A transition from coal-dominated to multi-source nitrate pollution limits air quality gains in China

Since 2013, China has implemented strict air cleaning policies, yet atmospheric nitrate (NO3−) pollution remains inadequately mitigated in many regions. It is widely recognized that enhanced atmospheric oxidation promotes NO3− formation, undercutting the effectiveness of coal combustion source (CCS) controls. However, the impact of contributions from multiple non-coal combustion sources (NCCS) has consistently been overlooked. Here, we quantify the formation pathways and sources of NO3− in China over the past twelve years through long-term measurements of the dual-isotope composition (δ15N and δ18O) of NO3− combined with machine learning. Our results reveal that, despite substantial shifts in NO3− formation pathways, its formation efficiency remains largely invariant. Moreover, while CCS has reduced its contribution to NO3− by 12.3% as of 2025 in China, four NCCSs, enhanced by the energy transition and climate warming, are emitting more NOx precursors in many regions. Individually, each of these NCCSs now contributes at a level comparable to that of CCS. We estimate a nationally averaged decrease in NO3− concentration of only 7.3% in China in the future, even if CCS’s contribution is further halved relative to 2025 haze day levels. Our study underscores the urgency of implementing coordinated multi-source NOx control strategies to achieve sustained improvements in air quality.