The development and evaluation of airborne in situ N<sub>2</sub>O and CH<sub>4</sub> sampling using a quantum cascade laser absorption spectrometer (QCLAS)

Abstract

Abstract. Spectroscopic measurements of atmospheric N 2 O and CH 4 mole fractions were made on board the FAAM (Facility for Airborne Atmospheric Measurements) large atmospheric research aircraft. We present details of the mid-infrared quantum cascade laser absorption spectrometer (QCLAS, Aerodyne Research Inc., USA) employed, including its configuration for airborne sampling, and evaluate its performance over 17 flights conducted during summer 2014. Two different methods of correcting for the influence of water vapour on the spectroscopic retrievals are compared and evaluated. A new in-flight calibration procedure to account for the observed sensitivity of the instrument to ambient pressure changes is described, and its impact on instrument performance is assessed. Test flight data linking this sensitivity to changes in cabin pressure are presented. Total 1σ uncertainties of 2.47 ppb for CH 4 and 0.54 ppb for N 2 O are derived. We report a mean difference in 1 Hz CH 4 mole fraction of 2.05 ppb (1σ = 5.85 ppb) between in-flight measurements made using the QCLAS and simultaneous measurements using a previously characterised Fast Greenhouse Gas Analyser (FGGA, Los Gatos Research, USA). Finally, a potential case study for the estimation of a regional N 2 O flux using a mass balance technique is identified, and the method for calculating such an estimate is outlined.

Extensive release of methane from Arctic seabed west of Svalbard during summer 2014 does not influence the atmosphere

“…Aircraft measurements during the campaign were performed as low as~15 m above the ocean, covering a larger area than the ship, for a short time (flights were around 4 h in duration). Figure 1d shows the "Facility of Airborne Atmospheric Measurements" (FAAM) aircraft path and height on 2 July 2014 in the area and the location of the flares identified (see Pitt et al [2016], O'Shea et al…”

Modification, Characterization and Evaluation of a Quantum/Interband Cascade Laser Spectrometer for simultaneous airborne in situ observation of CH<sub>4</sub>, C<sub>2</sub>H<sub>6</sub>, CO<sub>2</sub>, CO and N<sub>2</sub>O

“…Santoni et al (2014) reported on overall instrument performance for over 500 flight hours. Pitt et al (2016) found a strong cabin pressure dependence on retrieved methane mixing ratios. Recently, Gvakharia et al (2018) described a fast calibration strategy to overcome this cabin pressure dependence.…”

“…Since we are reporting retrieved mixing ratios relative to the WMO scale, however, only the working standard reproducibility contributes to the total uncertainty. Uncertainty on the other measured species is taken from the ACT-America dataset to allow for WMO A severe cabin pressure dependence in excess of 0.3 ppb hP a −1 in CH 4 mixing ratio has been previously reported for airborne Aerodyne TILDAS instrumentation (Pitt et al, 2016). We were not able to reproduce this large cabin pressure dependence during operation of the QCLS instrument aboard the C130 using the calibration strategy from Sect.…”

A Near-Field Gaussian Plume Inversion Flux Quantification Method, Applied to Unmanned Aerial Vehicle Sampling

“…The accuracy of UGGA concentration measurements was enhanced using a post-flight water vapour correction (see O'Shea et al [80] for details). Measurements were then further calibrated using a two term linear correction (derived from an average of eight calibration procedures, described in detail by Pitt et al [81]), such that dry methane concentration measurements were robustly traceable to the World Meteorological Organization greenhouse gas calibration scale [82]. Equation (A1) was used to calculate σ i ([CH 4 ]) values specific to the UGGA (required to calculate of uncertainty in q me in Equation (3)).…”