Sahara sand dust melts European snow

2023-08-29 05:00:10

A sky covered in orange colors, snow-capped peaks that turn ochre… Several times a year, sand dust from the Sahara goes up north and falls back on Europe. Although the phenomenon is common, the spatial variability of the amounts of dust deposited and their composition were still little known. A study conducted by several French laboratories – including the CNRS and the National Center for Meteorological Research (CNRM) – published on July 20 in the review Earth System Science Dataadds to this knowledge.

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After an extreme episode of Saharan dust fallout, on February 5, 2021, several scientists joined forces and launched a citizen science campaign. Citizens are called upon to collect as many samples as possible of Saharan dust deposited on the snow in the Alps and the Pyrenees.

“The idea came to us the same day, it’s not something that is usually done in sciencetell Mary DumontMeteo-France research director at the CNRM, in charge of the study. But, without the help of the inhabitants, it would have been difficult to obtain so many samples. » A successful attempt, according to Ghislain Picardprofessor at the University of Grenoble-Alpes: “In normal times, scientists are very attached to the rigor of sampling. However, they managed to urgently put in place a sampling protocol that works. » The participants had to take the layer of orange snow, as during a coring, using a container whose diameter they had previously noted, and take a photo of the location of the site in order to geolocate it.

Iron-rich particles

Thanks to citizen mobilization, one hundred and fifty-two samples of orange snow from seventy sites were collected. The scientists analyzed the mass of the dust, the size of the particles, their chemical composition and their optical properties. In the Pyrenees, crossed first by the cloud, the mass of dust and the size of the particles were greater than in the Alps. “These results correspond to the trajectory of the dust cloud. It’s quite logical, but, until then, we were unable to quantify it “, explains Marie Dumont. In the Pyrenees, 21.2 grams per square meter of dust were deposited. In the Alps Swiss, there were only 4.5 grams per square meter.

The particles, in the Pyrenees, were also richer in iron. “Iron changes the optical properties of dust. The more it contains, the darker and redder it becomes”, continues the researcher. However, this has consequences for the snowpack. “The more the dust is rich in iron, and therefore darker, the faster the snow will melt”says Ghislain Picard.

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