Measurement report: High contribution of N₂O₅ uptake to particulate nitrate formation in NO₂-limited urban areas

Abstract. Particulate nitrate (pNO₃^-) is a major component of fine particle in Chinese urban areas. However, the relative contributions of pNO₃^- formation pathways in NO₂-limited urban areas remain poorly quantified, hindering further particulate pollution control. In this study, comprehensive winter field observations were conducted in urban Xiamen, Southeast China. We observed significantly elevated nighttime pNO₃^- levels concurrent with increased N₂O₅ concentrations. Quantification using an observation-constrained model revealed that N₂O₅ uptake contributed 51.2 % to total pNO₃^- formation, which was comparable to that of the OH + NO₂ reaction. The N₂O₅ uptake was found to be mainly driven by nocturnal NO₃ oxidation capacity (modulated by NO₂ and O₃ levels) rather than by heterogeneous reaction conditions. Sensitivity simulations further demonstrated that pNO₃^- formation rate was more sensitive to NOx variations than to VOCs variations. Implementing NOx control measures at nighttime was shown to effectively reduce pNO₃^- by abating N₂O₅ uptake while simultaneously preventing daytime O₃ increase. Our findings enhance the understanding of pNO₃^- formation in NO₂-limited urban areas and provide valuable insights for developing joint PM_2.5 and O₃ mitigation strategies.