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).
- 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
startSampling
endDays
sampledMinimum wind
velocity (m s−1)Maximum wind
velocity (m s−1)Average wind
velocity (m s−1)2014-M3 24 November 2013 01 September 2014 281 1.9 13.6 8.7 ± 2.0 2014-M4 28 November 2013 27 January 2015 425 4.5 14.2 9.1 ± 1.8 2015-M3 22 November 2015 29 March 2016 432 0.9 12.7 6.7 ± 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 altitudeWinter: Traveled distance at
10 m s−1 winds from 3-km altitude100 400 438 km 75 km 200 1000 175 km 30 km 300 1500 117 km 20 km
Supplementary Materials
Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/4/12/eaau2768/DC1
Data file S1. Data on backward trajectories and grain-size distributions.
Additional Files
Supplementary Materials
Other Supplementary Material for this manuscript includes the following:
- Data file S1 (Microsoft Excel format). Data on backward trajectories and grain-size distributions.
Files in this Data Supplement:
