Albedo of dirty snow during conditions of melt

The evolution of spectrally averaged albedo (wavelengths between 0.28 μm and 2.8 μm) of snow surfaces treated with known initial concentrations of particles of submicron‐sized soot and air fall volcanic ash was investigated during conditions of natural melt. Depending on the particle type and concentration, the initial applications reduced the surface albedo to values ranging from 0.18 to 0.41 which were substantially lower than the albedo of the untreated natural snow (about 0.61). Many of the soot particles flushed through the snowpack with the meltwater, and surface concentrations of soot greater than about 5 × 10 −7 kg/kg did not persist for more than a few days. The migration of particles to depth caused the snow to brighten after the initial application, thus limiting the amount of albedo reduction and the consequent effects on melting. Nevertheless, the soot remaining near the surface had a substantial, long‐term effect. The residual concentration of 5 × 10 −7 kg/kg persisted for several weeks and, compared to the untreated surface, reduced the albedo by about 30% and increased melting by 50%. Particles of volcanic ash with diameters larger than about 5 μm remained at or near the snow surface. Although many of the smaller particles flushed through the snow with the meltwater, the surface albedo was not changed significantly by their removal. The different behaviors of the ash and soot are probably related to the difference in their particle size distributions in relation to the thickness of water films that form the transport paths under conditions of partial saturation that are characteristic of melting snow.