Molecular-Specific Aromatic Compounds Absorption Drives Divergent Radiative Forcing in High-Altitude Air and Snow over the Tibetan Plateau

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 PM2.5 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.