Effectiveness of ammonia reduction on control of fine particle nitrate

• Published in: Atmospheric chemistry and physics Vol. 18; no. 16; pp. 12241 - 12256
• Main Authors: Guo, Hongyu, Otjes, Rene, Schlag, Patrick, Kiendler-Scharr, Astrid, Nenes, Athanasios, Weber, Rodney J
• Format: Journal Article
• Language: English
• Published: Katlenburg-Lindau Copernicus GmbH 24-08-2018; Copernicus Publications
• Subjects: Acidity; Aerosol acidity; Aerosols; Agricultural production; Air pollution control; Air quality; Ammonia; Ammonium; Ammonium compounds; Ammonium nitrate; Analysis; Chemical properties; Cold season; Control; Emissions; Environmental aspects; Evaluation; Frameworks; Locations (working); Methods; Mineral nutrients; Nitric acid; Nitric acids; Nitrogen compounds; Nitrogen oxides; Nonlinear control; Nutrients; Oxides; Particulate matter; Particulate pollutants; pH effects; Photochemicals; Reduction; Toxicity
• ISSN: 1680-7324; 1680-7316; 1680-7324
• DOI: 10.5194/acp-18-12241-2018

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 (NOx) 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 NOx controls for a number of contrasting
locations, including Europe, the United States, and China. NOx 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 NOx 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 PM2.5 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
PM2.5 NH4NO3, such as increasing aerosol toxicity and
potentially altering the deposition patterns of nitrogen and trace nutrients.