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We demonstrated a 19-inches 6U-rack sensing system based on quartz-enhanced photoacoustic spectroscopy for simultaneous detection of methane, nitrous oxide, and ammonia in atmosphere. The system embeds two detection modules, each one equipped with a quantum cascade laser as light source and a spectrophone composed of a T-shaped quartz tuning fork coupled with acoustic resonator tubes. The sensing system results compact, lightweight, and portable, with a computer interface for an easy end-user operation. The calibration was performed in laboratory environment with certified concentrations. Because of the NH 3 large and permanent dipole moment, ammonia QEPAS signal was used to model and characterize adsorption/desorption processes, for an accurate and reliable sensor calibration. At 0.1 s signal integration time, detection limits of 28 ppb, 9 ppb, and 6 ppb were obtained for CH 4 , N 2 O, and NH 3 , respectively. Since these detection limits were well below the natural abundance of the three analytes in atmosphere, the sensor was employed to monitor CH 4 , N 2 O, and NH 3 concentrations in laboratory air. • Development of a compact 19-inches 6U-rack QEPAS sensor box for simultaneous detection of CH 4 , N 2 O, and NH 3 in atmosphere. • Minimum detection limits of 28.0 ppb, 9 ppb and 6 ppb at 0.1 s integration time for CH 4 , N 2 O and NH 3 detection, respectively. • Sensor tested by analysing ambient air. Mean concentrations of ∼ 1.81 ppm of CH 4 , 329 ppb of N 2 O and 83 ppb of NH 3 measured. [ABSTRACT FROM AUTHOR]