Project

N2O; stable isotopes; quantum cascade laser; spectrometry; spectroscopy

Lead
Lay summary
Nitrous oxide (N2O) is a stratospheric ozone depleting substance and one of the four most important greenhouse gases. Its major sink, stratospheric destruction, is well quantified, but the global budget is rather uncertain due to a limited understanding of the dominant N2O sources. The study of the three main stable isotopes (14N15N16O / 15N14N16O / 14N14N16O) is a powerful way to trace the biogeochemical cycle of N2O.Absorption spectroscopy in the mid-infrared is potentially the most powerful, direct method to distinguish between all relevant N2O isotopes because of their characteristic rotational-vibrational transitions. It allows the determination of both the N2O concentration and the isotope ratios (?15N? and ?15N?). However, up to now isotope measurements with the required precision of < 1 ‰ for ?15N were only possible at N2O concentration levels that are too high for environmental or atmospheric applications.Based on our latest improvements in laser spectroscopy, we expect a precision for ?15N of 0.1 ‰ at 90 ppm of N2O in a compact and field-deployable quantum cascade laser isotope spectrometer (QCL-IS). While this is adequate to study many biological and technical processes, we also intend to develop a liquid nitrogen-free, fully-automated preconcentration unit. This unit will then be coupled to the QCL-IS to allow continuous ambient air measurements (~320 ppb N2O) with a time resolution of 15 minutes.Studies based on the concentration of individual N2O isotopes and their ratio could significantly enhance our understanding of the global N2O budget. The key to this is source characterization, allocation and quantification of important processes, e.g. soil nitrification/denitrification, waste water treatment and combustion, which will become more accessible because of the novel analytical tool. Furthermore, the preconcentration unit and its coupling to QCLAS is a technique with a wide potential, since it might be used for other trace gases or isotopes with concentrations that are too low for currently available spectroscopy.

Direct link to Lay Summary

Last update: 21.02.2013

Active participation

Self-organised

Number	Title	Start	Funding scheme

Nitrous oxide (N2O) is a stratospheric ozone depleting substance and one of the four most important greenhouse gases. Its major sink, stratospheric destruction, is well quantified, but the global budget is rather uncertain due to a limited understanding of the dominant N2O sources. The study of the three main stable isotopes (14N15N16O / 15N14N16O / 14N14N16O) is a powerful way to trace the biogeochemical cycle of N2O. However, isotope-ratio mass-spectrometry (IRMS), the standard analytical tool for stable isotope ratios in trace gases, is generally a laboratory-based instrument and relies on discrete (flask) sampling which limits temporal and spatial resolution capabilities. Additionally, isotopomers such as 14N15N16O and 15N14N16O have the same mass and their quantification by IRMS is therefore a very challenging task, feasible by only a few laboratories.Absorption spectroscopy in the mid-infrared is a direct method to distinguish between all relevant N2O isotopes because of their characteristic rotational-vibrational transitions. It allows the determination of both the N2O concentration and the site specific isotope ratios (d15Na and d15Nb). However, up to now isotope measurements with the required precision of < 1 ‰ for d15N were only possible at N2O concentration levels that are too high for environmental or atmospheric applications. It was only in a recent feasibility study that we obtained a precision of 0.5 ‰ at concentrations of 90 ppm N2O, illustrating that major improvements are possible with mid-infrared quantum cascade laser absorption spectroscopy (QCLAS).In this follow-up project we project a precision for d15N of 0.1 ‰ at 10 ppm of N2O in a compact and field-deployable quantum cascade laser isotope spectrometer (QCL-IS) employing a continuous wave 4.6 müm QCL and a spectral ratio method. While this is adequate to study many biological and technical processes, we also intend to develop a liquid nitrogen-free, fully-automated preconcentration unit. This unit will then be coupled to the QCL-IS to allow continuous ambient air measurements (~320 ppb N2O) with a time resolution of 5 minutes. Finally, this project includes an exemplary field campaign to distinguish N2O source processes based on the site-specific isotopic composition of atmospheric N2O.The work is planned for a two-year study based on one full postdoc position, advised by the experts of our Laboratory for Air Pollution & Environmental Technology with long-term experience on spectroscopy, preconcentration traps and ambient air monitoring.