Site selective real-time measurements of atmospheric N₂O isotopomers by laser spectroscopy

Abstract

We describe the first high precision real-time analysis of the N2O site-specific isotopic composition at ambient mixing ratios. Our technique is based on mid-infrared quantum cascade laser absorption spectroscopy (QCLAS) combined with an automated preconcentration unit. The QCLAS allows for simultaneous and specific analysis of the three main stable N2O isotopic species, 14N15N16O, 15N14N16O, 14N14N16O, and the respective site-specific relative isotope ratio differences δ15Nα and δ 15Nβ. Continuous, stand-alone operation is achieved by using liquid nitrogen free N2O preconcentration, a quasi-room-temperature quantum cascade laser (QCL), quantitative sample transfer to the QCLAS and an optimized calibration algorithm. The N2O site-specific isotopic composition (δ15Nα and δ15Nβ) can be analysed with a long-term precision of 0.2‰. The potential of this analytical tool is illustrated by continuous N2O isotopomer measurements above a grassland plot over a three week period, which allowed identification of microbial source and sink processes.