The dual role of atmospheric HULIS in inducing climate forcing and oxidative stress in the northwestern Himalayas.

We report here dual impacts of atmospheric humic-like substances (HULIS) on climate forcing and oxidative potential in the northwestern Himalayas. Neutral HULIS (H-n) dominantly constrained the seasonal water-soluble organic carbon (WSOC) mass (53-63%) as well as bulk brown carbon (BrC) absorption (37-53%). The combined HULIS relative radiative forcing (RRF) at 300-400 nm ranged from 15±1% to 27±8% seasonally, and contributed 53-75% to BrC RRF. HULIS also accounted for more than half of the observed bulk volume-normalized oxidative potential (OPv), with greater contribution from H-n (31-37%) than that from H-a (14-18%). The intrinsic efficiency of reactive oxygen species (ROS) production (in terms of mass-normalized oxidative potential; OPm) was also higher for H-n than H-a (1.5-3.2 times), and a clear trend of increased OPm was observed in the photochemistry-dominated summer as compared to the primary emission-dominated winter. Three-dimensional fluorescence showed that H-n and H-a corresponded, respectively, to condensation-type HULIS associated with highly conjugated aromatic structures and degradation-type HULIS generated via photochemical and atmospheric processes. Clustered principal component analysis indicated that OPv of H-n was consistent with contributions from primary emissions and the degree of humification whereas that of H-a aligned more closely with secondary aerosol formation. These findings establish HULIS as a critical, yet overlooked, driver of both regional warming and oxidative stress in Himalayan environments.