Unveiling the Mechanism of Surface Particulate Nitrate Formation Dominated by Nocturnal N ₂ O ₅ Hydrolysis in Urban Areas Driven by Regional Atmospheric Transport

Particulate nitrate (pNO₃⁻) has recently emerged as a major component of fine particles during pollution episodes. However, the dominant formation pathway of surface pNO₃⁻ in urban areas remains controversial, and its driving factors are not fully understood. In this study, the WRF-Chem model, incorporating multiple heterogeneous reaction processes, was employed to investigate the surface pNO₃⁻ formation pathways in Beijing during autumn. Results show that nocturnal N₂O₅ hydrolysis gradually dominated surface pNO₃⁻ (27.1%-56.0%) on polluted days, driven by two key processes. First, local N₂O₅ hydrolysis was sustained by regional atmospheric transport supplying reactants (e.g., O₃ and N₂O₅) at night (23.4%-40.6%), and second, atmospheric transport of aged pNO₃⁻ derived from N₂O₅ hydrolysis occurred via vertical mixing (0.1%-15.1%) and other transport pathways (2.1%-10.5%). Building on the commonly recognized transport pathways of aged pNO₃⁻, our study reveals that regional atmospheric transport promotes local N₂O₅ hydrolysis by supplementing reactants (e.g., O₃ and N₂O₅). It acts as the main driver for N₂O₅ nocturnal chemistry to dominate urban pNO₃⁻ formation and thus explains the high contribution of N₂O₅ hydrolysis proposed by the isotope study. Considering the important role of regionally transported precursors, coordinated regional O₃ control strategies should be adopted to mitigate pNO₃⁻-driven PM2.5 pollution in urban areas.