 3-km altitude
    100400438 km75 km
    2001000175 km30 km
    3001500117 km20 km

Supplementary Materials

  • Supplementary Materials

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    Other Supplementary Material for this manuscript includes the following:

    • Data file S1 (Microsoft Excel format). Data on backward trajectories and grain-size distributions.

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