Assessment of spectroscopic instruments for continuous atmospheric nitrous oxide monitoring

In recent years, the field of laser spectroscopy has witnessed significant progress, leading to major advancements in the detection of atmospheric trace gases. This technological evolution is reflected in a growing number of commercial implementations, especially for prevalent atmospheric gases, such as carbon dioxide (CO2) and methane (CH4). The spectrum of detectable trace gases continues to expand, and manufacturers offer instruments with increasing performance in term of selectivity, sensitivity, power consumption, compactness and cost-effectiveness. In this presentation, we focus on recent instruments for the observation of atmospheric nitrous oxide (N2O). N2O is a major long-lived greenhouse gas which plays an important role in stratospheric ozone depletion, but still suffers from inadequate global data coverage. Therefore, the advent of more economical yet resilient instruments, demanding less space and power compared to conventional models, presents a welcome opportunity to broaden the N2O monitoring network. We provide an overview of laboratory tests carried out at Empa on a variety of commercial models. The evaluated techniques include (i) Mid-IR Tunable Diode Laser Spectrometry (TDLAS) with Interband Cascade Lasers (ICLs) and Quantum Cascade Lasers (QCL), (ii) Optical Feedback Cavity Enhanced Absorption Spectroscopy (OF-CEAS), (iii) Off-Axis Integrated Cavity Output Spectroscopy (OA-ICOS), and (iv) Cavity Ringdown Spectroscopy (CRDS). The tests assessed the suitability of the instruments for precise atmospheric N2O monitoring and provide insights into the operation, data handling and quality assurance / quality control procedures required for long-term operation. Particular attention was paid to the evaluation of the short-term precision and stability of the instrument response and the repeatability within days to weeks. Overall, the instrument performance is still superior for the most-established CRDS and OA-ICOS analyzers, which are widely used in the Global Atmosphere Watch (GAW) programme and the European Integrated Carbon Observation System Research Infrastructure (ICOS-RI). Nevertheless, the latest generation of TDLS and OF-CEAS instruments are cost-efficient alternatives, which may be suited for more extensive networks, such as the ones to be designed under the umbrella of World Meteorological Organization's new Global Greenhouse Gas Watch (G3W) programme. However, great care needs to be taken in terms of quality assurance and quality control (QA/QC) to ensure long-term accuracy and traceability. The most cost-efficient instrumental components still need to be identified as a function of the scientific targets and the related network design.