Photoacoustic spectroscopy based nitrous oxide measurement for field applications

The efficiency of fertilisers used worldwide is around 50%. It is a global environmental and economic problem, and intensive research is being conducted to find a solution. Nitrous oxide (N₂O) is one of the nitrogen compounds released from fertilised soils. N₂O is also emitted during the storage, treatment, and application of animal manure, in addition to fertilisers.

To reduce emissions, gas concentration and emission monitoring is important for accurate estimation of agricultural losses and to establish regulations for mitigation purposes. Laser spectroscopy-based methods provide in-situ, highly selective measurements with minimal maintenance, therefore they are promising techniques for monitoring N₂O. A photoacoustic system based on a quantum cascade laser emitting around 7.72 μm was developed for N₂O concentration measurement. Selectivity of the system was tested for water vapour (H₂O), carbon dioxide (CO₂) and methane (CH₄). No cross sensitivity was found for H₂O and CO₂, nevertheless for CH₄ it is not negligible, therefore a two-wavelength method is applied to correct for CH₄. The system has a minimum detectable concentration of 8.5 ppb with an averaging time of 10 seconds. The system was calibrated from 0.05 ppm to 10 ppm, the response was found to be highly linear over the calibrated range (R² = 0.9989).

A feasibility study was performed in a naturally ventilated free-stall dairy barn. Measurements were taken at a total of six measurement points, two of which were outside the barn and four inside the barn where spatial and temporal variations of N₂O concentration were measured. Measurements taken outside the barn were considered to be close to the background (333 ppb). There, the measured concentration was 388 ppb ± 11 ppb. The measured mean N₂O concentration inside the barn was 499 ppb ± 191 ppb during a three-hour period, and it varied between the near background concentration and 1 ppm. The system has a signal stability allowing for field applications; however, further tests are required to prove its applicability for quantifying biosphere-atmosphere exchanges of N₂O. In the future our measuring system will be applicable to monitor N₂O emission flux above crop fields and at livestock farms as well.