Particle pH: A Critical Air Quality Parameter (Invited Presentation)

In some regions, such as the western US in Winter, reducing aerosol ammonium nitrate concentrations may lead to substantial reductions in overall PM2.5 mass. This can be accomplished by: 1) Reductions in precursor emissions that lead to lower nitric acid that partitions to the aerosol. 2) Reductions in ammonia to lower particle pH and shift both nitrate and ammonium to gas phase nitric acid and ammonia. 3) Or reductions in both, or neither, may be optimal under certain conditions. The equilibrium of ammonia and nitric acid between the gas and particle phases depends on particle pH, which is currently most effectively predicted with a thermodynamic aerosol model constrained by observations of both gas and aerosol species. Based on a thermodynamic analysis, a critical pH of nominally 2 to 3 (there is also a liquid water dependence) has been estimated as a delineation between the most effective control approach. Ammonium nitrate partitioning is sensitive to pH, and hence to ammonia control, at fine particle pH below this threshold, and is not sensitive at higher pH values. If ammonium nitrate is a substantial fraction of the aerosol, reduction in emissions leading to nitric acid formation will be an effective strategy, whereas as a strategy designed on lowering particle pH (ammonia control) is more complex. It may reduce deleterious fine particle effects related to aerosol optics (i.e., reduce haze) and lower PM2.5 mass to achieve compliance with regulations, but lower pH to shift ammonium nitrate to the gas phase can result in other undesirable effects. Nitrogen deposition becomes more localized to the sources. Aerosol overall toxicity may increase with lower pH, for example, ammonium nitrate that is reduced is likely much less toxic than transition metal ions that are increased, such as ubiquitous iron that is converted to a highly toxic soluble form when aerosol pH is nominally lower than 3.