Fig. 1. (a) Absorption spectra of H₂O₂ and air from 1135 to 1393 cm⁻¹ with a 75 m path length at 296 K and 150 Torr. (b) Interference effects of individual air components (H₂O, N₂O, and CH₄) adjacent to H₂O₂ absorption lines acquired with a 75 m path length at 150 Torr.

Fig. 2. Schematic of the H₂O₂ generation, detection, and calibration system. M, mirror; FM, flip mirror; L, lens; MFC, mass flow controller; DAQ, data acquisition.

Fig. 3. (a) Output wavenumber of the QCL versus drive current at different temperatures. The orange line is the linear fitting line at 35°C. (b) Drive current versus wavenumber of the QCL (green line) across the target H₂O₂ line. (c) Peak values of 2f/1f signals at different modulation frequencies. (d) SNRs of the system under different modulation depths at a pressure of 150 Torr. (e) Peak values of 2f/1f signals across a pressure range of 50 to 250 Torr.

Fig. 4. (a) 2f signals measured at various light intensities for H₂O₂. (b) 2f/1f signals measured at various light intensities for H₂O₂.

Fig. 5. (a) 2f/1f signals at different concentrations. (b) Recorded 2f/1f signal amplitudes at different concentrations. (c) Peak values of 2f/1f signals versus H₂O₂ concentration values.

Fig. 6. (a) Time series concentrations of N₂ continuously measured (measurement session lasted for 32 min). (b) Allan deviation for the time series.

Fig. 7. (a) Measured data points at different humidity levels in the absence of H₂O₂; (b) 2f/1f signals of concentrations at different humidity levels in the absence of H₂O₂ and at a concentration of 2.8 ppm.