AAAR 36th Annual Conference October 16 - October 20, 2017 Raleigh Convention Center Raleigh, North Carolina, USA
Abstract View
On the Sensitivity of Fine Particle Nitrate to Ambient Ammonia Concentrations Assessed through a Detailed pH Analyses
HONGYU GUO, Athanasios Nenes, Rodney J. Weber, Georgia Institute of Technology
Abstract Number: 448 Working Group: Regional and Global Air Quality and Climate Modeling
Abstract Ammonium nitrate is a large fraction of ambient fine mode inorganic particles in some regions of US, Europe, and China. Ammonia (NH3) in the gas phase, mostly produced from agricultural activities and predicted to increase steadily in the future, is recognized as a key factor to control fine particle ammonium nitrate concentrations. NOx emissions also play an important role. Since fine particle mass is linked to public health, the argument is that agricultural emissions, and hence food productivity, may be relevant to improve air quality in many regions. In this research, we present a sensitivity study of ammonium nitrate levels to gas phase NH3 concentrations using a thermodynamic model, ISORROPIA-II. This model can accurately predict the effect of pH, particle water, and ambient temperature on gas-particle partitioning of semivolatile species, such as NH3-NH4+ and HNO3-NO3-. A consistent particle pH response to NH3 was found regardless of aerosol composition and meteorological conditions in which an order of magnitude increase in gas phase NH3 produces an increase in pH by about one unit. Combining with the “S curve” relationship between pH and the particle nitrate fraction of total nitrate (nitric acid plus nitrate), ε(NO3-), shows that reducing NH3 leads to a non-linear response in NO3- reduction, with the greatest efficiency when ε(NO3-) is approximate 50%. This condition depends on location. Besides NH3, we also show the thermodynamic response of particle mass and composition to NOx and SO2, which are important secondary aerosol precursors, based on observational data from the US, Europe, and China. The effectiveness of reducing these gas phase precursors on ammonium nitrate are compared for these various contrasting regions.