Blackbody Comparison Measurements for Improved Traceability of Longwave Downward Radiation Measurements
by Mr. Moritz Feierabend, Dr. Julian Gröbner, Dr. Ingmar Müller, Mr. Dirk Fehse, Mr. Max Reiniger, Dr. Christian Monte

Abstract

Longwave downward radiation refers to the infrared radiation emitted in the atmosphere that is incident on the surface of the Earth. Measurements of this radiation take place at many stations around the world. Within the Baseline Surface Radiation Network (BSRN) [1], the Tilted Bottom Cavity BB2007 [2] at PMOD/WRC has long served as a reference for tracing longwave downward radiation measurements to the SI. The traceability of the BB2007 is based on contact thermometry and optical simulations. Given the importance of the BSRN, a comparison of the BB2007 to an independent realization of a reference irradiance source is desirable from a metrological point of view and in terms of quality assurance. Therefore, comparison measurements were performed [3] between the BB2007 and an independent reference, namely the Hemispherical Blackbody (HSBB) [4]. The HSBB is a recently developed transportable reference instrument that was developed and built by PTB, the national metrology institute of Germany. It is a new tool aimed at improving the consistency of longwave downward radiation measurements within the BSRN. The measurements were carried out in two stages. In the first stage, the HSBB was characterized against an ammonia heat-pipe blackbody at PTB that serves as a national standard for the Radiation Temperature Scale of PTB [5]. This scale is directly traceable to the fixed points of the ITS-90 and hence to the SI. For the second stage, the HSBB was brought to PMOD/WRC, where comparison measurements were conducted between the BB2007 and the HSBB. The results show generally good agreement between the BB2007 and the HSBB in both radiation temperature and irradiance with respect to the target uncertainty of 0.5 W/m2 associated with the HSBB. The comparison has shown that a second independent path of traceability for the BB2007 is now possible. Acknowledgements This project has received funding from the EMPIR programme co-financed by the Participating States and from the European Union’s Horizon 2020 research and innovation programme. References [1] Baseline Surface Radiation Network (BSRN): structure and data description (1992-2017), In: Earth System Science Data, 10.3 (2018), pp. 1491-1501. [2] Operation and investigation of a tilted bottom cavity for pyrgeometer characterizations, In: Applied Optics, 47.24 (2008), pp. 4441-4447. [3] To be submitted to: Metrologia [4] To be submitted to: Optics Express [5] Non-contact temperature measurement at the Physikalisch-Technische Bundesanstalt (PTB), In: Quantitative InfraRed Thermography Journal, 18.3 (2021), pp. 187-212.