High-resolution simultaneous probing of site-specific N2O-isotopologues at 7.8 µm using cavity ring-down spectroscopy

Nitrous oxide, a linear asymmetric molecule, comprises site-specific isotopologues ¹⁴N¹⁵N¹⁶O and ¹⁵N¹⁴N¹⁶O, which provide insight into nitrous oxide production pathways. We explored the ν₁ symmetric stretching mode of nitrous oxide and identified a unique interference-free spectroscopic window spanning 0.065 cm⁻¹, in which ¹⁴N¹⁵N¹⁶O and ¹⁵N¹⁴N¹⁶O can be probed simultaneously with a single laser scan. Here, we employed an external-cavity quantum cascade laser (EC-QCL) emitting at 7.8 µm, coupled with cavity ring-down spectroscopy, to probe high-resolution ro-vibrational transitions of the ¹⁴N¹⁵N¹⁶O and ¹⁵N¹⁴N¹⁶O isotopologues. We prepared various concentrations of a sample mixture of ¹⁴N¹⁵N¹⁶O and ¹⁵N¹⁴N¹⁶O by mixing zero-air and then introduced it into the cavity at an optimized pressure of 8.1 torr. We systematically investigated the spectrometer response for various concentrations of the isotopologues probed within the spectroscopic window with a measurement rate of ∼1 Hz. We then performed a stability analysis, which exhibited a detection limit of 0.44 ppb for ¹⁴N¹⁵N¹⁶O with a 103 s integration time. In contrast, for ¹⁵N¹⁴N¹⁶O, the minimum detection limit was 0.37 ppb at a 102 s integration time, indicating highly sensitive detection of trace isotopologue levels. This work provides a unique spectroscopic window that could find applications in the investigation and characterisation of various biogeochemical pathways for atmospheric nitrous oxide.