Anthropogenic nitrogen impacts on global PM_2.5-O₃ co-pollution health burden

The growing recognition of the adverse health impacts posed by ambient fine particulate matter (PM_2.5) and ozone (O₃) has driven global efforts to implement stricter regulations on reactive nitrogen emissions, including nitrogen oxides (NO_x) and ammonia (NH₃). Nevertheless, effective mitigation strategies remain constrained by the gap between precursor emissions and health effects, especially in regions facing compounded PM_2.5-O₃ pollution. This work models the synergistic health burden from PM_2.5-O₃ co-exposure and its complex responses to anthropogenic nitrogen emission controls. Our modeling reveals a significant positive interaction between PM_2.5 and O₃, with co-exposure mortality 24.8% higher than non-interactive additive estimates. NO_x and NH₃ emissions contribute to 20.3% and 19.4% of these deaths, respectively, with relative importance varying across regions and seasons. To mitigate joint PM_2.5-O₃ health impacts, NO_x reduction is currently more effective than NH₃ control, though sustained benefits require coordinated cuts to both species. Importantly, due to a non-linear emission-health relationship, only aggressive emission reductions can generate proportionally significant health gains, exceeding 20.2% improvements over mild control scenarios. These findings challenge conventional uniform emission control measures and provide a scientific basis for developing dynamic air quality and health management frameworks.