Figure A1.

Measurement with the same setup but with a H-cell similar to the work of El-Safoury et al. [14]. The H-cell has a resonator length of 40 mm and a diameter of 3 mm. The buffer volumina have both a diameter and a length of 20 mm. In this setup, the offset is equivalent to 65.4 ppm NO${}_2$. Due to the high offset, the zero gas signal is more vulnerable to drift effects and therefore less stable.