1Department of Geological Sciences, Central Washington University, Ellensburg, Washington, USA
2Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, USA
3Laboratory of Atmospheric Chemistry, Paul Scherrer Institut, Villigen PSI, Switzerland
4Laboratory of Radiochemistry and Environmental Chemistry, Paul Scherrer Institut, Villigen PSI, Switzerland
Abstract. Black carbon (BC) and dust deposited on snow and glacier surfaces can reduce the surface albedo, accelerate snow and ice melt, and trigger albedo feedback. Assessing BC concentrations in snow and ice in the Himalaya is of interest because this region borders large BC sources, and seasonal snow and glacier ice in this region are an important source of water resources. Snow and ice samples were collected from crevasse profiles and snowpits at elevations between 5400 and 6400 m a.s.l. from Mera glacier located in the Solu-Khumbu region of Nepal on the southern slope of the Himalaya during spring and fall 2009. The samples were measured for Fe concentrations (used as a dust proxy) via ICP-MS, total impurity content gravimetrically, and BC concentrations using a Single Particle Soot Photometer (SP2). Measured BC concentrations underestimate actual BC concentrations due to changes to the sample during storage, and loss of BC particles in the ultrasonic nebulizer. BC and Fe concentrations peak during the winter–spring, and are substantially higher at elevations <6000 m due to post-depositional processes including melt and sublimation and greater loading in the lower troposphere. Because the largest areal extent of snow and ice resides at elevations <6000 m, the higher BC and dust concentrations at these elevations can reduce the snow and glacier albedo over large areas, accelerating melt, affecting glacier mass-balance and water resources, and contributing to a positive climate forcing. Radiative transfer modeling constrained by measurements indicates that BC concentrations in the winter–spring snow/ice horizons are sufficient to reduce albedo by 6–10% relative to clean snow, corresponding to instantaneous radiative forcings of 75–120 W m−2. The other bulk impurity concentrations, when treated separately as dust, reduce albedo by 40–42% relative to clean snow and give instantaneous radiative forcings of 490 to 520 W m−2. Adding the BC absorption to the other impurities results in additional radiative forcings of 3–10 W m−2. While BC contributes to accelerated snow and ice melt, the impact of BC is diminished in the presence of other light absorbing impurities. However, the time span of the BC exposure at the snow surface in the dry winter–spring season is likely a persistent forcing before impurity convergence, but is not addressed by these single measurements. Further observational studies are needed to assess the contribution of BC relative to other absorbing impurities to albedo reductions and snow and ice melt.