Exploring the analytical limits of the indirect CO2 balancing method: A round-robin study for quantifying CH4 and NH3 emissions

Abstract

Precise quantification of methane (CH 4 ) and ammonia (NH 3 ) emissions from naturally ventilated dairy barns (NVDBs) is important for climate and environmental assessment, yet the inter-laboratory reproducibility of the indirect CO 2 -balance method has not been quantified under field conditions. In this round-robin study, five independent laboratories simultaneously measured CO 2 , CH 4 , and NH 3 concentrations in an experimental NVDB using harmonised spatial sampling but laboratory-specific analysers and sampling systems, including CRDS and FTIR instruments. The choice of outdoor background sampling location was also evaluated using meteorological conditions and information on nearby contamination sources. Concentration-level differences between analysers were observed, although strong agreement was found for the derived ventilation rates (Q) and emissions (e CH4 , e NH3 ). The consortium means were 1052.3 ± 45.7 m 3 h −1 LU −1 for Q, 7.41 ± 0.29 g h −1 LU −1 for e CH4 , and 0.661 ± 0.096 g h −1 LU −1 for e NH3 (95% CI). CRDS analysers showed the most consistent performance, whereas FTIR performance depended strongly on correct spectral configuration and analyser-specific calibration. The results indicate that, under harmonised field conditions, the additional variability introduced by laboratory practice and analyser choice was comparatively small for Q and e CH4 , but greater caution remains necessary for e NH3 . This is the first field-based benchmark of inter-laboratory reproducibility for the indirect CO 2 -balance method in an NVDB. However, the accuracy of the indirect CO 2 -balance method still depends strongly on the P CO2 model, and the absence of a simultaneous direct reference method remains a major limitation that should be addressed in future work.