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Black carbon (BC) and dust deposited on snow and glacier surfaces can reducethe surface albedo, accelerate snow and ice melt, and trigger albedofeedback. Assessing BC and dust concentrations in snow and ice in theHimalaya is of interest because this region borders large BC and dustsources, and seasonal snow and glacier ice in this region are an importantsource of water resources. Snow and ice samples were collected from crevasseprofiles and snow pits at elevations between 5400 and 6400 m a.s.l. from Meraglacier located in the Solu-Khumbu region of Nepal during spring and fall2009, providing the first observational data of BC concentrations in snowand ice from the southern slope of the Himalaya. The samples were measuredfor Fe concentrations (used as a dust proxy) via ICP-MS, total impuritycontent gravimetrically, and BC concentrations using a Single Particle SootPhotometer (SP2). Measured BC concentrations underestimate actual BCconcentrations due to changes to the sample during storage and loss of BCparticles in the ultrasonic nebulizer; thus, we correct for theunderestimated BC mass. BC and Fe concentrations are substantially higher atelevations < 6000 m due to post-depositional processes includingmelt and sublimation and greater loading in the lower troposphere. Becausethe largest areal extent of snow and ice resides at elevations < 6000 m,the higher BC and dust concentrations at these elevations can reducethe snow and glacier albedo over large areas, accelerating melt, affectingglacier mass balance and water resources, and contributing to a positiveclimate forcing. Radiative transfer modeling constrained by measurements at5400 m at Mera La indicates that BC concentrations in the winter–springsnow/ice horizons are sufficient to reduce albedo by 6–10% relative toclean snow, corresponding to localized instantaneous radiative forcings of75–120 W m−2. The other bulk impurity concentrations, when treatedseparately as dust, reduce albedo by 40–42% relative to clean snow andgive localized instantaneous radiative forcings of 488 to 525 W m−2.Adding the BC absorption to the other impurities results in additionalradiative forcings of 3 W m−2. The BC and Fe concentrations were usedto further examine relative absorption of BC and dust. When dustconcentrations are high, dust dominates absorption, snow albedo reduction,and radiative forcing, and the impact of BC may be negligible, confirmingthe radiative transfer modeling. When impurity concentrations are low, theabsorption by BC and dust may be comparable; however, due to the low impurityconcentrations, albedo reductions are small. While these results suggest thatthe snow albedo and radiative forcing effect of dust is considerably greaterthan BC, there are several sources of uncertainty. Further observationalstudies are needed to address the contribution of BC, dust, and coloredorganics to albedo reductions and snow and ice melt, and to characterize thetime variation of radiative forcing

Seasonal and elevational variations of black carbon and dust in snow and ice in the Solu-Khumbu, Nepal and estimated radiative forcings