Traceability and Internal Consistency of Greenhouse Gases and Related Tracers within the Global Atmosphere Watch Programme

Reliable and traceable measurements of greenhouse gases (GHGs) and related tracers are essential for detecting trends, understanding sources and sinks, and supporting climate policy. The World Meteorological Organization’s (WMO) Global Atmosphere Watch (GAW) programme provides a Quality Management Framework to ensure that observations meet strict requirements for compatibility and traceability. As part of this initiative, Empa is operating the World Calibration Centre for Surface Ozone, Carbon Monoxide, Methane, Carbon Dioxide, and Nitrous Oxide (WCC-Empa) since 1996, providing independent verification of measurement quality across the GAW network.

A core activity of WCC-Empa is to conduct on-site system and performance audits at GAW stations. These audits include station visits, comparisons with travelling standards and extended parallel measurements, which are used to assess traceability to GAW reference scales. Over the past 30 years, WCC-Empa has performed over 120 audits worldwide, enhancing the quality and availability of data, particularly in regions with limited resources. Operator training and capacity building are also key components, ensuring sustainable improvements in measurement practices.

Recent audits demonstrate substantial progress in CH₄ and CO₂ measurements, driven by advancements in laser spectroscopy. The adoption of cavity-enhanced laser spectroscopy has led to a significant increase in compliance with WMO/GAW network compatibility goals. Over 80% of CH₄ and more than 50% of CO₂ comparisons now meet the respective targets (2 nmol mol⁻¹ for CH₄; 0.1 µmol mol⁻¹ for CO₂). This represents a roughly twofold improvement for CH₄ and a threefold improvement for CO₂ compared to previous gas chromatography (GC) and non-dispersive infrared (NDIR) techniques.

However, challenges remain for CO due to frequent amount fraction drifts in calibration standards, and for N₂O due to the limited availability of high-accuracy reference materials. These challenges result in uncertainties that exceed the network compatibility goals. Currently, only around 20% of CO audits and less than 10% of N₂O audits meet their respective goals (2 nmol mol⁻¹ for CO and 0.1 nmol mol⁻¹ for N₂O). Strengthening traceability chains for these gases is critical for harmonisation across networks.

This contribution will present traceability concepts within GAW, summarise WCC-Empa audit results and lessons learned, and outline ongoing efforts to improve the internal consistency and cross-network comparability of GHG and tracer measurements.