Estimating sources, sinks and fluxes of atmospheric reactive nitrogen within a forest canopy

Tomer Duman1, John T. Walker2, Jesse O. Bash3, Cheng-Wei Huang4, Khaled Ghannam5 and Gabriel G. Katul6

Canopy-scale flux measurements and inferential models are useful for developing estimates of net emission or deposition of trace gases and aerosols above forests.  However, more detailed measurements and models are needed to relate net fluxes to biological, physical, and chemical processes occurring within the air-canopy-soil system.  These processes occur over multiple time scales making direct measurements of sources or sinks difficult to conduct. However, measured concentration profiles can be used to infer the effective source-sink distribution if the flow field is known – commonly referred to as the solution of the inverse problem.  In-canopy and above-canopy multi-level concentration measurements of reactive nitrogen compounds (ammonia, nitric acid, nitrous acid), as well as other reactive gases that influence ammonia gas-to-particle partitioning (hydrochloric acid) and surface acidity (sulfur dioxide), are presented within a deciduous second-growth 180 year old oak-hickory forest situated within the Southeastern U.S. Three different approaches are used to solve for the source-sink distribution from the measured mean scalar concentration profiles: (1) a Eulerian high-order closure model that solves the scalar flux budget equation, (2) a Lagrangian localized near-field (LNF) method and (3) a new full Lagrangian stochastic model that estimates the dispersion matrix. As each of these methods is subject to different assumptions, the combination and comparison of the three can be used to constrain the solution to the inverse problem and permit inference on the origin and fate of reactive compounds within forested canopies.

 

1Nicholas School of the Environment, Duke University
2US Environmental Protection Agency, walker.johnt@epa.gov
3US Environmental Protection Agency
4Nicholas School of the Environment, Duke University
5Nicholas School of the Environment, Duke University
6Nicholas School of the Environment, Duke University