Atmospheric CO2 mole fraction is measured using a commercially available 'Li‐6252' analyser (Li‐Cor Inc.), which uses non‐dispersive infrared (NDIR) technology. The Li‐6252 is operated in differential mode, and is specifically tuned to the 4.26 μm absorption band for CO2 using a 150 nm optical filter. This ensures the analyser is insensitive to other infrared absorbing gases, and also to vibration, which is important for a shipboard measurement system.