A Missing Source of Atmospheric NO₂^-: Heterogeneous Dark Transformation of PAN on Industrial Mineral Dust

Understanding the heterogeneous uptake of peroxyacetyl nitrate (PAN) on mineral particles is crucial because this process significantly alters PAN's atmospheric lifetime and chemical state, thereby affecting its transport and transformation. Previous studies have primarily focused on uptake coefficients, while the underlying mechanism governing PAN heterogeneous transformation and its impact on the N cycle remain unexplored. Here, we report the overlooked particulate NO₂^- formation from the heterogeneous uptake of PAN on transition mineral dust (Fe₂O₃, CuO, MnO₂), especially significant for typical Fe₂O₃ particulates. Through the combination of online proton transfer reaction time-of-fight mass spectrometer (PTR-TOFMS), in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) spectroscopy, and density functional theory (DFT) calculations, we reveal that the electronic "donation-backdonation" covalent interaction between Fe 3d orbitals and the lone pair electrons of C═O groups in PAN facilitates Fe-PAN complex formation, resulting in an induced effect within the PAN molecule. Electrons shift from the nitro group toward the C═O group, weakening the O-NO₂ bond, thus leading to the formation of atmospheric NO₂^-. Our results demonstrate an unrecognized NO₂^- source driven by the interfacial conversion of PAN on transition metal dust, highlighting the crucial role of heterogeneous uptake in dark nitrogen chemistry in air.