Dust particles size distributions in snow and its importance for optical properties of snow

Light absorbing particles (LAPs) from various origins get deposited to the snow where they greatly influence its physical evolution, and most particularly its melt by changing the energy absorbed from solar radiation. Black Carbon and Dust are the most important such particles. How those particles change snow physics depends on their optical properties, which are a function of their chemical composition and size distribution.

In European mountain ranges, part of the deposited dust comes as sporadic Saharan dust outbreaks, which cause important dust layers in the snowpack. One such event was the object of a citizen science collaborative sampling campaign, which resulted in 150 samples collected over the Pyrenees, the Jura, and the French and Swiss Alps.

Dust in these samples was filtered out and weighted, in order to get total deposition fluxes. Size distributions were measured between 4 and 60 µm. This size fraction represents less than 10% of the total mass measured in those samples. Assuming lognormal distributions to extend the measured size distributions beyond 60 µm only explained a fraction of the missing mass.

The relative importance of the particles not measured between 4 and 60 µm for the optical properties of the snowpack depend strongly on their size. A rough estimate of the importance of the missing fraction was attempted by assuming that the overall optical effect scales with the surface area of the particles. Depending on the assumed diameter of particles in this fraction, the missing mass (~90% of the total mass) overall optical impact is estimated between 80 and 40% of the estimated total optical impact of the dust present in the snow.

A limited set of samples was used to assess the size of this missing fraction, using different methods. This preliminary assessment suggests the potential importance of grain aggregation in natural snow and the importance of unbiased size distribution measurements for dust in snow.