Alternative tracer gases for the ERUCT technique to estimate methane emission from grazing animals

The methane emission of grazing ruminants can be estimated using the Emissions from Ruminants Using a Calibrated Tracer (ERUCT) technique by applying sulphur hexafluoride (SF6) in low-release permeation tubes. The low background concentration of SF6 coupled with its low detection limits and solubility similar to methane made SF6 a good tracer. However, discovery of alternative tracer gases is desirable since SF6 itself is a greenhouse gas. The development of high-release permeation tubes has given new opportunities to use tracer gases with higher detection limits than SF6. An extended literature search for alternative tracer gases was undertaken screening approximately 300 potential gases. The suitability of stable isotope tracers was also screened. A large number of candidate gases were eliminated due to potential health risks, differences in their chemical properties relative to methane, being unsuited to delivery by permeation tubes or requiring high purchase or analytical costs. Laboratory evaluations of leading candidate gases (ethane, propane, butane) were conducted. It is concluded that ethane and stable isotopes of methane have the potential as alternative tracers for the ERUCT technique but optimisation of the permeation tube is required before validation in animal studies.