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

Precise and accurate observations of anthropogenic greenhouse gases (GHGs), especially carbon dioxide (CO₂) and methane (CH₄), are of utmost importance for the estimation of their emission strengths, flux changes and long-term monitoring. Furthermore, these measurements can be directly used for the verification of climate mitigation actions as demanded by the Paris COP21 agreement. Satellite observations are well suited for this task as they provide global coverage. However, like all measurements these need to be validated, particularly to avoid potential biases. The Total Carbon Column Observing Network (TCCON) performs ground-based observations of GHGs with reference precision using high-resolution Fourier Transform infrared (FTIR) spectrometers. TCCON data are of high accuracy as TCCON uses species dependent scaling factors derived from in-situ reference measurements to be calibrated to the World Meteorological Organization (WMO) reference scale. For several satellites measuring GHGs TCCON data are the main validation source.

Recently, in an effort to further improve the global coverage of ground-based FTIR spectrometers and complement TCCON in remote areas, the COllaborative Carbon Column Observing Network (COCCON) was established. This network utilizes the EM27/SUN FTIR spectrometer, a compact solar-viewing low-resolution instrument. Even though a COCCON spectrometer has recently been used in combination with two TCCON instruments to validate the Orbiting Carbon Observatory-2 (OCO-2) satellite, until now the main focus of COCCON was on the quality control of EM27/SUN spectrometers and dedicated campaigns to estimate emission strengths of CO₂ and CH₄ from local and regional sources, e.g. from cities, fracking areas, volcanoes or mining sites.

Here we present long-term observations with a spectrometer from the COCCON network. In 2015 the instrument was installed at the Gobabeb Namib Research Center in Namibia. Gobabeb is located at the center of the hyperarid Namib desert. Moreover, Gobabeb is situated next to the Kuiseb river, which marks the sharp transition zone between the gravel plains to the north and the sand desert to the south of the station. This high albedo site is especially interesting for satellite validation, as ground-based FTIR data with these characteristics are sparse. Furthermore, it is the first ground-based FTIR instrument measuring CO₂ and CH₄ on the African mainland.

We show long-term COCCON observations from Gobabeb and compare them to results obtained from the TCCON instrument at Reunion Island. Finally, we present a comparison with target mode observations from the Greenhouse Gases Observation Satellite GOSAT.