Standardisation of eddy-covariance flux measurements of methane and nitrous oxide

Abstract

Commercially available fast-response analysers for methane (CH4) and nitrous oxide (N2O) have recently become more sensitive, more robust and easier to operate. This has made their application for long-term flux measurements with the eddy-covariance method more feasible. Unlike for carbon dioxide (CO2) and water vapour (H2O), there have so far been no guidelines on how to optimise and standardise the measurements. This paper reviews the state-of-the-art of the various steps of the measurements and discusses aspects such as instrument selection, setup and maintenance, data processing as well as the additional measurements needed to aid interpretation and gap-filling. It presents the methodological protocol for eddy covariance measurements of CH4 and N2O fluxes as agreed for the ecosystem station network of the pan-European Research Infrastructure Integrated Carbon Observation System and provides a first international standard that is suggested to be adopted more widely. Fluxes can be episodic and the processes controlling the fluxes are complex, preventing simple mechanistic gap-filling strategies. Fluxes are often near or below the detection limit, requiring additional care during data processing. The protocol sets out the best practice for these conditions to avoid biasing the results and long-term budgets. It summarises the current approach to gap-filling.

Gap‐filling approaches for eddy covariance methane fluxes: A comparison of three machine learning algorithms and a traditional method with principal component analysis

“…Previous research has reported that u * and FCH 4 can have a physically based causal relationship by influencing the air–water gas exchange rate or the extent of the flux footprint (Herbst et al, ; Read et al, ). At the same time, low u * can cause systematic measurement bias of FCH 4 due to a decrease in diffusivity and an increase in advection loss (Nemitz et al, ). In other words, the relationship between u * and FCH 4 includes both actual and spurious relationships, but often the influence of spurious relationships can be reduced by introducing an u * filter.…”

“…In other words, the relationship between u * and FCH 4 includes both actual and spurious relationships, but often the influence of spurious relationships can be reduced by introducing an u * filter. However, u * filtering can also remove valid measurements of FCH 4 that were repressed by low turbulence, thus causing an overestimation of FCH 4 (Herbst et al, ; Nemitz et al, ). At the WET.BB site, CH 4 gradients from water to air were higher than typical peatland pools, and open water evasion flux significantly contributed to the total FCH 4 (D'Acunha, ).…”

“…This means nighttime turbulence con- u * and FCH 4 can have a physically based causal relationship by influencing the air-water gas exchange rate or the extent of the flux footprint (Herbst et al, 2011;Read et al, 2012). At the same time, low u * can cause systematic measurement bias of FCH 4 due to a decrease in diffusivity and an increase in advection loss (Nemitz et al, 2018).…”

“…Secondly, most of the variance in CO 2 and energy fluxes stems from variability at the diel and seasonal timescales; in other words, diel and seasonal timescales of the power spectra are the dominant contributor to the total variance of the fluxes (e.g., Baldocchi, Falge, & Wilson, 2001;Stoy et al, 2005). For FCH 4 , a larger part of the variance can occur at hourly or weekly-monthly timescales due to sporadic CH 4 emissions and the relationship between water table fluctuation (e.g., Chamberlain et al, 2018;Nemitz et al, 2018;Sturtevant et al, 2016). Some sites show a distinct diel pattern, but this is not always the case.…”

FLUXNET-CH4 Synthesis Activity: Objectives, Observations, and Future Directions

“…To this end, we are surveying, assembling, and synthesizing data from the EC community, in coordination with regional networks, including AmeriFlux's 2019 "Year of Methane" (http://ameriflux.lbl.gov/year-of -methane/year-of-methane/), FLUXNET initiatives, and other complementary activities. In particular, this work is being carried out in parallel with the EU's Readiness of ICOS for Necessities of Integrated Global Observations (RINGO) project, which is working to standardize protocols for flux calculations, quality control and gap-filling for CH 4 fluxes (Nemitz et al 2018). Methane-specific protocols are needed because of the added complexities and high variability of CH 4 flux measurements and dynamics (Nemitz et al 2018).…”