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<p>A high-sensitivity methane (<span class="inline-formula">CH₄</span>) and nitrous oxide (<span class="inline-formula">N₂O</span>) sensor based on mid-infrared continuous-wave (CW) cavity ring-down spectroscopy (CRDS) techniques was developed for environmental and biomedical trace-gas measurements. A tunable external-cavity mode-hop-free (EC-MHF) quantum cascade laser (QCL) operating at 7.4 to 7.8&thinsp;<span class="inline-formula">µ</span>m was used as the light source. The effect of temperature fluctuation on the measurement sensitivity of the CRDS experimental setup was analyzed and corrected, and a sensitivity limit of absorption coefficient measurement of <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M5" display="inline" overflow="scroll" dspmath="mathml"><mrow><mn mathvariant="normal">7.2</mn><mo>×</mo><msup><mn mathvariant="normal">10</mn><mrow><mo>-</mo><mn mathvariant="normal">10</mn></mrow></msup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="55pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="c97e2abf3c8068282d7ac6d8a379a80c"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="amt-12-2851-2019-ie00001.svg" width="55pt" height="14pt" src="amt-12-2851-2019-ie00001.png"/></svg:svg></span></span>&thinsp;cm<span class="inline-formula">⁻¹</span> was achieved at 1330.50&thinsp;cm<span class="inline-formula">⁻¹</span> with an average of 139 measurements or 21&thinsp;s averaging time and further improved to <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M8" display="inline" overflow="scroll" dspmath="mathml"><mrow><mn mathvariant="normal">2.3</mn><mo>×</mo><msup><mn mathvariant="normal">10</mn><mrow><mo>-</mo><mn mathvariant="normal">10</mn></mrow></msup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="55pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="52852bc0e7e417a7bda70cfccd0cd9b8"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="amt-12-2851-2019-ie00002.svg" width="55pt" height="14pt" src="amt-12-2851-2019-ie00002.png"/></svg:svg></span></span>&thinsp;cm<span class="inline-formula">⁻¹</span> with an average of 3460 measurements, or 519&thinsp;s averaging time. For the targeted <span class="inline-formula">CH₄</span> and <span class="inline-formula">N₂O</span>, absorption lines located at 1298.60 and 1327.07&thinsp;cm<span class="inline-formula">⁻¹</span> with temperature effect correction detection limits of 13 and 11&thinsp;pptv were experimentally achieved with 10.4 and 10.2&thinsp;s averaging times and could be further improved to 5 and 9&thinsp;pptv with 482.5 and 311&thinsp;s averaging times, respectively. Four spectral bands (1298.4 to 1298.9&thinsp;cm<span class="inline-formula">⁻¹</span>, 1310.1 to 1312.3&thinsp;cm<span class="inline-formula">⁻¹</span>, 1326.5 to 1328&thinsp;cm<span class="inline-formula">⁻¹</span>, and 1331.5 to 1333&thinsp;cm<span class="inline-formula">⁻¹</span>) in the spectral range from 1295 to 1335&thinsp;cm<span class="inline-formula">⁻¹</span> were selected for the separate and simultaneous measurements of <span class="inline-formula">CH₄</span> and <span class="inline-formula">N₂O</span> under normal atmospheric pressure, and all were in good agreements. The concentrations of <span class="inline-formula">CH₄</span> and <span class="inline-formula">N₂O</span> of atmospheric air collected at different locations and of exhaled breath were measured and analyzed. Continuous measurements of <span class="inline-formula">CH₄</span> and <span class="inline-formula">N₂O</span> concentrations of indoor laboratory air over 45&thinsp;h were also taken. It was found that anaerobic bacteria in the water and soil of wetlands might significantly increase the <span class="inline-formula">CH₄</span> concentration in the air. The measured <span class="inline-formula">N₂O</span> concentration in the central city area was somewhat lower than the reported normal level in open air. Our results demonstrated the temporal and spatial variations of <span class="inline-formula">CH₄</span> and <span class="inline-formula">N₂O</span> in the air.</p>