Quantifying the impact of Light Absorbing Impurities on snow properties and melting using mini-lysimeters in the Central Pyrenees

Abstract. The Pyrenees are experiencing an increasing frequency of atmospheric conditions favorable to African dust transport, while black carbon (BC) deposition is expected to decline in the coming decades. These two dominant light-absorbing impurities (LAIs) significantly alter snowpack properties by reducing snow surface albedo, accelerating melt, and disrupting mountain hydrology by modifying the surface energy balance. While the role of LAIs in enhancing snow metamorphism and melt is well recognized, quantifying their precise impact remains challenging due to the irregularity of deposition events, variability in impurity type and concentration, and the snowpack heterogeneity.

To better quantify and constrain the physical impacts of LAIs on snowpack, we implemented controlled field experiments using custom mini-lysimeters filled with natural snow, artificially doped with realistic concentrations of mineral dust (2, 5, 10 g/m²) and BC (0.1, 0.2 g/m²) and exposed to environmental conditions and sunlight for 3–4 hours in each set of experiments. Results revealed a systematic broadband albedo reduction of 0.2–0.3, accompanied by a marked decline in specific surface area (from ~10 to <4 m²/kg in dust-treated samples). Seasonal mean liquid water content increased from 3.1 % in clean snow to 6.2 % (2 g/m² dust) and up to 10.1 % (10 g/m² dust), with BC showing intermediate responses. However, the most striking effect was observed in meltwater production: even the lowest dust load frequently doubled melt rates relative to controls, and in several cases induced melt when control snow remained stable. Mineral dust outperformed BC in enhancing melt under comparable radiative conditions.

Overall, this study provides a simple and robust experimental framework for linking particle concentration to measurable changes in snow metamorphism and melt. Our results suggest that realistic concentrations of mineral dust, such as those frequently observed in the Pyrenees due to African dust deposition, can shorten snow duration and potentially reinforce the effects of ongoing climate warming.