A liquid nitrogen-free preconcentration unit for measurements of ambient N₂O isotopomers by QCLAS

Abstract. Important information about the biogeochemical cycle of nitrous oxide (N₂O) can be obtained by measuring its three main isotopic species, ¹⁴N¹⁵N¹⁶O, ¹⁵N¹⁴N¹⁶O, and ¹⁴N¹⁴N¹⁶O, and the respective site-specific relative isotope ratio differences δ¹⁵N^α and δ¹⁵N^β. Absorption laser spectroscopy in the mid-infrared is a direct method for the analysis of the ¹⁵N isotopic composition of N₂O, yet not sensitive enough for atmospheric N₂O mixing ratios (320 ppb). To enable a fully-automated high precision analysis of N₂O isotopic species at ambient mixing ratios, we built and optimized a liquid nitrogen-free preconcentration unit to be coupled to a quantum cascade laser (QCL) based spectrometer. During standard operation 10 l of ambient air are preconcentrated on a HayeSep D trap and desorbed in 50 ml of synthetic air. Rigorous tests were conducted, using FTIR, quantum cascade laser absorption spectroscopy (QCLAS), GC-FID and component-specific ozone and oxygen analysers to investigate recovery rates, conservation of isotopic signatures and spectral interferences after preconcentration. We achieve quantitative N₂O recovery of >99% with only minor, statistically not significant isotopic fractionation and no relevant spectral interferences from other atmospheric constituents. The developed preconcentration unit also has the potential to be applied to other trace gases and their isotopic composition.