Agricultural ammonia sensor using diode lasers and photoacoustic spectroscopy

A trace-gas sensor based on fibre-amplifier enhanced photoacoustic spectroscopy has been developed for measuring ambient ammonia in agricultural settings. The sensor was built in an enclosure for continuous, unattended operation in dusty and humid conditions. Lab testing yielded benchmark results of sub-ppm sensitivity with a measurement time of 1 min and a linearity of 99.99%. Field testing was performed in environmental chambers at UC Davis where the excreta from three Holstein cows were allowed to accumulate, providing a source of ambient ammonia. The photoacoustic sensor measured the ambient ammonia in the room as it increased from below the detection threshold, up to 8 ppm, operating over a three-day period. Intercomparison measurements with the Federal reference method (EPA 40 CFR, using sulfuric acid filled impingers to trap ammonia and subsequent analysis using ion chromatography) yielded good to excellent correlation.