The Hemispherical Blackbody (HSBB) for improved traceability of longwave downward radiation
In this work, the establishment of a new and metrologically solid traceability chain for the detection of atmospheric longwave downward radiation is presented. Key component of this traceability chain is a new reference cavity blackbody, the Hemispherical Blackbody of the Physikalisch-Technische Bundesanstalt. As a result, climate relevant atmospheric irradiance measurements are via the Hemispherical Blackbody strictly linked to the national standards of the Radiation Temperature Scale of the Physikalisch-Technische Bundesanstalt and consequently to the International System of Units. The new traceability chain serves as a much needed independent validation and metrological improvement of the already existing traceability of the reference for longwave downward radiation measurements. It will therefore pave the way for a consistent traceability of worldwide longwave downward radiation measurements independent of possible specific artefacts hold by a single institute. By this, the expanded uncertainties of longwave downward radiation measurements will then be significantly reduced from currently approximately 10 W/m2 to approximately 2 W/m2. More specifically, longwave downward radiation refers to the infrared radiation that is emitted in the atmosphere and incident on the surface of the Earth. It is directly related to the greenhouse effect of the Earth and therefore particularly interesting for climate related research in the context of climate change. Measurements of longwave downward radiation take place on weather stations and research institutes around the world in areas of different climate conditions. In order to ensure reliability and traceability of the worldwide measurement data, the detecting radiometers need to be calibrated regularly. In the past, a set of stable radiometers, the World Infrared Standard Group operated at the Physikalisch-Meteorologisches Observatorium Davos / World Radiation Center, provided the reference for global longwave downward radiation measurements. Efforts have been made in recent years to overcome specific issues through an improved traceability for longwave downward radiation measurements leading to substantially reduced measurement uncertainties. This will result in an enhancement of the credibility and significance of worldwide longwave downward radiation measurements and therefore an improved prediction accuracy of climate change related trends. The current reference for longwave downward radiation measurements is the well-established blackbody Tilted Bottom Cavity BB2007 operated at the Physikalisch-Meteorologisches Observatorium Davos / World Radiation Center. In order to complete the redefinition of the traceability of longwave downward radiation measurements and consequently effecting a scale change, the traceability of the Tilted Bottom Cavity BB2007 was in need of independent validation and more strict linkage to the International System of Units. Both aims have been accomplished in this work by using the Hemispherical Blackbody, thus providing the Tilted Bottom Cavity BB2007 with a new possible traceability chain. In detail, the Hemispherical Blackbody is an infrared radiation emitting calibration source and was developed, elaborately characterised and calibrated at the Physikalisch-Technische Bundesanstalt. It was developed specifically for the calibration of radiometers measuring longwave downward radiation. By providing a sufficiently constant effective emissivity for different viewing conditions, radiation thermometer measurements at normal incidence can be linked to calibration measurements with wide-angle viewing radiometers. These radiometers used for longwave downward radiation measurements have a hemispherical acceptance angle. The target irradiance standard uncertainty provided by the Hemispherical Blackbody amounts to 0.5 W/m2. Several cross check measurements are shown in this work to verify the consistency in the calibration of the Hemispherical Blackbody. In addition, application measurements were performed with typical radiometers for longwave downward radiation and a critical parameter for the calibration was identified. Finally, comparison measurements between the Tilted Bottom Cavity BB2007 and the Hemispherical Blackbody are shown. These measurements were performed with a dedicated radiation thermometer, an Infrared Integrating Sphere Instrument and a pyrgeometer. The large majority of the obtained data points show agreement within 0.5 W/m2. To conclude, the good agreement that was found in both radiation temperature and irradiance provided by the two reference blackbodies, the Tilted Bottom Cavity BB2007 and the Hemispherical Blackbody, serves as an independent validation of the traceability of the Tilted Bottom Cavity BB2007. These bilateral comparison measurements of the irradiance scales for longwave downward radiation were successful and have far-reaching consequences as outlined above. The typical blackbody operating temperatures representing the irradiance levels of longwave downward radiation range from -20 °C to 20 °C. The corresponding dominant infrared wavelength spectrum ranges from approximately 4 µm to 50 µm. Most of the content presented in this dissertation is based on two publications, namely [1,2], which were written throughout the research project presented in this work. [1] M. Feierabend et al. “Development and operation of the Hemispherical Blackbody (HSBB) for the calibration of infrared radiometers with a hemispherical acceptance angle”. In: Optics Express 30 (2022), pp. 46991-47003. [2] M. Feierabend et al. „Bilateral comparison of irradiance scales between PMOD/WRC and PTB for longwave downward radiation measurements”. In: Metrologia 60 (2023), p. 025010.
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