Effectiveness of ammonia reduction on control of fine particle nitrate

In some regions, reducing aerosol ammonium nitrate (NH₄NO₃) concentrations may substantially improve air quality. This can be accomplished by reductions in precursor emissions, such as nitrogen oxides (NO_x) to lower nitric acid (HNO₃) that partitions to the aerosol, or reductions in ammonia (NH₃) to lower particle pH and keep HNO₃ in the gas phase. Using the ISORROPIA-II thermodynamic aerosol model and detailed observational data sets, we explore the sensitivity of aerosol NH₄NO₃ to gas-phase NH₃ and NO_x controls for a number of contrasting locations, including Europe, the United States, and China. NO_x control is always effective, whereas the aerosol response to NH₃ control is highly nonlinear and only becomes effective at a thermodynamic sweet spot. The analysis provides a conceptual framework and fundamental evaluation on the relative value of NO_x versus NH₃ control and demonstrates the relevance of pH as an air quality parameter. We find that, regardless of the locations examined, it is only when ambient particle pH drops below an approximate critical value of 3 (slightly higher in warm and slightly lower in cold seasons) that NH₃ reduction leads to an effective response in PM_2.5 mass. The required amount of NH₃ reduction to reach the critical pH and efficiently decrease NH₄NO₃ at different sites is assessed. Owing to the linkage between NH₃ emissions and agricultural productivity, the substantial NH₃ reduction required in some locations may not be feasible. Finally, controlling NH₃ emissions to increase aerosol acidity and evaporate NH₄NO₃ will have other effects, beyond reduction of PM_2.5 NH₄NO₃, such as increasing aerosol toxicity and potentially altering the deposition patterns of nitrogen and trace nutrients.