# High-precision measurements of nitrous oxide and methane in air with cavity ring-down spectroscopy at 7.6 µm

A high-sensitivity methane (inline-formulaCH4) and nitrous oxide (inline-formulaN2O) 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 inline-formulaµm 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 inline-formula $M5inlinescrollmathmlnormal 7.2×{normal 10}^{-normal 10}$ 55pt14ptsvg-formulamathimgc97e2abf3c8068282d7ac6d8a379a80c amt-12-2851-2019-ie00001.svg55pt14ptamt-12-2851-2019-ie00001.png  cminline-formula−1 was achieved at 1330.50 cminline-formula−1 with an average of 139 measurements or 21 s averaging time and further improved to inline-formula $M8inlinescrollmathmlnormal 2.3×{normal 10}^{-normal 10}$ 55pt14ptsvg-formulamathimg52852bc0e7e417a7bda70cfccd0cd9b8 amt-12-2851-2019-ie00002.svg55pt14ptamt-12-2851-2019-ie00002.png  cminline-formula−1 with an average of 3460 measurements, or 519 s averaging time. For the targeted inline-formulaCH4 and inline-formulaN2O, absorption lines located at 1298.60 and 1327.07 cminline-formula−1 with temperature effect correction detection limits of 13 and 11 pptv were experimentally achieved with 10.4 and 10.2 s averaging times and could be further improved to 5 and 9 pptv with 482.5 and 311 s averaging times, respectively. Four spectral bands (1298.4 to 1298.9 cminline-formula−1, 1310.1 to 1312.3 cminline-formula−1, 1326.5 to 1328 cminline-formula−1, and 1331.5 to 1333 cminline-formula−1) in the spectral range from 1295 to 1335 cminline-formula−1 were selected for the separate and simultaneous measurements of inline-formulaCH4 and inline-formulaN2O under normal atmospheric pressure, and all were in good agreements. The concentrations of inline-formulaCH4 and inline-formulaN2O of atmospheric air collected at different locations and of exhaled breath were measured and analyzed. Continuous measurements of inline-formulaCH4 and inline-formulaN2O concentrations of indoor laboratory air over 45 h were also taken. It was found that anaerobic bacteria in the water and soil of wetlands might significantly increase the inline-formulaCH4 concentration in the air. The measured inline-formulaN2O 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 inline-formulaCH4 and inline-formulaN2O in the air.

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Tang, Jing / Li, Bincheng / Wang, Jing: High-precision measurements of nitrous oxide and methane in air with cavity ring-down spectroscopy at 7.6 µm. 2019. Copernicus Publications.

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