Science behind the eosAC

Introduction
Understanding the natural and anthropogenic flux of greenhouse gases from the soil to the atmosphere is critical for applications ranging from climate change research to environmental remediation. The illustration below briefly shows the dominant pathways by which each of the major greenhouse gases (Carbon Dioxide, Methane and Nitrous Oxide) are naturally produced (or consumed) in the soil and exchanged with the atmosphere.

AC_GHG_Cycles

The eosAC is an automated closed dynamic flux chamber designed to integrate with state of the art greenhouse gas analyzers to measure these gas fluxes with continuity, accuracy and precision.

In this blog post, I am going to take you through a few of the features we considered when designing the eosAC to maximize the chamber’s performance.

Minimize Artifacts During Long-Term Measurements
The eosAC is designed to minimize the impact of the chamber on the natural soil environment by automatically lifting off of the soil collar between measurements. This allows the measurement area to receive similar rain, sun and litter fall amounts as would be expected in the absence of a chamber, thereby maintaining near-natural conditions. We also minimized the required above-ground soil collar height to lessen the effect of the chamber on the air turbulence near the ground surface which can have significant effects on the emission of gases from the soil.

eosAC-in-the-field-puerto-rico

The eosAC lift-mechanism was designed to reduce the chamber’s operational footprint in the field, making it less likely that fallen branches or creeping vines will impede the chamber and prevent it from operating normally.

Chamber Venting
Chamber venting is critical to maintaining the chamber headspace at near-ambient atmospheric pressure conditions during measurement. Positive or negative pressures in the chamber caused by gas buildup, wind and rapid temperature changes can significantly bias flux measurements. The eosAC has an integrated vent tube that allows the chamber to maintain pressure equilibrium but is protected from wind and water. The vent tube is also designed to be long enough to prevent significant back diffusion of gases into the chamber from the atmosphere. This is particularly important for species such as Methane and Nitrous Oxide (as well as isotopologues of gases), which can be present at sub-ambient concentrations in the chamber headspace.

Chamber Sealing
The eosAC uses a baseplate and dual gasket system to seal between the chamber and the soil collar, and to prevent any disturbance caused by the motion of the chamber from being transferred into the soil via the collar. The gasket that seals the chamber to the baseplate during measurement is specifically designed as a face-seal to minimize pressure disturbances to the gas transport regime in the soil. By implementing the baseplate and dual gasket system the eosAC ensures a leak-free connection between the soil collar and the chamber while minimizing potential biases from the chamber opening and closing during the measurement cycle.

The face-seal (left) and collar-seal (right) on the eosAC

The face-seal (left) and collar-seal (right) on the eosAC highlighted by the green arrows.

Mixing of Chamber Air
Careful design of the gas inlet and outlet on the chamber leads to a well-mixed chamber atmosphere and unbiased measurements. We followed the recommendations of Pumpanen et al. (2004) on this one and designed a gas outlet “halo” to ensure mixing with minimal turbulence.

From Pumpanen et al. (2004):

Special attention should be paid to the mixing of air in the chamber since it can be a major source of error. Excessive turbulence inside the chamber can cause mass flow of CO2 between the soil and the chamber. However, when using non-steady-state chambers, proper mixing of the air is needed because the CO2 concentration must be evenly distributed within the chamber headspace to calculate the flux correctly.

The turbulence can be decreased by extracting the sample air and by pushing air from the analyser back into the chamber through a perforated manifold circulating around the chamber. This ensures a representative sample and adequate mixing of air with minimal turbulence.

Diffusion Gradient
Gases tend move from the soil to the atmosphere (or from atmosphere to soil) by diffusion along a concentration gradient. When measuring with a closed chamber, the concentration increase necessary to make an accurate measurement of flux disturbs the soil-to-atmosphere gas concentration gradient and the measurement of flux is thereby affected.

Simulated gas concentration disturbance caused by placing a chamber on the soil.

Gas concentration disturbance caused by placing a chamber on the soil. Colour scale is in micromoles per m3 N2O.

By designing a chamber with a comparatively small effective height (or volume to surface area ratio), gas concentrations increase quickly in the eosAC. This small effective height allows for a shorter measurement duration thereby disturbing the natural soil-to-atmosphere gas gradients as little as possible making the flux measurement more accurate.

linear-and-exponential-co2-fits-software

Even the eosAC can’t be perfect, so the eosAnalyze-AC software follows up by offering exponential data fitting algorithms. This exponential fit is based on gas diffusion theory, and corrects the flux estimates for any feedback-related disturbance that occurs during the chamber measurement period. All of this adds up to higher accuracy flux measurements.

Experts in environmental gas monitoring …

Eosense excels in providing innovative gas monitoring techniques that help you with your research needs. If you would like to learn more about chamber measurements please have a look at some of these recommended resources published by Eosense employees and other researchers:

Creelman, C., N. Nickerson and D. Risk. 2013. Quantifying lateral diffusion error in soil carbon dioxide respiration estimates using numerical modeling. Soil Sci. Soc. Am. J. 77: 699-708. (link)

Davidson, E.A., K. Savage, L.V. Verchot and R. Navarro. Minimizing artifacts and biases in chamber-based measurements of soil respiration. Agr. Forest. Meteorol. 113: 21-37. (link)

Venterea, R.T., K.A. Spokas and J.M. Baker. 2009. Accuracy and precision analysis of chamber-based nitrous oxide gas flux estimates. Soil Sci. Soc. Am. J. 73:1087-1093 (link)

Kutzbach, L., J. Schneider, T. Sachs, M. Giebels, H. Nykanen, N.J. Shurpali, P.J. Martikainen, J. Alm and M. Wilmking. 2007. CO2 flux determination by closed-chamber methods can be seriously biased by inappropriate application of linear regression. Biogeosciences. 4: 1005-1025. (link)

Hutchinson, G.L., G.P. Livingston, R.W. Healy and R.G. Striegl. 2000. Chamber measurement of surface-atmosphere trace gas exchange: numerical evaluation of dependence on soil, interfacial layer, and source/sink properties. J. Geophys. Res. 105: 8865-8875. (link)

Livingston, G.P., G.L. Hutchinson and K. Spartalian. 2005. Diffusion theory improves chamber-based measurements of trace gas emissions. Geophys. Res. Lett. 32(24). (link)