Reduced inorganic nitrogen (NH₃ + NH₄⁺) is an increasingly important contributor to the total nitrogen deposition budget, yet the bi-directional nature of NH₃ air-surface exchange makes incorporation of NH₃ measurements into dry deposition schemes in field-scale and regional chemical transport models (i.e. CMAQ) difficult. The purpose of this study is to develop a methodology for providing NADP with modeled NH₃ fluxes using bi-weekly AMoN concentrations. NH₃ fluxes derived from site specific NH₃ measurements (AMoN) and surface parameterizations (i.e., compensation points) will provide “best” estimates of NH₃ deposition for developing ecosystem specific deposition budgets, assessing sub-grid variability of fluxes within CMAQ, and characterizing the impact of bias correcting CMAQ NH₃ concentrations on NH₃ fluxes.  This effort will therefore improve the total nitrogen deposition estimates provided by TDEP, which does not currently use the AMoN NH₃ concentrations for deposition estimates.

The first phase of the project will focus on measurements while the 2^nd phase will focus on evaluation of the bi-directional air-surface exchange model.  During Phase I, a database of soil and vegetation chemistry, micrometeorology, and surface physical characteristics will be developed for 3 pilot sites: Chiricahua National Monument, AZ (desert); Bondville, IL (agricultural); and Duke Forest, NC (hardwood forest). These sites were selected based on available data (AMoN, CASTNET, and NADP/NTN), and differences in land-use type, vegetation and soil types, and average ambient NH₃ concentrations. Soil, live vegetation, and litter will be collected and analyzed for NH₄⁺ and pH to develop seasonal estimates of surface NH₃ emission potentials (Г) from which compensation points will be derived. Biogeochemistry will be combined with on-site meteorology and surface characteristics will be used to calculate net and component fluxes (i.e., foliage versus ground) using a 2-layer bi-directional flux model. Measurements will be used to assess model sensitivities to biogeochemical and meteorological inputs to develop a model suitable for implementation across the entire AMoN network.

Field measurements began in the summer of 2017 and will continue through spring 2018. Here we describe the field and laboratory measurement methods that have been designed to capture a robust biogeochemical dataset for the 3 AMoN pilot sites.  Aspects of model development and evaluation will also be discussed.