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

Abstract

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 1f and 2f wavelength modulation spectroscopy (WMS) method. The 1f WMS and 2f WMS signals are normalized by the linear and nonlinear 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 1f WMS and 4.05 m path length) and 45 ppb for CO2 (using 2f 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.