Giant mineral dust particles (>75 μm in diameter) found far from their source have long puzzled scientists. These wind-blown particles affect the atmosphere’s radiation balance, clouds, and the ocean carbon cycle but are generally ignored in models. Here, we report new observations of individual giant Saharan dust particles of up to 450 μm in diameter sampled in air over the Atlantic Ocean at 2400 and 3500 km from the west African coast. Past research points to fast horizontal transport, turbulence, uplift in convective systems, and electrical levitation of particles as possible explanations for this fascinating phenomenon. We present a critical assessment of these mechanisms and propose several lines of research we deem promising to further advance our understanding and modeling.

It is often assumed that the particle size of long-range transported mineral dust does not exceed 20 to 30 μm ( 9 – 12 ), and climate model simulations often limit particle diameters to only <10 μm ( 13 ). However, the incorporation of coarse particles is important as the radiative effect of dust is especially sensitive to the coarse dust mode. Coarse particles reduce the single-scattering albedo of shortwave radiation, increasing radiative absorption ( 3 ), and enhance the absorption of longwave radiation ( 14 ), possibly causing a net atmospheric warming as shown by Kok et al. ( 15 ). This latter study ( 15 ) also demonstrated a substantial effect on the atmospheric radiative balance when larger particles up to 20 μm are incorporated into climate models. If giant dust particles (>75 μm) would be considered, then the effects on atmospheric radiation budgets could be tremendous. In addition, the underrepresentation (or nonrepresentation) of particles larger than 10 μm in climate models and the distance these particles can travel affect total deposition fluxes over land and ocean. Giant mineral dust particles also play a role in the ocean carbon cycle, as they have a large ballasting potential for marine organic aggregates, making these aggregates denser and therefore aiding the transport of organic matter to the deep ocean ( 16 ). In addition, they influence cloud microphysics by acting as giant cloud condensation nuclei, which can accelerate the hydrological cycle through increasing precipitation rates ( 17 ). This demonstrates why a mechanism explaining the long-range transport of giant dust particles is urgently needed.

The Sahara is currently the largest single source of wind-blown sediments. Transport of Saharan dust across the Atlantic Ocean is subject to seasonal atmospheric changes in wind systems, blowing at different altitudes. In winter, low-level dust is carried toward the Atlantic with the northeasterly trade winds, or Harmattan, at altitudes between 0 and 3 km ( 5 ). In summer, the Saharan Air Layer (SAL) dominates dust transport. Upon reaching the west African coast, the SAL encounters a cool marine air mass that lifts the warm, dusty air to altitudes up to a maximum of 5 to 7 km ( 5 – 7 ). Wind velocities of up to 25 m s −1 associated with the Atlantic extension of the African easterly jet can lead to fast westward transport, particularly around 4 km ( 6 , 8 ).

About 30 years ago, scientists first observed so-called giant (>75 μm in diameter) wind-blown mineral dust particles at large (>10,000 km) distances from their source ( 1 ). These sand-sized mineral aerosols or dust particles, the largest of which (>200 μm) were all individual quartz grains, were transported from Asia to the remote Pacific Ocean. In Europe, giant dust particles were found >4000 km from their Saharan source ( 2 ), and dust particles up to 300 μm in diameter were sampled during aircraft campaigns over northwestern Africa ( 3 ). In marine sediment traps, positioned underneath the main Saharan dust plume in the Atlantic Ocean, giant particles are dominated by large quartz particles >100 μm, found at distances up to 4400 km from the west African coast ( 4 ).

RESULTS

New evidence for long-range transport of giant dust particles Here, we present new data from the same trans-Atlantic transect as van der Does et al. (4), this time collected directly from the atmosphere by Modified Wilson and Cooke (MWAC) samplers (see Materials and Methods), mounted on moored dust-collecting surface buoys at two stations in the tropical North Atlantic Ocean. The passive air samplers collected one discrete sample during periods between 2013 and 2016, comprising 281 to 432 days (Table 1) at approximately 3 m above sea level. These samples show giant dust particles (>75 μm; Fig. 1) that were collected at 2400 and 3500 km, respectively, from the west African coast at sampling stations M3 and M4 (Fig. 2). These are mostly well-rounded quartz particles up to 450 μm in diameter, with what appears to be high aspect ratios. As atmospheric samples, these sand-sized dust particles can only have been carried there by the wind. This observation adds further evidence to the ability of the atmosphere to transport giant particles over very long distances. 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. View this table: 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. The main reason why climate models do not incorporate dust particles >10 μm, despite the growing evidence for their existence far away from sources suggesting substantial residence times in the atmosphere, is related to the physical laws on which the models are based. The settling speed of small particles in air is usually calculated using equations based on Stokes’ law (10, 18). Settling velocities for particles >20 μm are overestimated by Stokes’ law due to turbulence created by the falling of larger particles and are therefore determined empirically (10, 19). Using the formula from Bagnold (19), we compare the settling velocities of giant mineral dust particles of 100, 200, and 300 μm (Table 2). These data show that, with such rapid settling velocities, it is not possible for giant particles to reach the sampling sites at 2400 and 3500 km from the west African coast (Fig. 2), even at high wind velocities of 25 m s−1. Some additional mechanisms are needed to keep these dust particles aloft. 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. View this table:

Potential mechanisms Several studies suggest such mechanisms including atmospheric vertical mixing (18, 20, 21) or large dust storms and turbulence (1, 9, 11), but the capacity of these mechanisms for long-range transport of giant dust particles over the ocean has not been explored. In addition, the shape of the mineral dust particles can influence their deposition, with aspherical particles having lower settling velocities than more spherical particles (22, 23). However, this does not seem to have a large effect on the giant dust particles observed at our Atlantic sampling locations, as these are almost exclusively spherical quartz minerals (Fig. 1). Here, we provide an integral discussion of the potential of four different mechanisms that can facilitate long-range transport of giant particles. First, strong winds causing fast horizontal transport greatly enhance the distance over which the dust travels. Second, transport of individual large dust particles is further aided by strong turbulence, keeping them in suspension for a longer time (24), although this could also have a negative effect on the dust particles, causing them to settle even quicker. Third, particle charge affects their dynamics and, for negatively charged particles, can offset a particle’s weight in a downward-directed electric field, so keeping it aloft for longer (25, 26). Last, thunderstorms or tropical cyclones can carry dust particles to great heights, strongly increasing their horizontal travel distance if the particles can leave the storm through the anvil or upper-level outflow region without being rained out. This mechanism will be particularly effective when multiple uplift cycles are encountered along active areas such as the intertropical convergence zone (ITCZ). In the following, we provide a plausibility analysis of these four factors to test whether they can explain our new observations of giant dust particles. We will discuss winter and summer situations separately.

Winter scenario In boreal winter, dust transport occurs at lower altitudes and at lower wind speeds (Table 2). There is insufficient convection over the sampling area to aid the long-range transport of dust particles, as the ITCZ is shifted southward (27). Backward trajectory calculations for February (see Materials and Methods) show that simple horizontal particle transport (no settling) within the boundary layer, where horizontal winds are fastest, would still take at least 48 hours to reach M3 and at least 72 hours to reach M4 (Fig. 2A). Transport at higher levels is unlikely, as winds become increasingly more westerly with height (28). Assuming the sedimentation velocities for particles of 100 μm given in Table 2, the shortest travel time to M3 and M4 would correspond to a total vertical sedimentation of ~70 and 100 km, respectively, strongly suggesting that other mechanisms must be involved. Fast horizontal transport events will be characterized by strong winds over the ocean but often also over land, as these tend to be associated with synoptic-scale subtropical highs (29), leading to large dust emissions. Given that transport is usually restricted to the boundary layer, we can assume highly turbulent conditions, stirred mechanically by high wind shear, even in the absence of buoyancy generation over the ocean. However, it is difficult to quantify the effect of turbulence on the likelihood of individual giant dust particles to stay suspended without any direct observations of this process. The third mechanism that may contribute to keeping giant particles aloft is via electrical forces. Many studies have found that atmospheric charging affects particle dynamics, with a vertical electrical force being able to potentially compensate a particle’s weight (25, 26). Renard et al. (26) found large particles (>40 μm) persisting over long distances over the Mediterranean region, without significant downwind trends in size. They speculated that this was due to particle charge, counteracting gravitational settling. Electric charge has also been shown to increase dust emission from source areas up to 10-fold (30, 31). Whether or not the electric field hinders or assists a particle in staying aloft depends on the relative polarity of the particle and the atmospheric electric field encountered. The downward-directed electric field always present in fair weather will drive positively charged particles downward, but the field direction can reverse in disturbed weather or during saltation events (32) and particles can readily carry both polarities of charge, so an upward electrical force is possible. The initial charge generated at dust emission is lost within hours (25), but charged particles have still been detected far away from sources. For instance, during Hurricane Ophelia of October 2017, which brought large amounts of dust and smoke particles to the southern United Kingdom, appreciable negative charge was detected in the dust plume after a transport time of tens of hours (33), showing that charge is also generated during transport. The most likely reason is that particles are more or less continuously charging through collisions, a process called triboelectrification (32, 34). This effect is facilitated in an atmospheric layer that is characterized both by a high dust concentration and strong turbulence, exactly the kind of strong-wind situation described above, which would sustain electric fields sufficiently to reduce the fall speeds of highly charged particles. A further consequence of a system of particles is to reduce the air’s electrical conductivity through removal of cluster ions, allowing the charge on the particle assembly to be sustained for longer than for an isolated charged particle. Two factors together determine the electrical effect on a particle of a given size: the local atmospheric electric field and the particle charge. The atmospheric electric field varies appreciably between fair weather conditions (E = −102 V m−1) and disturbed weather conditions such as convective clouds and thunderstorms in which substantial charge separation occurs meteorologically to generate strong electric fields of different polarities (E = ± ~104 V m−1). An individual particle’s charge aloft can also cover a wide range, resulting from its interactions with other particles and cosmic ray–generated ions or, exceptionally, its internal radioactivity. Figure 3A shows the number of particle charges necessary for the electric force on the particle to have the same magnitude as a particle’s weight in a range of typical atmospheric electric fields, that is, under which conditions it could become levitated. In the weak fair weather field (102 V m−1), only the smallest particles are affected, with ~2 elementary charges (e) required. For larger particles, the number of charges and field required increases. In electric fields characteristic of disturbed weather (~104 V m−1), particles of 100 μm typically require 107 or 108 e to offset their weight and reduce the particle’s fall speed (Fig. 3B). In situ measurements of individual particle charge aloft are not available for comparison, and related quantities near the surface are only poorly known, but charges found in dust devils are of ~106 e cm−3 (34) and resuspended dust on the order of 103 to 104 e per particle (or 1012 e cm−3) (32). Larger charges are likely for giant particles, scaling with particle area, but at very large charges and fields, charge emission would prevent further electrification (35). One highly relevant factor is particle composition (30, 32), as it is known that mineralogy affects the charging of particles. The giant dust particles found at the buoys are mostly quartz (Fig. 1), and it is found experimentally that quartz particles may charge more easily than clay minerals [(32) and references therein]. Any electrically assisted transport would require the sustained or fortuitous presence of strongly electrified clouds and particle charges of the appropriate relative polarities. More laboratory, field, and numerical studies are needed to quantify this effect in a fully turbulent dust layer with interacting particles of different sizes. 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 C D = 1.5).