Seasonal Dynamics and Radiative Impacts of Black Carbon in the Yumthang Valley in Sikkim Himalaya, India

Black carbon (BC) aerosols are a major concern in changing climatic scenario due to their ability to absorb solar radiation. This study investigates the temporal variation of BC and its impact on radiative forcing in the Sikkim Himalayan region. It aimed to understand the optical properties of BC and its radiative forcing at Yumthang Valley, a high-altitude (~3800 m) remote location in North Sikkim, India. In-situ measurements were undertaken to quantify BC mass concentration, using an Aethalometer, and satellite retrieval techniques were employed to assess the optical properties of BC during May 2022 to April 2024. The monthly mean BC concentration in the valley ranged from 1.03 ± 3.07 to 9.54 ± 16.44 µgm^-3, with an annual mean of 5.07 ± 16.54 µgm^-3. BC concentrations decrease during monsoon months due to limited long-range transport and effective wet scavenging. Biomass burning contributes significantly to BC levels, accounting for 88% of the total, with the highest contribution in September (55%) and the lowest in February (21%). Transport models indicate inputs from the Indo-Gangetic Plain and adjacent valleys with increased biomass burning activity. Seasonal variations reflect tourism-driven emissions and local wood burning, with the lowest levels observed in the early morning hours. BC-induced direct radiative forcing (DRF) was also calculated at the surface (SFC) and top of the atmosphere (TOA) using Santa Barbara DISORT Atmospheric Radiative Transfer (SBDART) model which estimate a substantial forcing at both the surface and top of the atmosphere. The BC-induced atmospheric heating rates suggest potential regional warming, which could accelerate glacier melt in the region.