Mid-infrared laser absorption spectroscopy of NO2 at elevated temperatures

- Spectroscopy of ten neighboring spin-split NO2 transitions in ν3 band.
- Collision broadening coefficients measured with Ar, N2, O2, CO2, and H2O.
- Temperature-dependence of broadening coefficients were measured up to 600 K.
- Sensor spectroscopy was validated using direct absorption spectroscopy.
- Performance of scanned WMS sensor was demonstrated at 600 K and 800 K in burner exhaust.

A mid-infrared quantum cascade laser absorption sensor was developed for in-situ detection of NO2 in high-temperature gas environments. A cluster of spin-split transitions near 1599.9 cm−1 from the ν3 absorption band of NO2 was selected due to the strength of these transitions and the low spectral interference from water vapor within this region. Temperature- and species-dependent collisional broadening parameters of ten neighboring NO2 transitions with Ar, O2, N2, CO2 and H2O were measured and reported. The spectral model was validated through comparisons with direct absorption spectroscopy measurements of NO2 seeded in various bath gases. The performance of the scanned wavelength modulation spectroscopy (WMS)-based sensor was demonstrated in a combustion exhaust stream seeded with varying flow rates of NO2, achieving reliable detection of 1.45 and 1.6 ppm NO2 by mole at 600 K and 800 K, respectively, with a measurement uncertainty of ±11%. 2σ noise levels of 360 ppb and 760 ppb were observed at 600 K and 800 K, respectively, in an absorption path length of 1.79 m.