In conjunction with the sediment traps described in van der Does et al. (4), two moored dust-collecting surface buoys were deployed at two of the sampling stations, M3 (12.39°N, 38.63°W) and M4 (12.06°N, 49.19°W) (Fig. 2), for two consecutive years (Table 1). These buoys were equipped with one MWAC (43) sampler each, a passive air sampler that sampled continuously over the time the buoys were deployed. Our MWAC samplers consisted of a plastic bottle with an inlet and outlet tube of 7.5 mm in diameter. They were installed vertically about 3 m above sea level, while a wind vane ensured windward orientation. The MWAC samplers have sampling efficiencies between 75 and 105% for dust with a median particle size of 30 μm, which means that, in some occasions, an oversampling would occur. However, Goossens and Offer (43) conclude that the MWACs are the least inefficient samplers. Sampling efficiency varies slightly with different wind speeds but without apparent trends (43). For sand-sized particles with median grain sizes between 132 and 287 μm, the samplers have slightly higher efficiencies of 90 to 120%, which are constant and independent of wind speed for velocities between 6.6 and 14.4 m s−1 (44). Maximum wind velocities at the sampling stations approximated 14 m s−1 (Table 1). We found these samplers to be best suited for our sampling purposes, as their sampling efficiencies are good, the mechanism is extremely simple, and the samplers are very inexpensive.

The MWAC samplers collected a discrete Saharan dust sample over the entire sampling period. Besides mineral dust, the samples inevitably also contained sea salts. These were removed by rinsing the sample bottle with Milli-Q water and subsequently filtrating over a 25-mm polycarbonate filter with a pore size of 0.4 μm (2014 samples) and 47-mm polycarbonate filters with a pore size of 0.2 μm (2015 samples). These filters were then qualitatively analyzed with a light microscope for the presence of giant mineral dust particles and photographed. Grain-size distributions of the complete MWAC samples were obtained using a laser particle size analyzer Coulter LS13 320, using the method described by van der Does et al. (4) (Fig. 4). Figure 4 shows the limitations of this method of obtaining information on giant particles, which are present in numbers that are below the detection limit of the laser particle sizer, and thus not registered in the grain-size distributions, which show particles up to only 80 μm.

We have tried to eliminate any possible contamination of giant particles to our dust samples. Light-microscope analysis (Fig. 1) confirms the giant particles to be mineral dust and not, for example, plastic fragments from the sampling bottle, not salt crystals (which have a very typical cubic mineral shape), nor glass shards from glass beakers and petridishes (which would have very angular shape, with typical glass fracture features). In addition, we can see that some of these particles have some sort of iron coatings, typical for Saharan dust particles. Other sources of sediment can be excluded since these samples were collected in the middle of the Atlantic Ocean, directly from the air. Therefore, the possibility of contributions from the ocean or riverine input can be excluded.

The moored dust-collecting surface buoys are also equipped with a carrousel of 24 filters through which a pump actively samples air, and as a result, the dust can be collected on these filters on a much higher resolution (45). Unfortunately, this active sampling proved to be unsuccessful as all the filters were retrieved ruptured, and future sampling campaigns will allow for a much more detailed study of the occurrence of giant mineral dust particles (seasonality, giant particle concentration in air, etc.). The sampling is done in parallel with meteorological observations such as wind speed and wind direction. In addition, future upgrades of the buoys will include wet deposition samplers.

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