Development of a field-deployable method for simultaneous, real-time measurements of the four most abundant N 2 O isotopocules.

Understanding and quantifying the biogeochemical cycle of N2 O is essential to develop effective N2 O emission mitigation strategies. This study presents a novel, fully automated measurement technique that allows simultaneous, high-precision quantification of the four main N2 O isotopocules (14 N14 N16 O, 14 N15 N16 O, 15 N14 N16 O and 14 N14 N18 O) in ambient air. The instrumentation consists of a trace gas extractor (TREX) coupled to a quantum cascade laser absorption spectrometer, designed for autonomous operation at remote measurement sites. The main advantages this system has over its predecessors are a compact spectrometer design with improved temperature control and a more compact and powerful TREX device. The adopted TREX device enhances the flexibility of the preconcentration technique for higher adsorption volumes to target rare isotope species and lower adsorption temperatures for highly volatile substances. All system components have been integrated into a standardized instrument rack to improve portability and accessibility for maintenance. With an average sampling frequency of approximately 1 h-1 , this instrumentation achieves a repeatability of 0.09, 0.13, 0.17 and 0.12 ‰ for δ15 Nα , δ15 Nβ , δ18 O and site preference of N2 O, respectively, for pressurized ambient air. The repeatability for N2 O mole fraction measurements is better than 1 ppb (parts per billion, 10-9 moles per mole of dry air).