Brown carbon (BrC) significantly influences climate by absorbing solar radiation, but the critical lack of molecular-level evidence from real-world vehicle emissions hinders accurate assessment of its radiative forcing in climate models. In this study, PM2.5 samples were collected in three urban tunnels in Northwest China and analyzed using ultrahigh-resolution mass spectrometry and UV-vis spectroscopy to elucidate the molecular composition, optical properties, and simple forcing efficiency (SFE) of vehicle-emitted water-soluble BrC. The results reveal that nitrogen-containing organics dominated vehicular BrC molecular formulas (33.5-43.9%), with unique molecular fingerprints distinct from other sources. The light absorption coefficient at 365 nm (b(365)) ranged from 3.07 to 5.86 Mm(-1), optical-based classification of the vehicular water-soluble BrC in the weak and moderate absorption category. The estimated SFE (1.17 to 2.37 W/g) indicates a non-negligible warming effect. Diurnal and spatial gradients in molecular unsaturation and optical properties highlighted divergent oxidative atmospheric processing by center dot OH and NO3 center dot radicals. Daytime BrC showed elevated aromaticity and nonmonotonic light absorption evolution, whereas nighttime oxidation drove persistent absorption enhancement. This study delivers field molecular evidence for vehicular water-soluble BrC and its in-tunnel chemical evolution, advancing the constraint of urban aerosol climatic impacts and supporting refined climate mitigation and air quality regulation.

来源平台：ENVIRONMENTAL SCIENCE & TECHNOLOGY