Intensity modulation-normalized calibration-free 1f and 2f wavelength modulation spectroscopy

This paper reports the development and validation of a field-deployable quantum cascade laser-based tunable diode laser spectroscopy (TDLS) sensor for in situ measurement of atmospheric CO and CO2 using a new calibration-free $1{f}$ and $2{f}$ wavelength modulation spectroscopy (WMS) method. The $1{f}$ WMS and $2{f}$ WMS signals are normalized by the linear and non-linear intensity modulation components respectively to make the signals immune to laser intensity variations. The advantage of this method is that the normalized signals appear on a background that is independent of the laser parameters. The error in mole fraction extraction is 0.2% and 0.98% for CO and CO2 respectively. The system has a single-pass detection limit of 3 ppb for CO (using $1{f}$ WMS and 4.05 m path length) and 45 ppb for CO2 (using $2{f}$ WMS and 20 cm path length) for an optimum integration time of 17 s and 69 s respectively. These values correspond to normalized detection limits of 12 ppb-m for CO and 9 ppb-m for CO2. The mean mole fraction of CO and CO2 measured in Gandhinagar, India from 19–25 July 2019 were found to be 477 ± 32 ppb and 486 ± 29 ppm respectively. These measurements demonstrate that this WMS algorithm and the sensor system are suitable for accurate long-term monitoring of greenhouse gases and air pollutants.