Abstract: Beijing’s PM2.5 pollution control has reached an implementation bottleneck, demanding mechanistic understanding of nitrate (NO3?) as the predominant secondary pollutant whose diurnal formation discrepancy and its dominated factors affect refined management. Utilizing an ENVINT-DEN08 diffusion tube-filter membrane system, we conducted diurnal observations of PM2.5 and precursor gases (NH3, HNO3,) in July 2024, quantifying gas-particle partitioning (RG/P) and oxidation efficiency (ROxi) to elucidate diurnal formation of nitrate dynamics. Key findings reveal that nocturnal nitrate concentrations (6.2±6.8 μg/m3) doubled daytime levels (3.2±2.1 μg/m3), contrasting with gaseous HNO3 showing 2.4-fold higher daytime concentrations (4.5±2.3 vs. 1.9±0.7 μg/m3). Gas-to-particle conversion (CG/P=0.56) exhibited a higher contribution to nitrate formation compared to oxidation processes (COxi=0.44), and was identified as the primary driver of diurnal variations in particulate NO3? concentrations. The oxidation efficiency during daytime (ROxi=0.25) was significantly higher than that observed at nighttime. NO2 demonstrated no statistically significant impact on oxidation products (p>0.05).Gas-to-particle conversion (RG/P=0.42) was predominantly regulated by temperature, humidity, and aqueous liquid water content (ALWC) (p≤0.001). NH4NO3 displayed a pronounced dissociation tendency, with 95% P_NH_3\ast P_HNO_3/Kp<1 samples. Metal ions (Na++Ca2?+Mg2?+K?) exhibited significant involvement in NO3? formation (p≤0.01). The gas-particle conversion efficiency at night (RG/P=0.61) was higher than that during the day and was regulated by temperature and humidity, ALWC and pH (p≤0.01). The oxidation efficiency at night (ROxi =0.19) decreased; Sensitivity analysis revealed daytime NO3? exhibited heightened sensitivity to variations in total ammonia (TNH3= NH3+ NH4+) and metal ions. Under 90% control scenarios, NO3? concentrations decreased by 1.5 μg/m3 and 0.6 μg/m3 for TNH3 and metal ions, respectively. At night, NO3? was mainly affected by TNH3. When the control ratio of TNH3 was 90%, NO3? decreased by 5.1μg/m3.This study establishes a diurnal control framework to guide Beijing’s refined PM2.5 governance.