## 2 Materials and Methods
### Experimental Setup  
The MPC-PAS architecture implemented for the experimental validation presented in this paper is shown in Figure 4.  
Figure 4: Experimental architecture MPC-PAS implemented. DFB-LD, DFB laser diode at 1532 nm; TC, temperature controller; IC, current controller; EDFA, optical amplifier; PD, reference photodetector; CH, chopper modulator; F, parabolic mirror; MPC, multipass circular cell; S, acoustic sensor; Loc-In, lock-in amplifier; AM/WMS, control, data acquisition, and processing system. 3D diagram of the implemented system. Figure 3: Ammonia absorption lines in the 1530 nm range. Simulated spectrum from the HITRAN database and measured by laser scanning [19]. The complete system is based on the classical PAS, which implements a semiconductor laser emitting in the NIR at a wavelength equal to the absorption footprint of the gas under study. A near-infrared distributed feedback laser (DFBLD-1530-20, Eblana, Dublin, Ireland) is used. The laser diode emits at 1532 nm with an output power of 20 mW, supporting a maximum operating current of 105 mA and an operating temperature range of 15 to 40 \({}^{\circ}\)C. It has a wavelength-temperature tuning coefficient of 0.1 nm/\({}^{\circ}\)C and a fine current coefficient of 10 pm/mA. In addition, it has a thermoelectric cooler for precise thermal stability and a thermistor for accurate temperature monitoring. A compact laser diode current and temperature driver (CLD1015, Thorlabs, Newton, NJ, USA) is used to precisely adjust these parameters. The operating temperature was set to 26 \({}^{\circ}\)C, and a current of 94.8 mA was applied to tune the emission to the NH\({}_{3}\) absorption line. Under these conditions, the measured optical output power was 10 mW. The optical signal was amplified using an erbium-doped fiber amplifier (EDFA) (CEFA-C-PB-HP, Keopsys, Lannion, France) at a 700 mA operating current, which provides an optical power gain of approximately 7 dB, to obtain an optical power of 50 mW. The laser beam was focused on the gas cell by a 100 mm focal length, gold-coated parabolic mirror (MPD269-M01, Thorlabs, Newton, NJ, USA). As previously described, a circular multipass cell was chosen, specifically, the IRcell-4m model from IRsweep (Stafa, Switzerland). The IRcell-4m is an instrument designed for infrared absorption spectroscopy and offers a 4 m optical path in a compact design. It uses highly reflective mirrors on its inner walls that allow multiple reflections of the light beam, thus extending the optical path without increasing the physical size of the cell. The bottom cover of the cell underwent modification, resulting in the drilling of three holes at equal distances from the center of the cell. These perforations enabled the installation of three sensors to detect the photoacoustic signal generated within the cell. This modification also permitted the adjustment of the sensors' height within the confines of the cell. Three identical capacitive microphones (FG-23329-P18, -53 dB, BW = 10 kHz Knowles Electronics, Itasca, IL, USA) were installed equidistantly (1 cm) from the center of the cell as shown in Figure 5. The MPC was installed on a custom-made support manufactured using a 3D printer, which facilitates straightforward installation and alignment. The top cover was outfitted with three gas inlets/outlets to regulate the concentration and flow within the cell. Two modes of operation were used. In the first, focused on cell characterization, the optical beam was modulated by a mechanical chopper (MC2000B with MC1F10 HP,  
Figure 5: Installation of the three electrical microphones inside the multipass cell, parallel to the optical beam. BW = 10 kHz, [PERSON], Newton, NJ, USA). This was controlled by a TTL reference signal at different frequencies provided by a lock-in amplifier (Lock-in HFZLI, Zurich Instruments, Zurich, Switzerland). A low-frequency voltage ramp (1 Hz) was applied to the current driver to scan the laser wavelength around the absorption line. The other mode was used for sensor characterization.