The thin layer of air that surrounds our planet is rich in particles of all kinds. Some of them come from desert areas, blown by the winds. However, a mystery continues to hold scientists in suspense: the now well-documented presence of giant particles transported far from their region of origin. Currently, we still know little about the mechanisms responsible for this transport that seems to challenge the law of gravity.
The Saharan desert is known to be an important source of mineral particles for the atmosphere. On average, they are transported westward by prevailing winds, and move over the tropics and subtropical Atlantic Ocean before reaching Latin America. But sometimes it happens that, under the influence of a southerly wind taking its source low in latitude, they are advected towards Europe. Their presence then gives a milky – or even orange – appearance to the European sky. If precipitation occurs, they are dissipated as a deposit of sand – particularly noticeable on car or exposed windows.
However, there is a mystery that has intrigued researchers for thirty years about this transport. It concerns the size of the dust particles. Numerous observations testify to the presence of giant specimens (> 75 micrometers in diameter) having been moved very far from their source region. Values greater than 300 micrometers could be measured. However, for the time being, there is no known effective support mechanism that would allow such large particles to be transported so far – i.e. several thousand kilometers away.
A paper published on December 12 in Science Advances magazine looked at the question. As part of the study, samples were taken in the northern tropical Atlantic between 2013 and 2016. The instruments were installed on buoys. They sampled the air at about 3 meters above sea level. Giant particles with a diameter of up to 450 micrometers were reported. They are made of quartz and have an almost spherical appearance. “This observation adds further evidence of the ability of the atmosphere to transport giant particles over very long distances,” the document says.
In a second step, an evaluation of the different hypotheses emitted to date has been carried out. Some of them highlight the role of turbulence in the middle and upper atmosphere or that of stormy ascents. Others focus instead on the impact of the electrification of particles by friction. The effect related to the shape of the dust is also advanced – the more asymmetrical ones falling less quickly. However, it does not seem to be relevant since the measurements reveal a quasi-spherical shape. In the end, it is likely that many of these mechanisms work together.
For example, in summer, there is a very turbulent air layer at altitude associated with the East African jet. It also has a high concentration of dust, allowing electrization by friction very effective. The effects of turbulence, strong wind and electrical charges would combine to transport the particles while offsetting gravity-driven surface drag.
Whatever the case may be, future research is needed because our knowledge is still pithy and the quantification of the various processes involved is almost unexplored. It is even possible that some have not yet been discovered.
In addition, the subject presents a major challenge in the field of climate modeling, since the models do not consider dust greater than 10 micrometers. This can lead to biased results given the major effect they have on the planet’s energy balance – through reflection / absorption of radiation or modification of cloud properties. Note also their impact on storm dynamics and tropical cyclones.
For the moment, it is not possible to include larger particles in the simulations, because the equations on which the models are based – Stokes’ law in particular – overestimate their speed of fall. To provide good values, one should know all the mechanisms involved and the weight of each. An empirical determination would then be possible. However, we have seen that this is not yet the case. A major challenge that remains to be met in the coming decades.