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Nitrate (NO 3 - ) is a key component of secondary inorganic aerosols and PM 2.5 . However, the contributions of nitrogen oxides (NO x ) emission sources to NO 3 - in PM 2.5 remain poorly constrained. This study measured nitrogen (N) isotopes of NO 3 - collected at Beijing in 2014. We observed that δ 15 N-NO 3 - ) in PM 2.5 collected at Beijing in 2014. We observed that δ 15 N-NO 3 - values in PM 2.5 (-2.3‰ - 19.7‰; 7.3 ± 5.4‰ annually) were significantly higher in winter (11.9 ± 4.4‰) than in summer (2.2 ± 2.5‰). The δ 15 N differences between source NO x and NO 3 - in PM 2.5 (hereafter as Δ values) were estimated by a computation module as 7.8 ± 2.2‰ - 10.4 ± 1.6‰ (8.8 ± 2.4‰). Using the Δ values and δ 15 N values of NO x of Beijing were mainly caused by those of NO x sources to NO 3 - in PM 2.5 were further estimated by the SIAR model. We found that seasonal variations of δ 15 N-NO 3 - values in PM 2.5 of Beijing were mainly caused by those of NO x contributions from coal combustion (38 ± 10% in winter, 20 ± 9% in summer). Annually, NO x from coal combustion, vehicle exhausts, biomass burning, and microbial N cycle contributed 28 ± 12%, 29 ± 17%, 27 ± 15%, and 16 ± 7% to NO 3 - in PM 2.5 , respectively, showing actually comparable contributions between non-fossil NO x (43 ± 16%) and fossil NO x (57 ± 21%). These results are useful for planning the reduction of NO x emissions in city environments and for elucidating relationships between regional NO x emissions and atmospheric NO 3 - pollution or deposition., (Copyright © 2019 Elsevier Ltd. All rights reserved.)