On the role of Microplastics as Light Absorbing Particles in seasonal snowpacks: First evidence from the Central Pyrenees

Microplastic particles (MP; plastic pieces with length < 5 mm) have already colonized every ecosystem on Earth, including cryospheric regions. The presence of MPs has been reported in the Artic, Antarctic and glaciers and seasonal snow at high mountain ranges from Europe, Asia and America. Still, their capacity to affect snow metamorphism and albedo, as light-absorbing impurities, remains unexplored. During the snow season of 2023/24 (from early February to early May 2024), 6 in situ experiments were conducted at the Spanish Central Pyrenees employing a set of mini-lysimeters containing surface snow doped with different concentrations of MP. In each experiment, a blank that remained exempt of particle addition was also included. Two types of polymers were used, low-density polyethylene (PE; ~300 µm) and polyurethane (PUR; ~450 µm) black pellets. The mini-lysimeters were exposed to atmospheric conditions for 3-4 hours to quantify changes in snow specific surface area (SSA), liquid water content (LWC), hyperspectral albedo (HA) and ultimately the total melted water after exposition. Results were very variable across the season. Thus, effective melting (> 40%) was observed only during the warmer days under high solar radiation on old snow. Still, changes in SSA, LWC and HA (calculated as percentage of change per hour) occurred almost in every experiment. SSA changes were quite variable, ranging from negative (mid-winter and old snow, with the lowest PUR concentrations used) to 33% h^-1 (spring and warmest day, old snow, all PUR concentrations used). Similarly, LWC increased from 0 the coldest day (with all MPs but the highest PE concentration used) to 500% h^-1 (early winter and old snow, with the highest concentration used). Changes in HA were modest, ranging from 1.1% h^-1 (mid-winter and old snow, low PUR concentrations) to 7.5% h^-1 (spring and warmest day, old snow, highest PUR concentration). Up to now, these results are the first evidence of MP’s capacity to act as light absorbing particles, triggering snow metamorphism and eventually snow melting. Forthcoming studies will test other types of MP and concentrations, under a wider range of snow conditions, to allow a more comprehensive spatial-temporal interpretation.