An essential component of quantifying, understanding and predicting changes in the Earth's carbon cycle is a global measurement system of key C-based greenhouse gases (notably, of carbon dioxide and methane), which feeds into model assimilation and inversion efforts to estimate local, regional and global carbon fluxes. In addition, such a system may contribute towards the monitoring of anthropogenic carbon emissions to sustain efforts towards their reduction. Carbon dioxide is typically measured through atmospheric surface layer in situ sampling and eddy covariance flux towers, but truly global maps can only be built by Earth Observing satellites. These observe key GHGs as total column amounts using remote sounding spectroscopic measurements, but diurnal and seasonal variations in the total column of CO2 are small (typically of the order of 1 ppm or less on a background of 400 ppm), and instruments therefore need to be highly precise. Satellite observations of CO2 are typically calibrated and validated against ground-based (Sun-viewing) total column measurements from the TCCON network, consisting of a global network of large FTIR spectrometers. Accurate total column (as opposed to in situ) CO2 measurements can also overcome some of the assimilation problems linked to uncertainties in the parameterization of cross-boundary layer fluxes, in other words, the exchange between the lowermost atmosphere and the free troposphere above, which might be advantageous.
Within the context of the Fiducial Reference Measurements for Greenhouse Gases (FRM4GHG) campaign co-funded by ESA, several novel, smaller and more rugged spectrometers have been deployed to a common TCCON reference site in Sodankylä in the Finnish Artic. From here, AirCore balloon sondes can also be launched, giving the group access to high-quality in situ atmospheric profiles from the ground up into the mid-stratosphere. FRM4GHG aims to compare and validate these instruments against the TCCON standard, generate new datasets for satellite product validation, and investigate future opportunities to create denser ground-based measurement networks using these novel sensors.
The RAL Space Laser Heterodyne Radiometer is one of the participating instruments to this campaign, lasting from early spring to autumn 2017. It has undergone a complete redesign to make it more compact, rugged, autonomous and easy-to-operate. The spectrometer has been set up to share the direct sunlight from a BIRA solar tracker, collecting radiation for the Vertex FTIR, also part of the campaign. The data will be used to compare instrument and retrieval processing performance and results, and to validate new instrumentation and satellites for ground and space such as Sentinel 5-p. This project was funded by the UKSA, STFC and ESA.
For more information please contact: RAL Space Enquiries
