Impacts of Black Carbon (BC) Pollution on Himalayan Glaciers

Black carbon (BC) is now well known as a solar absorptive aerosol and its higher contamination in the Indo-Gangetic Planes is a large concern. The BC deposited over Himalayan glaciers also likely contributes to snow darkening and heating the snow surface, finally leading to glacier retreats. However, still not so much observations on the snow-impurity-related topic over the Himalayas have been carried out yet and more observations are required. As the first preliminary study, we estimated at least how much BC can deposit over the southern slope of Himalayas during pre-monsoon season of 2006 from the observed meteorological data and equivalent BC concentration (eqBCC) in the atmosphere at Nepal Climate Observatory – Pyramid site (NCO-P, Nepal; lat. 27.958˚N, lon. 86.815˚E; 5079 m a.s.l.) with a realistic lower bound of dry deposition velocity (DDV). The BC deposition of 266 µg m^-2 was obtained and it can reduce snow albedo of 2.0-5.2% based the empirical equation based on the data from previous studies.

    However, the above estimation is the lower bound and actual deposition is expected to be more. Here we further estimated the range of dry deposition amount of BC during the same period based on the observed eqBCC and meteorological data at NCO-P with fixed slower DDV above, GOCART DDV code used in GOCART/GEOS-4 below, and another DDV theory. In addition, two types of outputs from chemical transport models coupled to general circulation models (GCMs: NASA GOCART/GEOS-4 and NIES/CCSR/FRCGC SPRINTARS). The two GCM outputs showed much higher dry depositions of BC. Comparing all the deposition estimations, the overestimations from the GCMs are probably due to stronger surface wind than the observation at NCO-P and without ice surface assumption for the DDV calculations. When we calculated DDVs with NCO-P based observations and ice surface assumption, the deposition velocities became slower than the outputs from GOCART/GEOS-4. It indicates that prescribed surface vegetation over glacial areas and stronger surface wind cause faster DDVs and overestimate BC deposition over glacial areas. For both fresh and aged snow cases, the estimation errors on the depositions themselves possibly cause the uncertainties on visible snow albedos of more than 5.6 and 8.0% for hydrophobic and hydrophilic BC cases, respectively. We can expect old snow and aged BC (considered as hydrophilic BC) for dry pre-monsoon period 2006. On this expectation, visible snow albedo reductions in broadband of 4.3-5.1% are considered to be reasonable range at the southeastern Himalayas.