Brown carbon is a significant contributor to aerosol climate forcing in the northwestern Himalayas.

We report here the climate-relevant optical properties and sources of water and organic-extractable brown carbon (BrCaq and BrCme) and its relative importance as a climate forcing agent in the northwestern (NW) Himalayan midlands (1044 m amsl). Diagnostic ratios coupled with trajectory and firespot-based analysis suggested that while biomass burning (BB) impacts are consistent in this region, a very significant influence is also exerted by the fossil fuel (FF)-dominated Indo-Gangetic Plain (IGP) outflow. BrC absorption peaked in winter, with ∼3-5 times enrichment in its absorption coefficient at 365 nm (babs_aq_365) over summer and post-monsoon and 5-15 times enrichment in the 400-500 nm absorption tail. Winter BrC exhibited much greater absorptivity (MAE365_aq: 1.85 ± 0.44 m2 g-1 and kaq: 0.08 ± 0.02) than summer and post-monsoon BrC (MAE365_aq: 0.78 ± 0.32-0.96 ± 0.43 m2 g-1; kaq: 0.03 ± 0.01-0.04 ± 0.02), suggesting compositional shifts potentially associated with seasonalities in source profiles and atmospheric processing. BrC MAE, especially in winter, was higher than that at other high-altitude Himalayan locations and was even comparable to that in source regions in the IGP. The relative radiative forcing (RRF) of BrCme was 15-30% of that of elemental carbon (EC) in the 300-2500 nm range and 62-121% in the 300-400 nm range, establishing clearly that BrC is as significant a climate forcer as EC in the low-UV range in the NW Himalayas. Optical source apportionment revealed contributions of 69% from FF and 31% from BB to BrC babs_365, with the FF source exhibiting MAE 3.7 times that of BB (2.6 m2 g-1vs. 0.7 m2 g-1). These findings underscore BrC as a major climate forcer in the NW Himalayas and call for its explicit consideration in regional climate assessments.