Development and testing of an ammonia bi-directional flux model for air quality models

Ammonia is an important contributor to particulate matter in the atmosphere and can significantly impact terrestrial and aquatic ecosystems. Surface exchange between the atmosphere and biosphere is a key part of the ammonia cycle. Agriculture, in particular, is a large source of ammonia emitted to the atmosphere, mostly from animal operations and fertilized crops, while dry and wet deposition are the primary sinks of atmospheric ammonia. Although, current air quality models consider all of these source and sink processes, algorithms for emissions from fertilized crops and dry deposition are too simplistic to provide accurate accounting of the net surface fluxes. New modeling techniques are being developed that replace current ammonia emission from fertilized crops and ammonia dry deposition with a bi-directional surface flux model. Nitrogen in fertilizer is converted by biochemical processes into ammonia that dissolves into soil water and is available for uptake by plants. A soil nitrification model is used to compute concentrations of ammonium (NH4⁺) and hydron (H⁺) in soil water, which determine the value of γ (NH4⁺/ H⁺). The soil γ is then used to compute ammonia gas concentration at the air-soil water interface, which is commonly called the compensation concentration. An analogous γ value for the leaf tissue is estimated according to plant type for computation of the stomatal compensation concentration. The difference in concentration between these compensation concentrations and the air concentration within the canopy define the direction and magnitude of the bidirectional fluxes to the leaves and soil. In addition, the model includes a deposition pathway (one-way) to the leaf cuticle that is a function of in-canopy ammonia concentration (to account for saturation effects) and relative humidity.

While the pathways and processes involved in bi-directional ammonia surface exchange are well represented by this model, the values of many key parameters are not well known. Therefore, a series of field experiments involving different crops have been instrumental for refining the model and many of its parameters. Comparisons between the model and field measurements of ammonia fluxes over heavily fertilized corn and lightly fertilized soybeans will be presented. We will also demonstrate and discuss sensitivities of the model to several key parameters. Successful deployment of this model in mesoscale air quality models will depend on further field campaigns in a variety of agricultural and natural environments to provide more guidance on parameterizations.