Black carbon aerosols over the Himalayas: direct and surface albedo forcing

Absorbing aerosols such as black carbon (BC) or dust over high-altitude Himalayan regions have potential implications on the regional climate and hydrological cycle over South Asia. Making use of extensive measurements of atmospheric BC from several Himalayan stations, an assessment of radiative forcing due to direct and snow-albedo darkening is examined. Generally, BC concentration in the atmosphere peaks during pre-monsoon season over the Himalayas and the climatological mean of atmospheric BC over Hanle (western Himalayas, 4.5 km msl) and Nepal Climate Observatory-Pyramid (central Himalayas, 5 km msl) are 106±27 ng m⁻³ and 190±95 ng m⁻³, respectively. Based on the optical and physical properties of composite aerosols measured at Hanle, clear sky direct radiative forcing (DRF) at the top of the atmosphere is estimated as 1.69 W m⁻² over snow surface and −1.54 W m⁻² over sandy surface during pre-monsoon season. The estimated amount of BC in the snow varied from 117 to 1.7 µg kg⁻¹ for wide range of dry deposition velocities (0.01–0.054 cm s⁻¹) of BC, snow depth (2–10 cm) and snow densities (195–512 kg m⁻³). Using a size-resolved wet scavenging parametrisation, the amount of BC on snow due to wet scavenging is estimated as 29 µg kg⁻¹ for an accumulated snow depth of 27 cm. For the range of 10–200 µg kg⁻¹ of BC in snow, the diurnally averaged forcing due to snow darkening has been found to vary from 0.87 to 10.2 W m⁻² for fresh snow and from 2.6 to 28.1 W m⁻² for the aged snow, which is significantly higher than the DRF. The direct and surface albedo radiative forcing could lead to significant warming over the Himalayas during pre-monsoon.