Measurement and modeling of the contribution of ammonia to total nitrogen deposition from canopy to regional scale

Measurement and modeling of the contribution of ammonia to total nitrogen deposition from canopy to regional scale

John T. Walker¹, Donna Schwede², Gary Lear³, Jeff Collett⁴, Bret Schichtel⁵, Jesse Bash², Robin Dennis², Tom Butler⁶

In North America, ammonia (NH₃) is increasingly being recognized not only for its role in atmospheric aerosol formation but also as an important component of atmospheric nitrogen deposition.  This has been driven by the evolution of policies to protect ecosystems from nitrogen over-enrichment, an expansion of research underpinning these policy efforts, and technological advances in measurement and modeling tools applied to these research needs. 

Ammonia measurements from satellites, nitrogen focused field campaigns, and the National Atmospheric Deposition Program's Ammonia Monitoring Network (AMoN) have advanced understanding of the processes controlling NH₃ air-surface exchange and the spatio-temporal behavior of NH₃ in the atmosphere.  These datasets have subsequently lead to improvements in NH₃ air-surface exchange models and therefore more accurate estimates of NH₃ deposition.

From a process standpoint, NH₃ differs from other nitrogen compounds such as nitric acid in that NH₃ is exchanged bi-directionally between the surface and atmosphere as regulated by a “compensation point”.  Because natural surfaces may be sources or sinks of atmospheric NH₃, and may alternate between emission and deposition on a timescale as short as hours, the deposition velocity concept does not accurately describe NH₃ air surface exchange.  Instead, a more mechanistic treatment of the nitrogen status and acidity of the surface must be employed, typically as a bi-directional framework that partitions the biosphere into atmosphere, vegetation and ground components.   Versions of this modeling framework have been successfully applied in canopy to regional scale modeling assessments of NH₃ emission and deposition and to assess the importance of NH₃ within the overall N deposition budget. 

This talk will present recent process-level NH₃ flux measurements and review advances in canopy- to regional-scale modeling of NH₃ air-surface exchange.   Examples of nitrogen deposition budgets, developed from measurements and modeling, that illustrate the contribution of NH₃ to total N deposition across a range of ecosystem and atmospheric chemical environments will also be presented.

 ¹U.S. EPA, Office of Research and Development, National Risk Management Research Laboratory, Durham, NC 27711.

²U.S. EPA, Office of Research and Development, National Exposure Research Laboratory, Durham, NC 27711.

³U.S. EPA, Office of Air and Radiation, Clean Air Markets Division, Washington, DC 20460.

⁴Colorado State University, Department of Atmospheric Science, Fort Collins, CO 80523.

⁵National Park Service-Air Resource Division,  Lakewood, CO 80523.

⁶Cooperative Institute for Research in the Atmosphere, Colorado State University, Fort Collins, CO 80523

⁷Cary Institute of Ecosystem Studies, Millbrook, NY 12545.

* Corresponding author, Email: walker.johnt@epa.gov, Telephone: (919) 541-2288