| Summary: | Methane (CH<sub>4</sub>) and nitrous oxide (N<sub>2</sub>O) are among the most important atmospheric greenhouse gases. A gas sensor based on a tunable 7.6 μm continuous-wave external-cavity mode-hop-free (EC-MHF) quantum cascade laser (from 1290 to 1350 cm<sup>−1</sup>) cavity ring-down spectroscopy (CRDS) technique was developed for the simultaneous detection of CH<sub>4</sub> and N<sub>2</sub>O in ambient air with water vapor (H<sub>2</sub>O) mostly removed via molecular sieve drying to minimize the impact of H<sub>2</sub>O on the simultaneous measurements. Still, due to the broad and strong absorption spectrum of H<sub>2</sub>O in the entire mid-infrared (mid-IR) spectral range, residual H<sub>2</sub>O in the dried ambient air due to incomplete drying and leakage, if not properly accounted for, could cause a significant influence on the measurement accuracy of the simultaneous CH<sub>4</sub> and N<sub>2</sub>O detection. In this paper, the impact of residual H<sub>2</sub>O on the simultaneous CH<sub>4</sub> and N<sub>2</sub>O measurements were analyzed by comparing the CH<sub>4</sub> and N<sub>2</sub>O concentrations determined from the measured spectrum in the spectral range from 1311 to 1312.1 cm<sup>−1</sup> via simultaneous CH<sub>4</sub> and N<sub>2</sub>O measurements and that determined from the measured spectrum in the spectral range from 1311 to 1313 cm<sup>−1</sup> via simultaneous CH<sub>4</sub>, N<sub>2</sub>O, and H<sub>2</sub>O measurements. The measured dependence of CH<sub>4</sub> and N<sub>2</sub>O concentration errors on the simultaneously determined H<sub>2</sub>O concentration indicated that the residual H<sub>2</sub>O caused an under-estimation of CH<sub>4</sub> concentration and over-estimation of N<sub>2</sub>O concentration. The H<sub>2</sub>O induced CH<sub>4</sub> and N<sub>2</sub>O concentration errors were approximately linearly proportional to the residual H<sub>2</sub>O concentration. For the measurement of air flowing at 3 L per min, the residual H<sub>2</sub>O concentration was stabilized to approximately 14 ppmv, and the corresponding H<sub>2</sub>O induced errors were −1.3 ppbv for CH<sub>4</sub> and 3.7 ppbv for N<sub>2</sub>O, respectively.
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