Optical Variation and Molecular Transformation of Brown Carbon During Oxidation by NO₃^• in the Aqueous Phase

The NO₃^•-driven nighttime aging of brown carbon (BrC) is known to greatly impact its atmospheric radiative forcing. However, the impact of oxidation by NO₃^• on the optical properties of BrC in atmospheric waters as well as the associated reaction mechanism remain unclear. In this work, we found that the optical variation of BrC proxies under environmentally relevant NO₃^• exposure depends strongly on their sources, with enhanced light absorptivity for biomass-burning BrC but bleaching for urban aerosols and humic substances. High-resolution mass spectrometry using FT-ICR MS shows that oxidation by NO₃^• leads to the formation of light-absorbing species (e.g., nitrated organics) for biomass-burning BrC while destroying electron donors (e.g., phenols) within charge transfer complexes in urban aerosols and humic substances, as evidenced by transient absorption spectroscopy and NaBH₄ reduction experiments as well. Moreover, we found that the measured rate constants between NO₃^• with real BrCs (k = (1.8 ± 0.6) × 10⁷ M_C^-1s^-1, expressed as moles of carbon) are much higher than those of individual model organic carbon (OC), suggesting the reaction with OCs may be a previously ill-quantified important sink of NO₃^• in atmospheric waters. This work provides insights into the kinetics and molecular transformation of BrC during the oxidation by NO₃^•, facilitating further evaluation of BrC's climatic effects and atmospheric NO₃^• levels.