Assessment of eight laser spectrometers for atmospheric nitrous oxide analysis

Recent advances in laser spectroscopy have substantially improved the detection of atmospheric trace gases, accelerating progress in environmental monitoring. The number of commercially available analyzers has grown, offering measurements for an expanding range of species. Manufacturers are increasingly prioritizing reduced power consumption, compact design, and cost-effectiveness, while aiming to maintain high sensitivity and selectivity. This presentation highlights recent instruments developed for monitoring atmospheric nitrous oxide (N₂O), the third most important long-lived greenhouse gas and the largest single contributor to stratospheric ozone depletion. Despite its global relevance, N₂O remains insufficiently observed, in part due to the high cost of established measurement technologies. The emergence of more economical, energy-efficient, and compact instruments presents an opportunity to strengthen the global N₂O monitoring network.

We assessed eight commercial analyzers at Empa using four spectroscopic techniques: (i) mid-infrared tunable diode laser absorption spectroscopy (TDLAS) with Interband Cascade Lasers (ICLs) and Quantum Cascade Lasers (QCLs), (ii) optical-feedback cavity-enhanced absorption spectroscopy (OF‑CEAS), (iii) off-axis integrated cavity output spectroscopy (OA‑ICOS), and (iv) cavity ring-down spectroscopy (CRDS). The tests revealed significant performance differences across techniques and considerable variability among instruments, particularly for mid-IR TDLAS systems using ICLs. The latter showed pronounced performance drift over time and, thus, were found unsuitable for sustained monitoring.

Overall, OA‑ICOS and CRDS analyzers remain the most robust solutions, consistent with their widespread adoption in World Meteorological Organization's Global Atmosphere Watch (GAW) program and the European Integrated Carbon Observation System Research Infrastructure (ICOS‑RI). While lower-cost alternatives exist, they usually involve trade-offs in precision and accuracy. Instrument selection should therefore balance cost, size, and performance requirements for the intended application.