Numerical Techniques for Correction of Biases In Greenhouse Gas Fluxes Determined Using Non-Steady State Chamber Methods.

Limitations of non-steady state (NSS) chamber methods for determining soil-to-atmosphere trace gas exchange rates have been recognized for several decades.  Of these limitations, the so-called “chamber effect” is one of the most challenging to overcome.  The chamber effect can be defined as the inherent tendency for NSS gas flux chamber methods to produce a biased estimate of the actual pre-deployment flux (PDF), where the PDF is defined as the flux occurring immediately prior to placement of the chamber on the soil surface.  This effect can be particularly important for greenhouse gases like nitrous oxide (N₂O) which often require more prolonged chamber deployment periods in order to facilitate analytical measurement.  Despite widespread recognition of this limitation, there is little consensus regarding practical approaches to either estimating the magnitude or reducing the significance of this effect.  Gas transport theory predicts that variations in both chamber methods and soil physical properties will affect not only the absolute magnitude of flux estimates, but also the relative difference in fluxes among soils that differ with regard to physical properties.  Since NSS chambers generally tend to underestimate the actual PDF, this raises the likelihood that current emissions assessments at the regional, national, and global scale are negatively biased.  This has important implications for effective management and mitigation of GHG emissions.  This presentation will discuss currently available options for quantifying and minimizing flux estimation errors resulting from the chamber effect including simulation studies using two-dimensional gas transport theory.