HighNITRO

High-precision nitrous oxide isotope measurements using preconcentration and laser spectroscopy

Nitrous oxide (N₂O) is a strong greenhouse gas and an important ozone depleting substance that is emitted from a range of anthropogenic sources, particularly from agricultural soils following nitrogen fertiliser addition. Emissions of N₂O are increasing, leading to an accelerating tropospheric growth rate, showing that effective mitigation policies are urgently needed. N₂O isotopic composition reflects production and consumption pathways and can be used to understand sources and sinks at different spatiotemporal scales. However, N₂O has a long lifetime and is only destroyed in the stratosphere, so high precision and accuracy are required to distinguish the small atmospheric isotopic changes. Preconcentration coupled to laser spectroscopy offers strong potential to achieve the required precision to constrain the N₂O budget during the anthropocene.

Schematic showing the principle of preconcentration and laser spectroscopy for isotopic measurements. a) During the trapping phase, whole air is passed through the cold trap to quantitively adsorb N₂O. b) Following trapping of sufficient N₂O, the trap is warmed and synthetic air is used to flush the concentrated N₂O gas mixture into the laser cell to a nominated pressure. The laser light makes many passes through the laser cell to achieve a long pathlength for high precision. c) The isotopocules of N₂O exhibit characteristic absorption lines, shown in the upper panel using data from the HITRAN database. The simulated, measured and fit spectra for a QCLAS measurement of N₂O are shown in the lower panel. 446 = ¹⁴N¹⁴N¹⁶O and analogously for 456, 546 and 448.

In the HighNITRO project, we are developing a preconcentration-QCLAS system optimized for automated, high precision position-specific N₂O isotopic analysis, known in short as RAPTOR-QCLAS (RAPid nitrous oxide separaTOR). The RAPTOR system uses an absorption trap cooled to below −145°C to collect N₂O from an ambient air sample. The collected N₂O will then be released into the cell of a MIRO Quantum Cascade Laser Absorption Spectrometer (QCLAS) instrument, where the mixing ratios of the four major isotopocules of N₂O are measured based on absorption of light at specific wavelengths.

The HighNITRO and MIRO teams setting up the new QCLAS in the lab in Bern. L-R: Peter Nyfeler, Phillip Agredazywczuk, Jonas Bruckhuisen, Jiří Hlubuček.

We will use the RAPTOR-QCLAS to measure N₂O isotopic composition at background sites, in order to constrain trends and spatiotemporal variability in N₂O isotopic composition in the present day and over the past century. Using this data, we expect to gain new insight into global and regional N₂O budgets at different temporal scales. We will use the data and modelling approaches to constrain the impacts of climate change on the strength of stratosphere-troposphere exchange. These results underpin our understanding of anthropogenic N₂O sources, and will aid in the development of effective mitigation strategies to reduce N₂O emissions while maintaining food security.

Project scientists

Climate and Environmental Science, University of Bern: Prof. Dr. Eliza Harris (Project PI), Dr. Phillip Agredazywczuk, Peter Nyfeler

This project is funded by the Swiss National Science Foundation R'Equip program (206021_229469) as well as start-up funds from the University of Bern.