Black carbon over a central Himalayan Glacier (Satopanth): Pathways and direct radiative impacts.

Continuous measurement of Black Carbon (BC) concentration was carried out during May-October 2018 periods over Satopanth Glacier in the central Himalayas. BC concentrations varied between 28 and 287 ngm on different days during the observational period. High concentration of BC was observed in the month of May (monthly mean of 221 ± 79 ngm), and a lower concentration was observed in August (monthly mean of 92 ± 58 ngm). Biomass burning was found to contribute up to 58% of BC mass over the region, with lower contribution during June and higher during the month of May. Compensation parameter (K) values were found to vary between -0.005 and 0.005 in different months, asserting the presence of aged BC in June to October months and relatively fresh BC in the month of May. Concentration weighted trajectory (CWT) analysis showed that the air mass from Indo Gangetic Plains (IGP) was responsible for the majority of transported BC in July & August months (up to 65%) and partially in September (up to 40%). However, the transport from Middle East and far north-western regions was found to be the major contributor to BC concentrations in other months. The estimated BC direct radiative forcing was found to induce 4.5 to 7.6 Wm reduction of radiation at the surface (SFC) and the forcing was +2.3 to +3.5 Wm at the Top of the Atmosphere (TOA). The BC induced atmospheric heating rates were found to be up to 0.35 k day over the region. The sensitivity of snow albedo to radiative forcing was studied, and it is found that BC albedo changes tend to decrease albedo with an increase in BC-snow deposition, leading to a decrease in atmospheric absorption.