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 δ¹⁵N^α and δ ¹⁵N^β. 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 (δ¹⁵N^α and δ¹⁵N^β) 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.