A Comparative Evaluation of Box Models for Simulating Ammonium Fluxes between Atmosphere and Biosphere

Ammonia (NH3) is a major alkaline species that has several environmental impacts including modulating the nitrogen cycle, neutralization of acidic gases and particles, and contributing to atmospheric aerosol formation. Similar to other gases, NH3 can deposit onto the ground through diffusion and dispersion processes near surface. It, however, can also emit from plants and soil back to the atmosphere. The bi-directional exchange of NH3 between the atmosphere and biosphere makes its modeling more challenging than other gases. In this study, four box models are being compared to study their capabilities in reproducing observed NH3 fluxes over various surfaces such as fertilized soybean and corn fields. The four models include two single-layer models that treat NH3 dry deposition processes only in one canopy layer near the surface, one two-layer model that considers the competing processes of deposition to the leaf cuticle, bi-directional stomatal exchange, and exchange with soil but with a simplified parameterization, and one multi-layer model that also treats NH3 bi-directional exchange but considers the nonlinear processes of NH3 fluxes explicitly in multiple canopy layers.

Our preliminary results using measurements over soybean during the summer showed that the multi-layer model can generally reproduce the diurnal variation of NH3 fluxes due to deposition and emission, whereas the single-layer model showed negative fluxes (due to deposition) that are in much smaller magnitude than the observations. The intercomparison among the four models will provide insights into the strengths and weaknesses of each model along with recommendations regarding the best candidate model for incorporation into the 3-D air quality models to accurately simulate the fate of NH3 in the real atmosphere.