First Assessment of NO_x Sources at a Regional Background Site in North China Using Isotopic Analysis Linked with Modeling

Nitrogen oxides (NO_x, including NO and NO₂) play an important role in the formation of atmospheric particles. Thus, NO_x emission reduction is critical for improving air quality, especially in severely air-polluted regions (e.g., North China). In this study, the source of NO_x was investigated by the isotopic composition (δ¹⁵N) of particulate nitrate (p-NO₃^-) at Beihuangcheng Island (BH), a regional background site in North China. It was found that the δ¹⁵N-NO₃^- (n = 120) values varied between -1.7‰ and +24.0‰ and the δ¹⁸O-NO₃^- values ranged from 49.4‰ to 103.9‰. On the basis of the Bayesian mixing model, 27.78 ± 8.89%, 36.53 ± 6.66%, 22.01 ± 6.92%, and 13.68 ± 3.16% of annual NO_x could be attributed to biomass burning, coal combustion, mobile sources, and biogenic soil emissions, respectively. Seasonally, the four sources were similar in spring and fall. Biogenic soil emissions were augmented in summer in association with the hot and rainy weather. Coal combustion increased significantly in winter with other sources showing an obvious decline. This study confirmed that isotope-modeling by δ¹⁵N-NO₃^- is a promising tool for partitioning NO_x sources and provides guidance to policymakers with regard to options for NO_x reduction in North China.