Black carbon concentrations and atmospheric forcing in a high-altitude Himalayan valley

Black carbon (BC), a strong light-absorbing aerosol, is a significant contributor to atmospheric warming, particularly over high-altitude cryospheric regions. This study presents BC measurements using a seven-wavelength Aethalometer (AE-33), at Yumthang Valley (∼3800 m a.s.l.), a high-altitude region in Eastern Himalaya. Monthly mean BC concentrations ranged from 1.04 μg m⁻³ (August) to 9.55 μg m⁻³ (April), with peaks during the pre-monsoon period and minima in the monsoon months. Elevated pre-monsoon concentrations are attributed to regional biomass burning, long-range transport, and seasonally enhanced atmospheric stability, while reduced concentrations during monsoon is due to wet scavenging. Fossil fuel combustion accounts for 58% of the total BC, with peak contribution in February (78%) and lowest in September (40%). Long-range transport models indicate an influx of pollutants from the Indo-Gangetic Plain and adjacent valleys with increased biomass burning activity. Seasonal variations reflect tourism-driven emissions and local biomass burning, with the lowest concentration observed in the early morning hours. The Santa Barbara DISORT Atmospheric Radiative Transfer (SBDART) model results an estimated increase in temperature at rates of 0.83 K day⁻¹ in the region due to BC induced atmospheric heating. These results underscore the vulnerability of Himalayan high-altitude valleys to transported pollutants and highlight the importance of BC in altering radiative forcing and accelerating cryospheric melt.