Infrared photoacoustic spectroscopy in animal houses: Effect of non-compensated interferences on ammonia, nitrous oxide and methane air concentrations

Reduction of ammonia and greenhouse gas emissions is a major goal in countries where the livestock sector has a high social and economic importance. Estimates of emissions per building and per annum usually have uncertainties higher than the reduction targets. Therefore, mitigation strategies and public funding efficiency will benefit from progress and standardisation in measuring and modelling emissions from livestock buildings. Quantification of livestock emissions requires accurate measurements of trace gas concentrations. Infrared photoacoustic spectroscopy has been used for decades because of its selectivity, reduced drift in calibration, the existence of industrial analysers ease of use and the reduced cost per measurement. Its use can be further improved through better knowledge of measurement uncertainty. Although the infrared bands selected by optical filters are narrow, the risk of interference bias exists due to the high number and diversity of gas species. The emitted gases depend on the management of animals, feed, manure, heating and motorised equipment. Non-compensated interferences between gases induce an overestimation of gas concentrations, and a cascade effect induces underestimation. This study demonstrates two examples in broiler and dairy cow buildings, which lead either to overestimation (e.g. 12 instead of 2 mg [NH3] m−3 and 4 instead of 1 mg [N2O] m−3 or underestimation (e.g. 2 instead of 4 mg [CH4] m−3). The detection of interference bias and the reduction of uncertainty in concentration measurements are discussed. These results can be used to provide uncertainty estimates for previously published emission factors that were based on infrared photoacoustic spectroscopy.

Graphical abstract Interference effect in a dairy cow house mechanically ventilated with constant ventilation rate: a high gap between two analysers can occur: PAS-A2 (empty symbols; NH3 concentrations without cross-compensation) and PAS-A3 (black symbols; NH3 concentrations with cross-compensation with ethanol, acetic acid, propanol); different conditions: period A (with cows in the room, slurry and silage); period B (no cows, no silage with slurry); period C (no cows, no slurry with silage); the increase in concentration around 80% with PAS-A2 occurred after feeding with silage but the flux increase is not greatly explained by the known factors that determine ammonia emission in animal houses. It is assumed here that ethanol emitted by silage induced the overestimation of NH3 by PAS-A2.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► Non-compensated interferences can induce large overestimates of gas concentration. ► A cascade effect can lead to large underestimates of gas concentration. ► Non-compensated interferences can be detected through multigas kinetics inside and outside.