Effectiveness of ammonia reduction on control of fine particle nitrate

In some regions, reducing aerosol ammonium nitrate (NH 4 NO 3 ) 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 (HNO 3 ) that partitions to the aerosol, or reductions in ammonia (NH 3 ) to lower particle pH and keep HNO 3 in the gas phase. Using the ISORROPIA-II thermodynamic aerosol model and detailed observational datasets, we explore the sensitivity of aerosol NH 4 NO 3 to gas phase NH 3 and NO x controls for a number of contrasting locations, including Europe, the US, and China. NO x control is always effective, whereas the aerosol response to NH 3 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 NH 3 control. We find that regardless of the locations examined, it is only when ambient particle pH drops below approximately 3 that NH 3 reduction leads to an effective response in PM 2.5 mass. The required amount of NH 3 reduction to efficiently decrease NH 4 NO 3 at different sites is assessed. Owing to the linkage between NH 3 emissions and agricultural productivity, substantial NH 3 reduction required in some locations may not be feasible. Finally, controlling NH 3 emissions to increase aerosol acidity and evaporate NH 4 NO 3 will have other effects, beyond reduction of PM 2.5 NH 4 NO 3 , such as increasing aerosol toxicity and changing the deposition patterns of nitrogen and trace nutrients.