Photoacoustic measurement with infrared band-pass filters significantly overestimates NH₃ emissions from cattle houses due to volatile organic compound (VOC) interferences

Abstract

Infrared (IR) photoacoustic spectroscopy (PAS) using band-pass filters is a widely used method for measurement of NH3 and greenhouse gas emissions (CH4, N2O and CO2) especially in agriculture, but non-targeted gases such as volatile organic compounds (VOCs) from cattle barns may interfere with target gases, causing inaccurate results. This study made an estimation of NH3 interference in PAS caused by selected non-targeted VOCs which were simultaneously measured by a PAS and a PTR-MS (proton-transfer-reaction mass spectrometry). Laboratory calibrations were performed for NH3 measurement, and VOCs were selected based on a headspace test of the feeding material (maize silage). Strong interferences of VOCs were observed on NH3 and greenhouse emissions measured by PAS. Particularly, ethanol, methanol, 1-butanol, 1-propanol and acetic acid were found to have the highest interferences on NH3, giving empirical relationships in the range of 0.7 to 3.3 ppmv NH3 per ppmv VOC. A linear response was typically obtained, except for a nonlinear relation for VOCs on N2O concentration. The corrected online NH3 concentrations measured by PAS in a dairy farm (with empirical relationships 2.1±0.8 and 2.9±1.9 for Location 1 and Location 2, respectively) were confirmed to be correlated (R2=0.73 and 0.79) to the NH3 concentration measured simultaneously by the PTR-MS when the empirical corrections obtained from single VOC tests were applied.