Pinpointing and Characterizing Large Anthropogenic Emitters of Atmospheric Reactive Nitrogen in China Using Multisource Satellite Observations.

Atmospheric reactive nitrogen (Nr) emissions, including nitrogen oxides (NOx) and ammonia (NH3), exert substantial yet often underestimated impacts on air quality and public health. However, comprehensive long-term monitoring of facility-level Nr emissions across industrial sectors remains limited, hindering effective regulation. Here, we identified 1372 NO2 and 1069 NH3 facility-level emission sources in China during 2018-2022 using a satellite-based algorithm that integrates multisource data sets, including NO2 and NH3 concentrations, fire points, and high-resolution imagery. Emissions were concentrated in the Yangtze River Delta and North China Plain, with 76% of NO2 and NH3 point sources co-located at the same facilities. NH3 emissions were more prevalent in western China, while NO2 sources dominated in the northeast. Among sectors, steel facilities impacted the most people yet emitted the least Nr pollution per unit energy, showing the highest emission-control efficiency, whereas cement and chemical plants released far more per unit energy and, thus, had much lower efficiency. Facility-level changes, including capacity expansion, shutdowns, and adoption of selective catalytic reduction technology, have shaped emission trends, with notable cases, such as ammonia slip at Hebei steel plants. Broader patterns reflected the influence of policies, COVID-19, technological maturity, production shifts, and atmospheric processes. Provincial disparities were mainly driven by industrial structures, economic activity, policy enforcement, and environmental efficiency. This study advances the understanding of facility-level Nr emissions in China, offering critical insights for emission management and supporting targeted strategies for air quality improvement.