Spectro-analytical techniques are used to identify and quantify concentrations of greenhouse and other gases in the atmosphere. These techniques are based on the unique spectral ‘finger prints’ generated by molecular interactions with specific radiation selected in the near and mid infrared wavelengths of the electromagnetic spectrum.
Satellite borne spectral instruments calibrated pre-launch require on-going performance validation, which is provided by a network of ground based instruments situated in remote locations – one of the largest of many ground based monitoring networks is the Total Carbon Column Observing Network, TCCON. Checking the performance of these remote Earth-based instruments is currently achieved using the World Meteorology Organisation (WMO) in-situ scale, as traceability directly to the SI is both difficult and expensive to achieve.
Spectral measurements from different altitude satellite, air craft and ground based spectrometer measurements made at different temperatures and pressures. These require different pressure and temperature correction procedures for spectrum evaluation before comparison using research databases to produce gas ‘fingerprints’. This enables identification of the different gases present and their concentrations. However, only a very small subset of the spectral data in research databases is underpinned by measurements traceable to the SI units, leading to unnecessarily high levels of uncertainty in atmospheric models.
Accurate spectral data and improved ground station instrument traceability directly to the SI is required to reduce measurement uncertainties and generate more robust data for climate predictions.
The ESA ENVINET satellite launch is leading a significant European investment in atmospheric monitoring for greenhouse gases and in support of this the EMRP project Spectral reference data for atmospheric monitoring has enabled development and commissioning of a new validated spectral line data measurement facility capable of greenhouse gas line data generation with improved traceability to the SI.
The spectral data generated within the project is due to be included in an upcoming revision of the HITRAN database, one of the most widely used spectral databases in the world. Originally compiled in the 1960s, the current HITRAN database is maintained by the Harvard-Smithsonian Center for Astrophysics and contains over 7 million spectral lines for 47 different molecules. However, some of the data accumulated over the years lacks the quality required for robust long-term climate trend analysis.
The project’s contribution will mark a significant increase in the amount of traceable spectral data available to researchers using HITRAN. One of the key users of the database is TCCON, made up of 23 ground-based atmospheric monitoring stations distributed across the globe.