# Terahertz Rotational Spectroscopy of Greenhouse Gases Using Long Interaction Path-Lengths
## 3 Results and Discussion
### THz Measurements of CH\({}_{4}\) in a White Type Multi-Pass Cell
MultiCHARME was designed to be coupled with different spectrometers covering a very large spectral range from visible to sub-mm wavelengths. N\({}_{2}\)O was chosen as a test molecule to characterize the performance of MultiCHARME on three frequency decades. In particular, the linearity of the absorption was checked with rovibrational measurements up to 480 m in the near-IR and with rotational measurements up to 240 m in the THz domain [33]. In Figure 4, a rotational absorbance of 32% at 577.58 GHz (P = 0.7 mbar) is obtained by measuring in CHARME 400 ppm of residual N\({}_{2}\)O traces with a THz path-length adjusted to 200 m (configuration 4 \(\times\) 5 on the field mirror). Measurements of ozone at a trace level of 200 ppm were also performed in the same configuration with an absorbance of 25% at 577.58 GHz (P = 1 mbar). We benefited from the possibility to simultaneously modulate the amplitude and frequency of the AMC. It was of particular utility to minimise the effects of the standing waves disturbing the baseline spectra measured in MultiCHARME. A rapid frequency modulation was applied with a depth corresponding to the FSR of the interaction length. The absorption profiles of the targeted lines displayed a collisional broadening in excess of this FSR. The profiles were measured using a significantly slower amplitude modulation and a lock-in amplifier to extract the correct modulation frequency. In the right part of Figure 4, the absorbances of N\({}_{2}\)O and O\({}_{3}\), respectively at typical tropospheric and stratospheric concentrations have been simulated with HITRAN on the web [24]. Absorption levels lower than \(1.0\times 10^{-8}\) cm\({}^{-1}\) and \(2.5\times 10^{-7}\) cm\({}^{-1}\) should be reached to measure, respectively, tropospheric N\({}_{2}\)O and stratospheric O\({}_{3}\). With a THz path-length of 200 m, the associated absorbances are respectively 2000 times and 50 times weaker that those measured with MultiCHARME. The capability to monitor N\({}_{2}\)O and O\({}_{3}\) with typical atmospheric concentrations depend now on a correct modelisation of the subsisting baseline variations due to long path FP effects. By removing these baseline oscillations, we can reasonably hope to reach sufficient SNR to monitor O\({}_{3}\) in CHARME at stratospheric concentration with the same setup. The measurement of tropospheric N\({}_{2}\)O requires to improve by at least two orders of magnitude the LOD. Finally, we can notice that the LOD actually reached using THz spectroscopy in MultiCHARME is already sufficient to monitor N\({}_{2}\)O collected in specific polluted atmosphere such as dental office where the waste anesthetic N\({}_{2}\)O exposure could reach several hundreds of ppm without adapted extractors [34]. Figure 3: Left: Scheme of the MULTICHARME Chernin cell with pictures, inside CHARME, of the 6 \(\times\) 6 arrangement highlighted with a He-Ne laser on the 2 fields and 3 objective mirrors. Right: external view of CHARME: CHamber for Atmospheric Reactivity and Metrology of the Environment located in the LPCA, Dunkirk, France.