Analysis of N2O isotopic composition at a tall tower in central Switzerland to identify emission sources and hot spots

Abstract:

The isotopic composition of nitrous oxide (N₂O) provides important information on N₂O sources, because microbial processes exhibit characteristic isotopic signatures [1]. Therefore, quasi-continuous measurements of ambient N₂O concentration and isotopic composition in combination with atmospheric modelling can be used to identify local and regional emission hotspots and disentangle production pathways [2]. Quantum cascade laser absorption spectroscopy (QCLAS) in combination with preconcentration allows simultaneous and high-precision analysis of the four main stable N₂O isotopologues (¹⁴N¹⁴N¹⁶O, ¹⁴N¹⁵N¹⁶O, ¹⁵N¹⁴N¹⁶O and ¹⁴N¹⁴N¹⁸O) at ambient mixing ratios [3].

In the presented project, we will initiate quasi-continuous measurements of N₂O isotopologues at the Beromünster tall tower in Central Switzerland [4]. For the two inlet heights 12 and 212 m, maximum changes in the N₂O mixing ratios of 50 and 10 ppb are anticipated, which would result in delta value changes in the order of 3 and 0.6 ‰, assuming a 20 ‰ depletion in delta values for the main source processes. To resolve differences in N₂O isotopic composition at the highest inlet the repeatability of delta value measurements has to be improved to < 0.1 ‰. In an ongoing laboratory phase we significantly enhanced the long-term stability of the laser spectrometer by reducing the dependency of cell temperature on ambient temperature changes from 100 mK K^-1 to 10 mK K^-1. Thereby, the required precision level is maintained for a 10 times longer period, which results in an accuracy of < 0.1‰ for the targeted isotope ratios. In addition, a novel preconcentration device [5] was set up and optimized with respect to temporal resolution. First results from the Beromünster tall tower, which are anticipated for fall 2015, will be presented and followed by atmospheric transport simulations and a biogeochemical soil model developed at IMK-IFU to simulate N₂O isotopomer surface fluxes.

References

[1] S. Toyoda et al. Mass Spectrometry Reviews, DOI: 10.1002/mas.21459, (2015).
[2] S. Park S. et al. Nature Geosci, 5, 4, 261 (2012).
[3] B. Wolf et al. Biogeosciences, 12, 8, 2517 (2015).
[4] B. Oney et al. Atmos Chem Phys Discuss, 15, 9, 12911 (2015).
[5] E. Simon et al. submitted to Atmos Meas Tech Discuss, (2015).