Air pollutant detection and chlorinated very short-lived substance characterization with a prototype terahertz time-domain spectrometer

In this work, within the realm of apparatus for in-field detection of gaseous pollutants a complementary portable prototype for gas sensing based on terahertz time-domain spectroscopy (THz-TDS) that directly probes low-energy molecular transitions serving as unique spectral fingerprints for gas identification, offering high selectivity and the ability to simultaneously detect multiple analytes is addressed. To exploit these advantages in practical scenarios, a compact and portable THz-TDS system capable of performing remote, real-time measurements is developed. The prototype was used to characterise for the first time in literature over a broad THz range, pure dichloromethane and chloroform, two chlorine-containing pollutants known as very short-lived substances (VSLS) that strongly contribute to ozone depletion. The THz range allows selective detection of their absorption lines related to the rotational molecular motion, for which the theoretical confirmation is also presented. Then, the optical response of a multi-component mixture achieved with the two aforementioned chlorine-based compounds mixed with two widely distributed volatile pollutants (acetone and methanol) was investigate. For these first measurements, a set-up specifically for laboratory conditions in which the substances are directly injected into a custom gas-cell circuit is investigated. Finally, the prototype was modified to ensure that the ambient atmosphere is drawn directly into the gas cell via a long pipe and a suction system opportunely developed. The analysis of the mixtures in both laboratory and in-field conditions demonstrates that the prototype, together with the approach employed in this work, can simultaneously identify and quantify single components in complex atmospheric mixtures. These findings highlight the potential of the portable THz-based sensor as a powerful and complementary technique for environmental monitoring, particularly where high selectivity, remote measurement, and multi-component detection are critical.