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Measurement and modeling of the contribution of ammonia to total nitrogen deposition from canopy to regional scale
Citation:
Walker, Johnt. Measurement and modeling of the contribution of ammonia to total nitrogen deposition from canopy to regional scale. Presented at American Meteorological Society, phoenix, AZ, January 04 - 08, 2015.
Impact/Purpose:
This invited talk provides an overview of the state of the science with respect to ammonia air-surface exchange research in the context of total nitrogen deposition.
Description:
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.