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.