Marvin Knapp, Ralph Kleinschek, Benedikt Hemmer, Ralph Pfeifer, Frank Hase, Anna Agustí-Panareda, Antje Inness, Jerome Barre, Stefan Kinne, and André Butz
Validation opportunities for model data and satellite observations in the short-wave infra-red spectral range are still sparse above the oceans. To provide such opportunities, we qualify a Fourier-transform spectrometer (FTS) for the regular use on ships. We use the EM27/SUN FTS  in direct-sunlight measurement geometry to retrieve total column densities of carbon dioxide (CO2), methane (CH4), and carbon monoxide (CO)  with solar absorption spectroscopy.
Performing direct-sunlight measurements from a moving platform poses significant challenges to the solar tracking. We use a solar tracker that compensates the vessel's movements in real time, keeping the pointing of the instrument relative to the center of the sun better than 0.05° for more than 99 % of the time . The solar tracker is part of a newly developed enclosure that allows automated measurements and withstands environmental factors such as rain, humidity, and sea spray.
The instrument was deployed on board the German research vessel RV Sonne during the MORE-2 (Measuring Oceanic REferences 2) campaign on a longitudinal transect from Vancouver (Canada) to Singapore in June 2019. During the campaign we recorded 33800 direct sunlight spectra from which column-averaged dry-air mole fractions of CO2, CH4, and CO are retrieved. Our results are calibrated against World Meteorological Organization standards and the columns achieve a relative precision of 0.06 %, 0.06 %, and 1.02 % for CO2, CH4, and CO, respectively.
We compare our records to coincident observations of the Greenhouse gases Observing SATellite (GOSAT), the Orbiting Carbon Observatory-2 (OCO-2), and the TROPOspheric Monitoring Instrument (TROPOMI). Our CO2 records show a mean offset of -3.2 ± 1.1 ppm to OCO-2 and -1.4 ± 1.7 ppm to GOSAT observations. Furthermore, we find a mean CH4 offset of 17 ± 6 ppb to GOSAT and a mean CO offset of 3.5 ± 2.6 ppb to TROPOMI. The Copernicus Atmosphere Monitoring Service (CAMS) provided us with model data of CH4 and CO. We could show that the CO data agree well with our measurements, showing an offset of 3.5 ± 3.6 ppb.
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