On surface fluxes at night – Application of the virtual chamber approach to ammonia flux measurements above a corn canopy in central Illinois

Abstract:

Quantification of nighttime emission rates of various atmospheric gases from small areas with large fluxes remains a challenge confronting agricultural science (e.g. ammonia-NH₃) and climate science (e.g. carbon dioxide-CO₂ and methane-CH₄). Standard micrometeorological methods are rarely appropriate because of their inability to relate to the surface of relevance unless the fetch is uniform and large. Alternative chamber methods result in a variety of answers depending largely on the configuration of the chambers themselves, but also on their interference with natural exposure. A statistical approach is proposed, in which micrometeorological field data are used to replicate the likely characteristics of a chamber experiment, yielding estimates of surface fluxes at the surface itself with a reduced requirement for adequate fetch. Two alternative methods are discussed in this study, corresponding to the use of both closed and vented chambers.

This new approach was evaluated with NH₃ flux measurements obtained by the flux-gradient (FG) and relaxed eddy accumulation (REA) methods in a fertilized corn field in central Illinois in 2014. The study field was treated with urea ammonium nitrate (UAN). Results showed that NH₃ data derived from the virtual chamber approach yielded averages of the same order as the FG and REA approaches.

Although further validation is desirable, the “virtual chamber” concept is an important first step as it allows us to address the surface exhalation question in conditions of stable stratification, e.g. such as over land at night and often over inland water bodies in daytime.