Saharan dust impacts on the surface mass balance of Argentière Glacier (French Alps)

Saharan dust deposits frequently turn alpine glaciers orange and darken their surface. Together with other light-absorbing particles, mineral dust reduces snow albedo, increases snow melt rate, and lowers the surface mass balance of glaciers. Since the surface mass balance drives the evolution of alpine glaciers, assessing the impact of impurities helps to understand their current and future evolution. The location of impurities within the snowpack and their effect on snow albedo can be estimated through physical modelling. In this study, we quantified the impact of dust, taking into account mineral dust and black carbon in snow, on the Argentière Glacier over the period 2019–2022. Our results show that during the three years preceding 2022, the contribution of mineral dust to the annual decrease in surface mass balance was between 0.31–0.45 m w.e., while it reached the double in 2022 with 0.63 m w.e. [0.54, 0.69] (median, [Q10–Q90]), and up to 1.2 m w.e. [0.9, 1.4] at specific locations. The impact of dust in snow was unevenly distributed over the glacier, especially in 2022. The highest simulated impacts occurred where firn layers from previous years were exposed after the total melt of the snowpack of the previous winter. The gravitational redistribution of the snow from avalanches was not taken into account, which can reduce the impact of dust at specific locations. Increasing the modelled scavenging efficiency of black carbon can double the impact of dust alone at the glacier scale. In general, the contribution of mineral dust to the melt represents between 8 % and 16 % of Argentière Glacier summer melt depending on the year. Hence, we recommend accounting for impurities to simulate the distributed surface mass balance of glaciers.