Interfacial Electric Fields Transform Brown Carbon Formation: Accelerate Radical Coupling toward Strong Light-Absorbing Products

The formation of brown carbon (BrC) in atmospheric aerosols significantly influences air quality and climate, yet the underlying chemistry governing its interfacial genesis remains poorly understood. Here, we reveal that the intense electric field at the air-water interface of deliquescent nitrite aerosols promotes the oxidation of methoxyphenol, a key biomass burning constituent. The formation of conjugated oligomers, such as C₁₆H₁₄O₆ and C₁₅H₁₃NO₇, exhibits substantial light absorption (mass absorption coefficients ∼4 m² g^-1)─a level that surpasses many previously reported pathways. The aging rate can be increased by up to 2 orders of magnitude compared to bulk solutions due to selective radical-radical coupling polymerization. This is driven by promoted intermolecular energy transfer between direction-oriented excited methoxyphenol molecules and reduced energy barriers via stabilized dimer intermediates in the presence of a strong interfacial electric field, as evidenced by high-resolution spectroscopy and quantum calculations. Our study underscores the catalytic role of electric fields in the browning process, reshaping our understanding of atmospheric photochemical aging and highlighting an overlooked driver of BrC formation with far-reaching implications for aerosol reactivity and atmospheric processes.