Molecular-Specific Aromatic Compounds Absorption Drives Divergent Radiative Forcing in High-Altitude Air and Snow...

Aromatic compounds (ACs) are key brown carbon constituents, yet their radiative forcing (RF) in high-elevation regions remains poorly constrained. Here, we report year-round characterization of ACs (nitrated phenols (NPs), polycyclic aromatic hydrocarbons (PAHs), and nitrated PAHs (NPAHs)) in PM_2.5 at Mt. Gongga, Tibetan Plateau (TP). AC concentrations were higher in the dry season (1.38 vs 0.94 ng m^-3), but mass absorption coefficients at 365 nm (MAC₃₆₅) peaked in the wet season from higher aerosol liquid water content and elevated pH. Clear-sky direct RF (DRF) was positive in the wet season (5.7 × 10^-4 W m^-2) driven by NPs (single scattering albedo (SSA) = 0.81; imaginary part (k) = 0.055 at 365 nm) and stronger in the dry season (6.8 × 10^-3 W m^-2) due to higher AC concentrations and solar radiation. Under all-sky conditions, multiple scattering further amplifies DRF, highlighting the cloud modulation of BrC climatic effects. Dry-season biomass-burning transport from South and Central Asia enhanced AC deposition, elevating median snow RF ∼19-fold (4.6 × 10^-4 vs 8.6 × 10^-3 W m^-2) through snow darkening and aging. Overall, trace ACs over the TP exert dual radiative impacts through atmospheric heating and cryospheric forcing via long-range transport and deposition.