Nitrate (NO) 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 NO in PM and PM in Beijing in 2022. The results indicate that the contribution of vehicle exhaust, coal combustion, biomass burning, and soil emissions to NO in PM were 33.9%, 20.5%, 29.8%, and 15.9%, respectively, while for PM, the contributions were 30.6%, 21.6%, 29.9%, and 17.9% respectively. An analysis of δO-NO values indicated that the contribution of NO hydrolysis to NO in PM and PM over the year was 64.0% and 75.6%, respectively, highlighting its predominant role in nitrate formation. Nevertheless, the gas-phase reaction of NO with ·OH radicals was notably more pronounced in summer. Compared to PM, the gas-phase reaction of NO with ·OH radicals contributes more to NO in PM. These results offer a vital foundation for further research into the sources and formation mechanisms of atmospheric NO and provide scientific support for measures to prevent and control air pollution.