Intercomparison of greenhouse gas measurements using whole air and synthetic standards at UK tall tower sites

Greenhouse gas (GHG) measurements are crucial for understanding climate change. Therefore, the World Meteorological Organisation (WMO) identified them as critical for global monitoring. There is a need for an infrastructure that can provide traceable atmospheric measurements to underpin the fulfilment of internationally agreed emissions reduction targets.    

High accuracy gas Reference Materials (RMs) are required to underpin GHG composition measurements for long term temporal and spatial trend analysis. These are prepared by compressing whole-air into high pressure cylinders for direct use following measurement by a central calibration laboratory (CCL) to provide traceability to international scales. A span of GHG amount fractions targeted for assessing instrument linearity can be made by addition of pure gases or removal from whole-air. Synthetic RMs, traceable to the SI, prepared gravimetrically from individual components to a target amount fraction, offer a potential alternative way to compare instrument performance without the difficulties of preparing certified whole air standards. However, use of synthetic RMs present their own challenges, namely in adequately matching the matrix to atmospheric composition in order to prevent measurement biases from pressure broadening when using techniques such as Cavity Ring Down Spectroscopy (CRDS).  

The UK-DECC network is comprised of tall tower monitoring sites measuring amount fractions of major GHGs using whole-air calibrated CRDS instruments. Whole-air RMs have been previously used to understand site-to-site differences and study possible instrumental reasons for differences. However, the use of synthetic-air RMs to quantify variations between monitoring sites in reported amount fractions has not been reported previously for this network.  

In this work a set of synthetic-air and whole-air RMs containing CH₄, CO₂ and N₂O were prepared and measured in an intercomparison at UK-DECC network sites. The results show promise for the use of synthetic RMs in efficiently explaining measurement offsets across a network. Synthetic RMs, prepared appropriately, offer a potentially cost effective and more convenient route to help quality control atmospheric monitoring programmes.