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• A QEPAS-based sensor with V-T relaxation self-calibration was developed. • A dual-gas detection was realized based on two-mode combined vibration of a QTF. • A self-calibration method for the CH 4 V-T relaxation was provided. • The ppb-level self-calibration CH 4 sensor has a simple setup and small sensor size. • Continuous measurements near a landfill in Shanxi province, China, were performed. A quartz-enhanced photoacoustic spectroscopy methane (CH 4) sensor with vibrational to translational (V-T) relaxation self-calibration was realized and tested for atmospheric CH 4 detection near a landfill. To normalize the influence of H 2 O vapor on the CH 4 energy relaxation rate, CH 4 and H 2 O concentrations were detected simultaneously by means of a frequency division multiplexing technique, in which a custom quartz tuning fork was operated in the fundamental and first overtone combined vibration mode. A continuous wave, thermoelectrically cooled distributed feedback interband cascade laser emitting at 3.3 μm and a near-infrared DFB laser emitting at 1.37 μm were used as the excitation source for CH 4 and H 2 O detection, respectively. A theoretical model of V-T relaxation and self-calibration method were developed to allow this CH 4 sensor to have a simple setup and a small sensor size. Continuous field measurements were carried out near the largest sanitary landfill in Shanxi province, China, to demonstrate the stability and ruggedness of the realized CH 4 sensor. [ABSTRACT FROM AUTHOR]