N2O concentration and flux measurements and complete isotopic analysis by FTIR spectroscopy

Abstract

We report the development and application of analytical techniques for atmospheric N₂O based on Fourier transform infrared (FTIR) spectroscopy. Using mobile low resolution (1 cm⁻¹) FTIR spectroscopy in the field, the technique delivers mixing ratio measurements of precision ±0.3 ppbv (0.3 nmol mol⁻¹) N₂O and in situ soil–atmosphere flux chamber measurements of fluxes less than 1 ngN m⁻² s⁻¹ (0.04 nmol N₂O m⁻² s⁻¹) with a time resolution of 30 min. The method offers the additional advantages of being simultaneously able to measure CO₂, CH₄ and CO mixing ratios in air to high precision (±0.15 μmol mol⁻¹, ±1 nmol mol⁻¹, ±0.3 nmol mol⁻¹, respectively). By a similar analysis procedure, but with laboratory-based high resolution (0.012 cm⁻¹) FTIR spectroscopy, the N₂O isotope ratios δ¹⁵N, δ¹⁸O and δ¹⁷O are determined simultaneously for a single sample, with current precision of $\text{±1.0‰,}\text{±2.5‰}$ and $\text{±4.4‰}$, respectively. FTIR also resolves the individual contributions of the ¹⁵N¹⁴N¹⁶O and ¹⁴N¹⁵N¹⁶O to overall δ¹⁵N. The resolution of these two isotopomers is not possible using conventional isotope ratio mass spectrometry (IRMS). We present laboratory results demonstrating precision, and N-positionally resolved δ¹⁵N and δ¹⁸O measurements of UV-photolysed N₂O in which a distinct asymmetric ¹⁵N positional effect is observed.