response to Review of acp-2018-869 (Anonymous Referee #2)

Abstract

We estimated the black carbon (BC) concentration over the Hindu Kush Himalayan region (HKH), its impact on snow albedo reduction, and sensitivity on annual glacier runoff over the identified glaciers. These estimates were based on free-running aerosol simulations (freesimu) and constrained aerosol simulations (constrsimu) from an atmospheric general circulation model, combined with numerical simulations of a glacial mass balance model. BC concentration estimated from freesimu performed better over higher altitude (HA) HKH stations than that over lower altitude (LA) stations. The estimates from constrsimu mirrored the measurements well when implemented for LA stations. Estimates of the spatial distribution of BC concentration in the snowpack (BC c) over the HKH region led to identifying a hot-spot zone located around Manora Peak. Among glaciers over this zone, BC c (> 60 µg kg −1) and BC-induced snow albedo reduction (≈ 5 %) were estimated explicitly being high during the pre-monsoon for Pindari, Poting, Chorabari, and Gangotri glaciers (which are major sources of fresh water for the Indian subcontinent). The rate of increase of BC c in recent years (i.e., over the period 1961-2010) was, however, estimated to be the highest for the Zemu Glacier. Sensitivity analysis with a glacial mass balance model indicated the increase in annual runoff from debris-free glacier areas due to BC-induced snow albedo reduction (SAR) corresponding to the BC c estimated for the HKH glaciers was 4 %-18 %, with the highest being for the Milam and Pindari glaciers. The rate of increase in annual glacier runoff per unit BC-induced percentage SAR was specifically high for Milam, Pindari, and Sankalpa glaciers. The source-specific contribution to atmospheric BC aerosols by emission sources led to identifying the potential emission source being primarily from the biofuel combustion in the Indo-Gangetic Plain south of 30 • N, but also from open burning in a more remote region north of 30 • N.