# Terahertz Rotational Spectroscopy of Greenhouse Gases Using Long Interaction Path-Lengths
## 4 Conclusions  
The present article highlights the best performances reached by rotational submm-wave/THz long-path absorption spectroscopy of important atmospheric GHGs. The results obtained on CH4 and CF4 demonstrate that the weakly intense centrifugal distortion-induced rotational lines are measurable with a high degree of accuracy thanks to the progress of THz gas phase high-resolution spectroscopy. Several percents of methane absorbances with a path-length of 20 m may be measured above 2 THz by a cw-THz photomixing source coupled to a White-type multi-pass cell. Thanks to a frequency metrology based on a frequency comb, the rotational line centers are measured with an accuracy competitive with those of electronic synthesizers and few cw-THz measurements are able to improve the ground state low-order molecular parameters of the CH4 spherical top [13]. The study of CH4 has been focused on THz line position analysis but absolute intensities measurements are required for quantitative spectrosocopy of methane. Such analysis requires the improvement of the spectrometer sensitivity. For this purpose, photomixers using a metallic mirror-based FP cavity could be tested at short term in order to increase significantly the power of the source [37]. Next a new generation of spectrometers has to be imagined allowing technological breakthroughs in the THz frequency gap. The use of cavity enhanced techniques initially developed in the IR hold great promise, as demonstrated by the results on CF4 presented in this study where the tetrahedral splitting of CF4 rotational lines have been measured and resolved by FP-TAS in the submm-wave domain. This technique based on a high finesse cavity allows reaching km effective path-length in a compact gas cell. According to the CF4 lines measured in Figure 6 (lower panel), FP-TAS with a finesse >3200 should be able to measure rotational lines with SNR > 3 for transitions with minimal intensities up to \(10^{-28}\) cm\({}^{-1}/(\text{m molec.cm}^{-2})\). The results presented in this article constitute a first step of a larger study of the CF4 pure rotational spectrum by FP-TAS: numerous lines in the 600-650 GHz frequency range (see Figure 6, middle panel) with absorbances >1% have to be measured, especially the ground state lines. The measured line frequencies will be included in a global fit of the CF4 ground state and \(v_{3}=1\) molecular parameters in order to update the TFMeCaSDa database [38]. No doubt that the THz spectroscopy studies of GHGs such as CH4 and CF4 have to be continued. In addition, new spectroscopic data may be also obtained by considering the asymmetric isotopologues of non polar GHGs. In particular, with the high sensitivity offered by our THz cavity, it will probably possible to measure the R(27) and R(28) rotational transitions of asymmetric \({}^{16}O^{12}C^{18}O\) and to improve its ground state constants fitted only with low \(J\) value cm- and mm-wave transitions [39]. On the other hand, the trace gas detection of non or weakly polar GHGs at atmospheric concentration can not be seriously considered. In the case of the polar stable N2O and unstable O3 GHG the situation is different since we demonstrated our ability to measure these molecules at trace levels. For the first time, THz spectroscopy was used for a direct monitoring of molecular species in an atmospheric simulation chamber. Thanks to an especially designed Chernin multi-pass cell allowing to adjust a THz pathlength from 120 m to 240 m, quantitative rotational measurements of N2O and O3 traces were performed. Right now, the accessible detection levels for both compounds are limited to tens of ppm. We are working on a correct baseline modelization in order to remove its variations due to multiple interfering stationary waves in the Chernin cell. Anyway, the first results obtained in CHARME with the coupling of a THz source to the MultiCHARME setup opens new possibilities especially for the monitoring of stratospheric reaction processes at low-pressure.