Measurement report: High contribution of N ₂ O ₅ uptake to particulate nitrate formation in NO ₂ -limited urban areas

Abstract. Particulate nitrate (pNO3-) is a major component of fine particles in Chinese urban areas. However, the relative contributions of pNO3- formation pathways in urban areas remain poorly quantified, particularly under the NO2-limited regime that governs its formation (as defined by the NO2/O3 ratio), which hinders effective 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 PM2.5 and O3 mitigation strategies.