<p>Nitrate (NO₃^–) is a crucial component of atmospheric pollutants, and understanding its sources and formation mechanisms holds significant importance for air pollution control. In this study, stable isotope techniques and Bayesian Mixing Models (Mix SIAR) were applied to analyze the primary sources and formation processes of NO₃⁻ in PM_2.5 and PM₁₀ in Beijing in 2022. The results indicate that the contribution of vehicle exhaust, coal combustion, biomass burning, and soil emissions to NO₃⁻ in PM_2.5 were 33.9%, 20.5%, 29.8%, and 15.9%, respectively, while for PM₁₀, the contributions were 30.6%, 21.6%, 29.9%, and 17.9% respectively. An analysis of δ¹⁸O-NO₃⁻ values indicated that the contribution of N₂O₅ hydrolysis to NO₃⁻ in PM_2.5 and PM₁₀ over the year was 64.0% and 75.6%, respectively, highlighting its predominant role in nitrate formation. Nevertheless, the gas-phase reaction of NO₂ with ·OH radicals was notably more pronounced in summer. Compared to PM₁₀, the gas-phase reaction of NO₂ with ·OH radicals contributes more to NO₃⁻ in PM_2.5. These results offer a vital foundation for further research into the sources and formation mechanisms of atmospheric NO₃⁻ and provide scientific support for measures to prevent and control air pollution.</p>