Testing and evaluation of a new airborne system for continuous N₂O, CO₂, CO, and H₂O measurements: the Frequent Calibration High-performance Airborne Observation System (FCHAOS)

Abstract

We present the development and assessment of a new flight system that uses a commercially available continuous-wave, tunable infrared laser direct absorption spectrometer to measure N2O, CO2, CO, and H2O. When the commercial system is operated in an off-the-shelf manner, we find a clear cabin pressure–altitude dependency for N2O, CO2, and CO. The characteristics of this artifact make it difficult to reconcile with conventional calibration methods. We present a novel procedure that extends upon traditional calibration approaches in a high-flow system with high-frequency, short-duration sampling of a known calibration gas of near-ambient concentration. This approach corrects for cabin pressure dependency as well as other sources of drift in the analyzer while maintaining a ∼ 90 % duty cycle for 1 Hz sampling. Assessment and validation of the flight system with both extensive in-flight calibrations and comparisons with other flight-proven sensors demonstrate the validity of this method. In-flight 1σ precision is estimated at 0.05 ppb, 0.10 ppm, 1.00 ppb, and 10 ppm for N2O, CO2, CO, and H2O respectively, and traceability to World Meteorological Organization (WMO) standards (1σ) is 0.28 ppb, 0.33 ppm, and 1.92 ppb for N2O, CO2, and CO. We show the system is capable of precise, accurate 1 Hz airborne observations of N2O, CO2, CO, and H2O and highlight flight data, illustrating the value of this analyzer for studying N2O emissions on ∼ 100 km spatial scales.