Atmospheric N₂O and CH₄ total columns retrieved from low-resolution FTIR spectra (Bruker Vertex 70) in the mid-infrared region

Abstract. Nitrous oxide (N₂O) and Methane (CH₄) are two important greenhouse gases in the atmosphere. In 2019, mid- infrared (MIR) solar absorption spectra were recorded by a Bruker Vertex 70 spectrometer and a Bruker IFS 125HR spectrometer at Sodankylä, Finland at spectral resolutions of 0.2 cm⁻¹ and 0.005 cm⁻¹, respectively. The N₂O and the CH₄ retrievals from high-resolution MIR spectra have been well investigated within the Network for Detection of Atmospheric Composition Change (NDACC), but not for MIR spectra gathered with instruments operating at low spectral resolution. In this study, N₂O and CH₄ retrieval strategies and retrieval uncertainties from the Vertex 70 MIR low-resolution spectra are discussed and presented. The accuracy and precision of the Vertex 70 N₂O and CH₄ retrievals are assessed by comparing them with the co-located 125HR retrievals and AirCore measurements. The relative differences between the N₂O total columns retrieved from 125HR and Vertex 70 spectra are -0.3±0.7(1σ)% with a correlation coefficient (R) of 0.93. Regarding CH₄ total column, we first used the same retrieval microwindows for 125HR and Vertex 70 spectra, but there is an underestimation in the Vertex 70 retrievals, especially in summer. The relative differences between the CH₄ total columns retrieved from the 125HR and Vertex spectra are -1.3±1.1 % with a R value of 0.77. To improve the Vertex 70 CH₄ retrievals, we propose an alternative retrieval microwindows. The relative differences between the CH₄ total columns retrieved from the 125HR and Vertex spectra in these new windows become 0.0±0.8 %, along with an increase in R value to 0.87. The co-located AirCore measurements confirm that the Vertex 70 CH₄ retrievals using the latter window choice are better, with the relative mean differences between the Vertex CH4₄ retrievals and AirCore measurements of -1.9 % for the standard NDACC mircrowindows, and of 0.13 % for the alternative microwindows. This study provides an insight into the N₂O and CH₄ retrievals from the low-resolution (0.2 cm⁻¹) MIR spectra observed with a Vertex 70 spectrometer, and demonstrates the suitability of this kind of instruments for contributing to satellite validation, model verification, and other scientific campaigns with the advantage of their transportability and lower cost compared to standard NDACC-type FTIR instruments.