The development and evaluation of airborne in situ N2O and CH4 sampling using a quantum cascade laser absorption spectrometer (QCLAS)

[摘要] Spectroscopic measurements of atmospheric N₂O and CH₄ molefractions were made on board the FAAM (Facility for Airborne AtmosphericMeasurements) large atmospheric research aircraft. We present details of themid-infrared quantum cascade laser absorption spectrometer (QCLAS, AerodyneResearch Inc., USA) employed, including its configuration for airbornesampling, and evaluate its performance over 17 flights conducted duringsummer 2014. Two different methods of correcting for the influence of watervapour on the spectroscopic retrievals are compared and evaluated. A newin-flight calibration procedure to account for the observed sensitivity ofthe instrument to ambient pressure changes is described, and its impact oninstrument performance is assessed. Test flight data linking thissensitivity to changes in cabin pressure are presented. Total 1σuncertainties of 2.47 ppb for CH₄ and 0.54 ppb for N₂O are derived. We report a mean difference in 1 Hz CH₄ mole fraction of 2.05 ppb (1σ =  5.85 ppb) between in-flight measurements made using theQCLAS and simultaneous measurements using a previously characterised FastGreenhouse Gas Analyser (FGGA, Los Gatos Research, USA). Finally, apotential case study for the estimation of a regional N₂O flux using a mass balance technique is identified, and the method for calculating such an estimate is outlined.

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