An acid rain-friendly NH₃ control strategy to maximize benefits toward human health and nitrogen deposition

Ammonia (NH₃) abatement remains controversial in China owing to its effectiveness in reducing PM_2.5 pollution and nitrogen deposition but with the potential risk of promoting acid rain formation, necessitating scientific guidance. Here, we propose a novel method for designing an NH₃ control strategy to mitigate both air pollution and nitrogen deposition without significantly exacerbating acid rain. This method involves extending the response surface model (RSM) to deposition using a delicately developed polynomial response function of deposition (i.e., dep-RSM). The Yangtze River Delta (YRD) dep-RSM application reveals that 16 out of 41 cities have NH₃ control potentials from 15 % to 71 %. Excellent NH₃ control potentials have been noted between April and June (78 %-92 %). From 2013 to 2017, the effective SO₂ and NO_x control significantly reduced wet sulfur and oxidized nitrogen deposition, providing considerable NH₃ abatement potentials (15 %-24 %) to further reduce PM_2.5 and nitrogen deposition by up to 2 % and 9 %, respectively, without acid rain exacerbation (the wet neutralization factor was maintained). Additionally, 57 % and 73 % NH₃ emission reduction potentials were obtained under acid rain constraints with 75 % and 86 % reductions in the other precursors to reduce the average PM_2.5 concentration below 25 and 15 μg/m³, and an additional 8408 and 14,459 premature deaths could only be avoided at an extra cost of 8.7 and 19.7 billion CNY, respectively. Meanwhile, the N deposition considerably reduced by 10 and 13 kgN/ha·yr. However, the YRD region could still simultaneously obtain substantial amounts of PM_2.5 and N deposition mitigation using the strategy proposed herein. The expanded optimization system can be directly adopted by policymakers to implement coordinated control in regions or countries facing the same NH₃ control conundrum.