Carbon Monoxide (CO) and Nitrous Oxide (N₂O) QCL Instrument
The Aerodyne CS-108 quantum cascade laser (QCL) carbon monoxide (CO) and nitrous oxide (N2O) TILDAS instrument operates on the principle of direct absorbance of infrared radiation near 4.6 µm. The optical spectrometer consists of a thermoelectrically cooled QCL laser source, alignment and wavelength calibration optics, photodetector, and an approximately 0.5 L Herriott cell that provides a 76 m path length. The optical bench is housed in an unsealed, vibrationally isolated aluminum box.
To improve accuracy of airborne observations, the In-situ Photochemical Tracers group (within the ACOM Laboratory) implemented a continuous purge of the optical housing using dry air scrubbed in order to remove residual CO and thereby eliminate the contribution of ambient pressure CO and N2O absorbance to the background spectral fit.
Measurement Characteristics: Upon optimization of the CO-scrubbed dry purge gas flow rate and additional optimization of spectral fit parameters for improved CO quantification, the instrument underwent EOL environmental chamber and WE-CAN null test flight characterization to confirm adequate performance in airborne applications. This instrument was first deployed during the WE-CAN 2018 experiment. In 2019 it was certified for GV operation in order to undergo high altitude GV flight tests in preparation for the 2022 ACCLIP experiment. In laboratory and airborne operation, a 1-s, 1-sigma precision of 0.3 ppbv is observed for each measurand. During airborne operations the instrument typically performs with a ± 1 ppbv overall uncertainty (2 sigma) in N2O and CO determinations.
Figure 1. Frank Flocke helping set up the Aerodyne QCL instrument for characterization in the EOL Altitude/Temperature Chamber.
Hourly in-flight calibrations of 2-4 minute duration are typically conducted with 1-2 working standards and in coordination with other sensors. Working standard concentrations are traceable to WMO network by laboratory calibration against NOAA GMD primary standard suite maintained by RAF.