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

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, (NNO)-N-14-N-15-O-16, (NNO)-N-15-N-14-O-16, (NNO)-N-14-N-14-O-16, and the respective site-specific relative isotope ratio differences delta N-15(alpha) and delta N-15(beta). 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 (delta N-15(alpha) and delta N-15(beta)) can be analysed with a long-term precision of 0.2 parts per thousand. 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.