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Here we present the first known data set on black carbon (BC) and mineral dust concentrations in snow from the Juneau Icefield (JIF) in southeastern Alaska, where glacier melt rates are among the highest on Earth. In May 2016, concentrations of BC (0.4-3.1 mu g/L) and dust (0.2-34 mg/L) were relatively low and decreased toward the interior of the JIF. The associated radiative forcing (RF) averaged 4 W/m(2). In July, after 10 weeks of exposure, the aged snow surface had substantially higher concentrations of BC (2.1-14.8 mu g/L) and dust (11-72 mg/L) that were not spatially distributed by elevation or distance from the coast. RF by dust and BC ranged from 70 to 130 W/m2 (87 W/m2 average) across the JIF in July, and RF was dominated by dust. The associated median snow water equivalent reduction in the July samples is estimated at 10-18 mm/day, potentially advancing melt on the scale of days to weeks. Aging of the snow surface in summer likely resulted in a positive feedback of melt consolidation, enhanced solar absorption and melting, and further concentration of surface particles. Regional projections of warming temperatures and increased rain at the expense of snow make it likely that summer season darkening will become a more important contributor to the high melt rates on the JIF. Further studies are needed to elucidate the spatiotemporal occurrence of various light-absorbing particles on the JIF, and models of ice field wastage should incorporate their associated RF.

Plain Language Summary In May and July 2016, we collected snow samples from the Juneau Icefield, where glacier melt rates are among the highest on Earth. Analyses of black carbon, a by-product of biomass and fossil fuel burning, and dust, small mineral particles that are deposited on the ice field, indicate that these particles darken the surface enough to be a significant factor in the ice field's melt. Because white snow reflects solar radiation whereas dark particles absorb it, these particles enhance melting of the snow beyond that which is caused simply by warming temperatures. As the length of time between spring and autumn snowfalls expands and light-absorbing particles accumulate at the surface throughout the summer months, the particles are likely to become increasingly important players in the ice field's energy balance. The role of these and other light-absorbing particles should be further investigated and incorporated into melt models.