Long-term column-averaged greenhouse gas observations using a COCCON spectrometer at the high-surface-albedo site in Gobabeb, Namibia

Frey, Matthias M.; Hase, Frank; Blumenstock, Thomas; Dubravica, Darko; Groß, Jochen; Göttsche, Frank; Handjaba, Martin; Amadhila, Petrus; Mushi, Roland; Morino, Isamu; Shiomi, Kei; Sha, Mahesh Kumar; de Mazière, Martine; Pollard, David F.

In this study, we present column-averaged dry-air mole fractions of inline-formulaCO2 (inline-formulaXCO2), inline-formulaCH4 (inline-formulaXCH4) and inline-formulaCO (inline-formulaXCO) from a recently established measurement site in Gobabeb, Namibia. Gobabeb is a hyperarid desert site at the sharp transition zone between the sand desert and the gravel plains, offering unique characteristics with respect to surface albedo properties. Measurements started in January 2015 and are performed utilizing a ground-based Fourier transform infrared (FTIR) EM27/SUN spectrometer of the COllaborative Carbon Column Observing Network (COCCON). Gobabeb is the first measurement site observing inline-formulaXCO2 and inline-formulaXCH4 on the African mainland and improves the global coverage of ground-based remote-sensing sites. In order to achieve the high level of precision and accuracy necessary for meaningful greenhouse gas observations, we performed calibration measurements for 8 d between November 2015 and March 2016 with the COCCON reference EM27/SUN spectrometer operated at the Karlsruhe Institute of Technology. We derived scaling factors for inline-formulaXCO2, inline-formulaXCH4 and inline-formulaXCO with respect to the reference instrument that are close to 1.0. We compare the results obtained in Gobabeb to measurements from the Total Carbon Column Observing Network (TCCON) sites at Réunion Island and Lauder. We choose these TCCON sites because, while 4000 km apart, the instruments at Gobabeb and Réunion Island operate at roughly the same latitude. The Lauder station is the southernmost TCCON station and functions as a background site without a pronounced inline-formulaXCO2 seasonal cycle. We find a good agreement for the absolute Xinline-formulagas values, apart from an expected inline-formulaXCH4 offset between Gobabeb and Lauder due to significantly different tropopause height, as well as representative intraday variability between TCCON and COCCON. Together with the absence of long-term drifts, this highlights the quality of the COCCON measurements. In the southern hemispheric summer, we observe lower inline-formulaXCO2 values at Gobabeb compared to the TCCON stations, likely due to the influence of the African biosphere. We performed coincident measurements with the Greenhouse Gases Observing Satellite (GOSAT), where GOSAT observed three nearby specific observation points, over the sand desert south of the station, directly over Gobabeb and over the gravel plains to the north. GOSAT H-gain inline-formulaXCO2 and inline-formulaXCH4 agree with the EM27/SUN measurements within the 1inline-formulaσ uncertainty limit. The number of coincident soundings is limited, but we confirm a bias of 1.2–2.6 ppm between GOSAT M-gain and H-gain inline-formulaXCO2 retrievals depending on the target point. This is in agreement with results reported by a previous study and the GOSAT validation team. We also report a bias of 5.9–9.8 ppb between GOSAT M-gain and H-gain inline-formulaXCH4 measurements which is within the range givenpage5888 by the GOSAT validation team. Finally, we use the COCCON measurements to evaluate inversion-optimized CAMS model data. For inline-formulaXCO2, we find high biases of 0.9 inline-formula± 0.5 ppm for the Orbiting Carbon Observatory-2 (OCO-2) assimilated product and 1.1 inline-formula± 0.6 ppm for the in situ-driven product with inline-formulaR2inline-formula> 0.9 in both cases. These biases are comparable to reported offsets between the model and TCCON data. The OCO-2 assimilated model product is able to reproduce the drawdown of inline-formulaXCO2 observed by the COCCON instrument at the beginning of 2017, as opposed to the in situ-optimized product. Also, for inline-formulaXCH4, the observed biases are in line with prior model comparisons with TCCON.

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Frey, Matthias M. / Hase, Frank / Blumenstock, Thomas / et al: Long-term column-averaged greenhouse gas observations using a COCCON spectrometer at the high-surface-albedo site in Gobabeb, Namibia. 2021. Copernicus Publications.

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