Challenges in N2O isotope measurements using CRDS analysers

Nitrous oxide (N₂O) has a significant global warming potential of about 300 times that of CO₂ and a steadily rising atmospheric concentration. Therefore, understanding N₂O production and consumption pathways in major source ecosystems, such as agricultural soils, coupled with accurate quantification of associated N₂O emissions, is critically important in the context of climate change.

The relative abundance of ¹⁵N and ¹⁸O substituted N₂O isotopocules (e.g.,¹⁴N¹⁵N¹⁶O, ¹⁵N¹⁴N¹⁶O, ¹⁴N¹⁴N¹⁸O) to the predominant isotopic form (¹⁴N¹⁴N¹⁶O) serves as valuable tracers for the distinction between important biogeochemical soil processes, such as nitrification and denitrification, which in turn enhances our understanding of N₂O emissions. In this regard, advances in cavity-ring-down-spectroscopy (CRDS) have enabled precise measurement of isotopic species in ambient N₂O, which holds key advantages over isotope ratio mass spectrometry in its ability to measure online, on-site and site-specific with respect to ¹⁵N substitution in the N₂O-molecule.

Despite the CRDS technique's ease in measuring the isotopic composition of N₂O-isotopocules, the apparent isotope data requires significant post-processing, since spectral fitting is controlled by a complex interplay between fundamental physical parameters, instrumental parameters, gas matrix composition, instrumental drift, and fitting algorithms, some of which also depend on the absolute N₂O concentration. Therefore, to retrieve accurate and comparable results, it is necessary to apply appropriate reference gases with minimal differences in gas composition to the sample in the measurement sequence. Remaining deviations between sample and reference have to be post-corrected using predefined, analyser-specific correction functions.

This work provides a comprehensive and detailed correction and calibration protocol, exemplified by the reduction of N₂O isotopic data obtained from a Picarro G5131-i isotopic and gas concentration analyser. This protocol outlines the theoretical and mathematical framework for the necessary corrections and suggests a logical order for applying these corrections. Moreover, the protocol provides a standalone MATLAB code for streamlined and automatic data reduction that can be employed once the required analyser-specific correction functions are established. The developed algorithms were validated on a suite of target gases, which accounts for concentration-based interferences from various species.

With this protocol, we aim to enable researchers to accurately and efficiently acquire high-quality N₂O isotope data from CRDS instruments and similar devices and contribute to standardized community guidelines for post-processing N₂O isotope data. In a prototype application, we analysed N₂O from automated flux chambers to track biogeochemical processes in agricultural soils. Subsequently, these insights will be integrated into soil biogeochemical models, to enable upscaling of emission data.