Real time monitoring of H2S emissions at the Pisciarelli fumarolic field (Campi Flegrei caldera) using a compact Quartz-Enhanced Photoacoustic sensor

Monitoring the volcanic plume emissions of dormant volcanoes and restless calderas reveals essential information on the sub-surface magmatic and hydrothermal processes, providing an essential tool for improving the surveillance of active volcanoes, especially during the unrest phases. Together with the predominant emissions of water vapor (H₂O) and carbon dioxide (CO₂), monitoring the sulfur degassing, in terms of hydrogen sulphide (H₂S) and sulfur dioxide (SO₂), is a priority due to its significant atmospheric and climatic impacts. To achieve reliable and continuous monitoring under the challenging conditions typical of volcanic environments, compact and robust sensors are required, capable to guarantee high selectivity and sensitivity with detection limits in the part-per-million (ppm) range in the complex and variable volcanic plume. Moreover, a fast response time on the order of seconds is a precious asset for effectively tracking rapid changes in gas emissions. Quartz Enhanced Photoacoustic Spectroscopy (QEPAS) sensors fulfil these requirements by using a quartz tuning fork to detect sound waves generated by the interaction of the target gas with infrared modulated light. In addition, QEPAS sensors overcome the cross-interference and long recovery-time limitations, offering an advantageous alternative to conventional electrochemical sensors.

Here we report on the realization of a multi-gas sensor system composed of an electronic hygrometer for temperature and H₂O monitoring, a commercially available CO₂ sensor and a compact QEPAS sensor for the detection of H₂S in volcanoes environment. The QEPAS sensor employed a DFB diode laser targeting the H₂S absorption line at 3792.90 cm^-1 and an acoustic module composed of a T-shaped quartz tuning fork coupled with micro-resonator tubes. The QEPAS sensor was optimized and calibrated in laboratory, reaching a 1-σ minimum detection limit of 1.6 ppm with an integration time of 1 s, at a working pressure of 100 Torr. Field tests were carried out through continuous, real-time measurements at the Pisciarelli fumarolic field (Campi Flegrei caldera, southern Italy) with the system operating for several hours for three days. Measurements were taken at varying distance from the main fumarolic vent (from few to tens of meters), demonstrating the sensor capability to track rapid fluctuations of H₂S concentrations within the plume. At the closest distance, H₂S peaks of tens of ppm were detected and positive correlation with the CO₂ emission was retrieved. These results fully demonstrated the applicability of the multi-gas system for monitoring H₂S concentrations and the CO₂/H₂S ratio in volcanic environments. Based on these results, further measurement campaigns will be conducted at the Campi Flegrei on February 2026 with an additional QEPAS sensor for CH₄ and SO₂ detection.