Evaluation of Density Corrections to Methane Fluxes Measured by Open-Path Eddy Covariance over Contrasting Landscapes

Corrections accounting for air density fluctuations due to heat and water vapour fluxes must be applied to the measurement of eddy-covariance fluxes when using open-path sensors. Experimental tests and ecosystem observations have demonstrated the important role density corrections play in accurately quantifying carbon dioxide ((hbox {CO}_{2})) fluxes, but less attention has been paid to evaluating these corrections for methane ((hbox {CH}_{4})) fluxes. We measured (hbox {CH}_{4}) fluxes with open-path sensors over a suite of sites with contrasting (hbox {CH}_{4}) emissions and energy partitioning, including a pavement airfield, two negligible-flux ecosystems (drained alfalfa and pasture), and two high-flux ecosystems (flooded wetland and rice). We found that density corrections successfully re-zeroed fluxes in negligible-flux sites; however, slight overcorrection was observed above pavement. The primary impact of density corrections varied over negligible- and high-flux ecosystems. For negligible-flux sites, corrections led to greater than 100% adjustment in daily budgets, while these adjustments were only 3–10% in high-flux ecosystems. The primary impact to high-flux ecosystems was a change in flux diel patterns, which may affect the evaluation of relationships between biophysical drivers and fluxes if correction bias exists. Additionally, accounting for density effects to high-frequency (hbox {CH}_{4}) fluctuations led to large differences in observed (hbox {CH}_{4}) flux cospectra above negligible-flux sites, demonstrating that similar adjustments should be made before interpreting (hbox {CH}_{4}) cospectra for comparable ecosystems. These results give us confidence in (hbox {CH}_{4}) fluxes measured by open-path sensors, and demonstrate that density corrections play an important role in adjusting flux budgets and diel patterns across a range of ecosystems.