Contents I. Introduction
THz SPECTROSCOPY has been used for several decades to obtain valuable information by probing the pure rotational transitions of polar compounds or low-frequency vibrational transitions of molecules with large amplitude motions. At low pressure, under Doppler broadening conditions, the rotational line-width at THz frequency is typically less than 3 MHz, offering an exceptional degree of selectivity. Indeed at the Doppler limit several thousands of individual lines can be resolved in a bandwidth of a few GHz [1]. Moreover, absolute molecular concentration can be deduced by straightforward determination of rotational line-strengths provided that the spectral parameters are well known and thus no calibration step is required. Therefore, THz gas phase spectroscopy is an attractive approach for trace gas detection, identification and quantification of polar molecules. Recent progress of sources and detectors in this frequency band has enabled the development of such THz sensor. This is illustrated by previous studies including analysis of mainstream cigarette smoke [2], [3], a submillimeter gas sensor employing a sample pre-concentration to reach parts per trillion (ppt) sensitivity [4], millimeter wave environmental and industrial monitoring [5], [6], or the broadband trace gas sensor based on chirped-pulse terahertz spectroscopy [7] that presents great potential. Such developments are very promising stimulating the interest of many fields like medical diagnostics, pollution monitoring and global security systems. In this paper, multicomponent time-resolved quantification is performed by a THz spectrometer monitoring the photodecomposition of the formaldehyde (H2CO), a key volatile organic compound occurring in the atmosphere. This molecule is known to be an important indoor pollutant and a secondary product formed in atmospheric reactions [8]. The dominant process responsible for the removal of H2CO from the atmosphere is photolysis by sunlight, in particular with wavelengths shorter than 370 nm. After absorption of the UV radiation, the relaxation process leads to two distinct dissociation pathways [9]: \begin{align} H_{2}CO+h{v}_{<330nm}\rightarrow&HCO+H \\ H_{2}CO+h{v}_{<370nm}\rightarrow&CO+H_{2} \end{align}
