High-precision measurements of nitrous oxide and methane in air with cavity ring-down spectroscopy at 7.6 µm
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 7.8  µ 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
 7.2 × 10 - 10  cm −1 was achieved at 1330.50 cm −1 with
an average of 139 measurements or 21 s averaging time and further
improved to 2.3 × 10 - 10  cm −1 with an average of 3460
measurements, or 519 s averaging time. For the targeted CH4 and
 N2O , absorption lines located at 1298.60 and 1327.07 cm −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 cm −1 , 1310.1 to
1312.3 cm −1 , 1326.5 to 1328 cm −1 , and 1331.5 to
1333 cm −1 ) in the spectral range from 1295 to 1335 cm −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 45 h 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.