The Reduction Gas Analyser (RGA3, Trace Analytical)) at Weybourne Atmospheric Observatory is used to make quasi-continuous measurements of molecular hydrogen (H2) and carbon monoxide (CO) in atmospheric air. Measurements are made using a modified commercial Reduction Gas Analyser (RGA3, Trace Analytical, Inc., California, USA), which includes gas chromatography followed by the reduction of mercuric oxide. Mercury vapour from this reaction is detected by UV-absorption. Samples are analysed every six minutes and with reference to the MPI2009 scale and the NOAA2004 scale for H2 and CO, respectively. In addition the instrument can determine the H2 and CO concentration is discrete samples (i.e. whole air samples, SilcoCans).