Optimal reactive nitrogen control pathways identified for cost-effective PM2.5 mitigation in Europe.

Excess reactive nitrogen (Nr), including nitrogen oxides (NOx) and ammonia (NH3), contributes strongly to fine particulate matter (PM2.5) air pollution in Europe, posing challenges to public health. Designing cost-effective Nr control roadmaps for PM2.5 mitigation requires considering both mitigation efficiencies and implementation costs. Here we identify optimal Nr control pathways for Europe by integrating emission estimations, air quality modeling, exposure-mortality modeling, Nr control experiments and cost data. We find that phasing out Nr emissions would reduce PM2.5 by 2.3 ± 1.2 μg·m-3 in Europe, helping many locations achieve the World Health Organization (WHO) guidelines and reducing PM2.5-related premature deaths by almost 100 thousand in 2015. Low-ambition NH3 controls have similar PM2.5 mitigation efficiencies as NOx in Eastern Europe, but are less effective in Western Europe until reductions exceed 40%. The efficiency for NH3 controls increases at high-ambition reductions while NOx slightly decreases. When costs are considered, strategies for both regions uniformly shift in favor of NH3 controls, as NH3 controls up to 50% remain 5-11 times more cost-effective than NOx per unit PM2.5 reduction, emphasizing the priority of NH3 control policies for Europe.