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

Abstract

A high-sensitivity methane (CH4) and nitrous oxide (N2O) 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 7.8μm 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 7.2×10-10 cm-1 was achieved at 1330.50cm-1 with an average of 139 measurements or 21s averaging time and further improved to 2.3×10-10 cm-1 with an average of 3460 measurements, or 519s averaging time. For the targeted CH4 and N2O, absorption lines located at 1298.60 and 1327.07cm-1 with temperature effect correction detection limits of 13 and 11pptv were experimentally achieved with 10.4 and 10.2s averaging times and could be further improved to 5 and 9pptv with 482.5 and 311s averaging times, respectively. Four spectral bands (1298.4 to 1298.9cm-1, 1310.1 to 1312.3cm-1, 1326.5 to 1328cm-1, and 1331.5 to 1333cm-1) in the spectral range from 1295 to 1335cm-1 were selected for the separate and simultaneous measurements of CH4 and N2O under normal atmospheric pressure, and all were in good agreements. The concentrations of CH4 and N2O of atmospheric air collected at different locations and of exhaled breath were measured and analyzed. Continuous measurements of CH4 and N2O concentrations of indoor laboratory air over 45h were also taken. It was found that anaerobic bacteria in the water and soil of wetlands might significantly increase the CH4 concentration in the air. The measured N2O 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 CH4 and N2O in the air.