Ppb-Level Self-Calibrating Ozone Detection Using a T-Type Multipass Enhanced Photoacoustic Sensor with a 9.46 μm Quantum Cascade Laser

A sensitive, real-time, and accurate ozone (O₃) sensor system is developed based on the combination of multipass enhanced photoacoustic (MPPA) and direct multipass absorption spectroscopy with a mid-infrared quantum cascade laser (QCL). The QCL with an emission wavelength of 9.46 μm was used to probe the O₃ absorption lines without interference from the absorption of water and carbon dioxide in the flowing mixtures. The MPPA sensor was constructed with a T-type cell composed of a vertical cylinder and a horizontal cavity which were designed as an acoustic resonator and for multipass absorption enhancement, respectively. By periodically on-off switching the modulation of the laser wavelength, rapidly switched measurements of direct absorption and PA spectra can be achieved for real-time and accurate calibrations of the second harmonic (2f) PA signals with the direct absorbance spectra of O₃. Moreover, a detection limit of O₃ of 6 ppb at an average time of 300 s was achieved, and a short sensor response time of 16 s was also obtained in the flow mixtures with a flow rate of 50 sccm. This work provides a reliable method for O₃ detection with capabilities of parts-per-billion-level sensitivity and on-site real-time concentration calibration, thus holding promise for in situ ozone monitoring under various environments.