An official website of the United States government.

This is not the current EPA website. To navigate to the current EPA website, please go to This website is historical material reflecting the EPA website as it existed on January 19, 2021. This website is no longer updated and links to external websites and some internal pages may not work. More information »

Research Fellowships

Combining Measurements and Modeling to Better Understand Ammonia Air-Surface Exchange Processes

EPA Office of Research and Development

EPA National Science Foundation Graduate Research Internship Program (GRIP) & Graduate Research Fellowship Program (GRFP) Opportunities


Opportunity Title:

Combining Measurements and Modeling to Better Understand Ammonia Air-Surface Exchange Processes

Research Area:


EPA Lab/Center/Office:

National Risk Management Research Laboratory (NRMRL)


Durham, NC


12 months

Brief Summary:

The objective of this project is to characterize the sensitivity of EPA’s bi-directional ammonia flux model to inputs of meteorology, air concentrations and biogeochemistry, focusing on the role of soil and vegetation compensation points.

Opportunity Description:

Ammonia air-surface exchange is an important yet poorly characterized part of the reactive nitrogen cascade. Accurate estimates of ammonia deposition are needed to develop complete budgets of atmospheric nitrogen deposition. These deposition budgets are, in turn, needed to determine when and where sensitive ecosystems are receiving nitrogen deposition in excess of their “critical load”. EPA and other federal and state natural resource managers use this information to assess the adequacy of current regulations to protect ecosystem health.  Over the past decade, EPA scientists and their collaborators have greatly improved our understanding of the role of ammonia in nitrogen deposition and subsequently have adopted a bi-directional framework for modeling ammonia air-surface exchange. While this approach is mechanistically much more representative of the exchange processes than previous modeling approaches, certain aspects of the model remain uncertain due to a lack of data to improve parameterizations, particularly for the soil and leaf compensation points responsible for the “bi-directional” nature of ammonia fluxes over natural surfaces. Recent availability of ammonia measurements across the U.S. as part of the National Atmospheric Deposition Program (NADP) Ammonia Monitoring Network (AMoN) as well as new measurements of relevant soil and vegetation chemistry in North American ecosystems offer opportunities for further model development and testing.

Ammonia concentrations are measured at more than 100 AMoN sites across the U.S. EPA is adapting a bi-directional flux model that will combine bi-weekly ammonia concentrations from AMoN with modeled ammonia compensation points to produce ammonia flux estimates at each site, collocated with NADP wet deposition measurements. The modeling component of the project will be two fold, including an assessment of the sensitivity of the model to inputs of air concentrations, meteorology and soil/vegetation chemistry across a range of AMoN site conditions, and a literature and online data search to expand current datasets of biogeochemical model inputs. Additionally, the candidate will assist with measurements of soil and leaf chemistry as part of a field study at Duke Forest near the EPA Research Triangle Park Laboratory in North Carolina. These measurements will be incorporated into the model evaluation. Ultimately, the model sensitivity analysis will be used to identify the most important model uncertainties and prioritize future field studies to improve model parameterizations.

Opportunities for Professional Development:

The research project is unique in that it will include both measurements and modeling, thereby offering a mix of opportunities for professional development. The candidate will interact with a broad group of scientists including atmospheric chemists, ecologists and meteorologists. This research effort directly supports the review of the secondary National Ambient Air Quality Standards (NAAQS) for oxides of nitrogen and sulfur and, as such, provides an opportunity to learn about the science/policy interface between EPA’s Office of Research and Development and the Program Offices that develop and implement the NAAQS.

Point of Contact or Mentor:

John Walker (

For more information about EPA Research Fellowship opportunities, visit: