Characterizing Nitrogen Stable Isotope Composition of Typical Industry Emitted NO _<i>x</i> and Implications for the Sources of Atmospheric NO _<i>x</i> in a Polluted City in Northern China

Abstract The nitrogen stable isotope composition ( δ 15 N) of atmospheric nitrate and nitrogen oxides (NO x ) is widely used to quantify their sources. However, the δ 15 N signatures from some important anthropogenic NO x sources, for example, the coke and cement industries, are unknown, and reports on the signatures of δ 15 N(NO x ) from coal‐fired power plants and other fossil fuel combustion related industrial processes are limited. Here, we report a comprehensive investigation of δ 15 N(NO x ) from various emission sources in Taiyuan, a megacity in Northern China where coal dominated the energy consumption before 2018. The different sources were investigated in 2023–2024, and the δ 15 N(NO x ) signatures for coal‐fired and gas‐fired power plants are (20.0 ± 2.3) ‰ and (−19.8 ± 0.8) ‰ (mean ± 1 σ ), respectively. While for residential gas furnaces, municipal solid waste incineration plants, coke industry, and cement industry, the δ 15 N(NO x ) signatures were measured as (−18.5 ± 0.8) ‰, (11.7 ± 2.4) ‰, (−10.9 ± 1.9) ‰, and (8.1 ± 2.3) ‰, respectively. Among them, the coke and cement industries have not been quantified before. In addition to these emission sources, we further collected ambient NO x and found in Taiyuan the ambient δ 15 N(NO x ) values were significantly lower in winter ((−14.1 ± 4.2) ‰) than those in summer ((−4.2 ± 1.7) ‰). This is because in winter Taiyuan natural gas is the predominant NO x emission source. That the winter δ 15 N(NO x ) was lower than that in summer appears to be opposite to previously reported seasonality of δ 15 N(NO 3 − ) values in Northern China which was suggested to be caused by seasonal δ 15 N(NO x ) differences, and this is likely because after the implementation of the coal‐to‐gas project and clean air policy in 2018, natural gas combustion became the dominant NO x emission source in winter which is the heating season with extensive demand of fuel consumption.