New calibration procedures for airborne turbulence measurements and accuracy of the methane fluxes during the AirMeth campaigns

Hartmann, Jörg; Gehrmann, Martin; Kohnert, Katrin; Metzger, Stefan; Sachs, Torsten

Low-level flights over tundra wetlands in Alaska and Canada have been conducted during the Airborne Measurements of Methane Emissions (AirMeth) campaigns to measure turbulent methane fluxes in the atmosphere. In this paper we describe the instrumentation and new calibration procedures for the essential pressure parameters required for turbulence sensing by aircraft that exploit suitable regular measurement flight legs without the need for dedicated calibration patterns. We estimate the accuracy of the mean wind and the turbulence measurements. We show that airborne measurements of turbulent fluxes of methane and carbon dioxide using cavity ring-down spectroscopy trace gas analysers together with established turbulence equipment achieve a relative accuracy similar to that of measurements of sensible heat flux if applied during low-level flights over natural area sources. The inertial subrange of the trace gas fluctuations cannot be resolved due to insufficient high-frequency precision of the analyser, but, since this scatter is uncorrelated with the vertical wind velocity, the covariance and thus the flux are reproduced correctly. In the covariance spectra the inline-formula M1inlinescrollmathml - normal 7 / normal 3 28pt14ptsvg-formulamathimg8ad467e5ebb510fb36fcc80fab34490b amt-11-4567-2018-ie00001.svg28pt14ptamt-11-4567-2018-ie00001.png drop-off in the inertial subrange can be reproduced if sufficient data are available for averaging. For convective conditions and flight legs of several tens of kilometres we estimate the flux detection limit to be about 4 mg minline-formula−2 dinline-formula−1 for inline-formula M4inlinescrollmathml true w chem normal CH normal 4 normal ‾ 38pt17ptsvg-formulamathimg40f7e3995732df4f9fdead43c35867bc amt-11-4567-2018-ie00002.svg38pt17ptamt-11-4567-2018-ie00002.png , 1.4 g minline-formula−2 dinline-formula−1 for inline-formula M7inlinescrollmathml true w chem normal CO normal 2 normal ‾ 39pt17ptsvg-formulamathimg3266f04424dc9429a3a2c3fb05aadb82 amt-11-4567-2018-ie00003.svg39pt17ptamt-11-4567-2018-ie00003.png and 4.2 W minline-formula−2 for the sensible heat flux.

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Hartmann, Jörg / Gehrmann, Martin / Kohnert, Katrin / et al: New calibration procedures for airborne turbulence measurements and accuracy of the methane fluxes during the AirMeth campaigns. 2018. Copernicus Publications.

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