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Accurate measurements of greenhouse gases – what we can learn from over 100 audits in 25 years
by Dr. Christoph Zellweger, Dr. Martin Steinbacher, Dr. Lukas Emmenegger, Dr. Brigitte Buchmann


Empa operates the World Calibration Centre for Carbon Monoxide, Methane, Carbon Dioxide and Surface Ozone (WCC-Empa) since 1996 as a Swiss contribution to the Global Atmosphere Watch (GAW) programme of the World Meteorological Organization (WMO). WCC-Empa strongly supports sustaining and improving the data quality and data availability required for climate and environmental research. One of the main tasks of WCC-Empa is the quality control of GAW stations through on-site system- and performance audits. Audits include a station visit by WCC-Empa staff, comparison of travelling standards, and parallel measurements over the course of one to three months. Until today, we have conducted over 100 audits as independent checks of the measurement's traceability to the accepted standards of the WMO/GAW programme, which are hosted and distributed by Central Calibration Laboratories (CCLs). The audits further focus on operator training and capacity building to increase the data availability and data quality, especially in less developed regions. The presentation will give an overview of audits results of the last 25 years at GAW stations for all parameters of the scope of WCC-Empa. The results show a significant improvement of the accuracy due to improved measurement techniques and quality control procedures. In addition, WCC-Empa re-analyzed the reference standards obtained from the CCL over the past 25 years with respect to internal consistency and drift. We found that the recent revision of the carbon dioxide calibration scale by the CCL resulted in an improved internal consistency. However, the long-term stability of gas mixtures is dependent on the molecule, and drift of calibration standards remains one of the most pressing issue especially for carbon monoxide. The presentation will focus on potential solutions to overcome this issue by using higher amount fraction standards in combination with zero air, and taking advantage of the improved capabilities of spectroscopic techniques. Based on 25 years of experience with on-site comparisons during WCC-Empa audits and investigations on calibration scale stability and consistency of the major greenhouse gases and carbon monoxide, we will propose recommendations from a metrological viewpoint to improve measurement capabilities for atmospheric research.

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Topic : Theme 2: Accuracy requirements for atmospheric composition measurements across economic sectors, and temporal and spatial scales.
Reference : T2-A3

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