Mobile ground-based remote sensing of atmospheric CO2, CH4, and CO column densities above the Pacific Ocean
Authors: Knapp Marvin , Kleinschek Ralph , Hemmer Benedikt , Pfeifer Ralph , Hase Frank , Agustí-Panareda Anna , Inness Antje , Barre Jerome , Kinne Stefan , Butz André 
Published: March 2020

&lt;p&gt;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 [1] in direct-sunlight measurement geometry to retrieve total column densities of carbon dioxide (CO&lt;sub&gt;2&lt;/sub&gt;), methane (CH&lt;sub&gt;4&lt;/sub&gt;), and carbon monoxide (CO) [2] with solar absorption spectroscopy.&lt;br&gt;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&amp;#176; for more than 99 % of the time [3]. 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.&lt;br&gt;The instrument was deployed on board the German research vessel &lt;em&gt;RV Sonne&lt;/em&gt; 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 CO&lt;sub&gt;2&lt;/sub&gt;, CH&lt;sub&gt;4&lt;/sub&gt;, 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 CO&lt;sub&gt;2&lt;/sub&gt;, CH&lt;sub&gt;4&lt;/sub&gt;, and CO, respectively.&lt;br&gt;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 CO&lt;sub&gt;2&lt;/sub&gt; records show a mean offset of -3.2 &amp;#177; 1.1 ppm to OCO-2 and -1.4 &amp;#177; 1.7 ppm to GOSAT observations. Furthermore, we find a mean CH&lt;sub&gt;4&lt;/sub&gt; offset of 17 &amp;#177; 6 ppb to GOSAT and a mean CO offset of 3.5 &amp;#177; 2.6 ppb to TROPOMI. The Copernicus Atmosphere Monitoring Service (CAMS) provided us with model data of CH&lt;sub&gt;4&lt;/sub&gt; and CO. We could show that the CO data agree well with our measurements, showing an offset of 3.5 &amp;#177; 3.6 ppb.&lt;/p&gt;&lt;p&gt;&lt;br&gt;[1] Gisi, M. et al.: XCO2-measurements with a tabletop FTS using solar absorption spectroscopy, Atmos. Meas. Tech., 5, 2969-2980, 2012&lt;br&gt;[2] Hase, F. et al.: Addition of a channel for XCO observations to a portable FTIR spectrometer for greenhouse gas measurements, Atmos. Meas. Tech., 9, 2303-2313, 2016&lt;br&gt;[3] Klappenbach, F. et al.: Accurate mobile remote sensing of XCO2 and XCH4 latitudinal transects from aboard a research vessel, Atmos. Meas. Tech., 8, 5023&amp;#8211;5038, 2015&lt;/p&gt;