Accurate mobile remote sensing of XCO2 and XCH4 latitudinal transects from aboard a research vessel

[摘要] A portable Fourier transform spectrometer (FTS), model EM27/SUN, was deployedonboard the research vessel Polarstern to measure the column-averagedry air mole fractions of carbon dioxide (XCO₂) and methane(XCH₄) by means of direct sunlight absorption spectrometry. We reporton technical developments as well as data calibration and reduction measuresrequired to achieve the targeted accuracy of fractions of a percent inretrieved XCO₂ and XCH₄ while operating the instrument underfield conditions onboard the moving platform during a 6-week cruise on the Atlantic from Cape Town (South Africa,34° S,18° E; 5 March 2014) to Bremerhaven (Germany,54° N, 19° E; 14 April 2014). We demonstratethat our solar tracker typically achieved a tracking precision of better than0.05° toward the center of the sun throughout the ship cruisewhich facilitates accurate XCO₂ and XCH₄ retrievals evenunder harsh ambient wind conditions. We define several quality filters thatscreen spectra, e.g., when the field of view was partially obstructed by shipstructures or when the lines-of-sight crossed the ship exhaust plume. Themeasurements in clean oceanic air, can be used to characterize a spuriousair-mass dependency. After the campaign, deployment of the spectrometeralongside the TCCON (Total Carbon Column Observing Network) instrument atKarlsruhe, Germany, allowed for determining a calibration factor that makesthe entire campaign record traceable to World Meteorological Organization(WMO) standards. Comparisons to observations of the GOSAT satellite andconcentration fields modeled by the European Centre for Medium-Range WeatherForecasts (ECMWF) Copernicus Atmosphere Monitoring Service (CAMS) demonstratethat the observational setup is well suited to provide validationopportunities above the ocean and along interhemispheric transects.

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