Atmospheric CO in situ measurements are carried out at the Iza a (Tenerife) global GAW (Global Atmosphere Watch Programme of the World Meteorological Organization – WMO) mountain station using a Reduction Gas Analyser (RGA). In situ measurements at Iza a are representative of the subtropical Northeast Atlantic free troposphere, especially during nighttime. We present the measurement system configuration, the response function, the calibration scheme, the data processing, the Iza a 2008–2011 CO nocturnal time series, and the mean diurnal cycle by months. We have developed a rigorous uncertainty analysis for carbon monoxide measurements carried out at the Iza a station, which could be applied to other GAW stations. We determine the combined standard measurement uncertainty taking into consideration four contributing components: uncertainty of the WMO standard gases interpolated over the range of measurement, the uncertainty that takes into account the agreement between the standard gases and the response function used, the uncertainty due to the repeatability of the injections, and the propagated uncertainty related to the temporal consistency of the response function parameters (which also takes into account the covariance between the parameters). The mean value of the combined standard uncertainty decreased significantly after March 2009, from 2.37 nmol mol 1 to 1.66 nmol mol 1, due to improvements in the measurement system. A fifth type of uncertainty we call representation uncertainty is considered when some of the data necessary to compute the temporal mean are absent. Any computed mean has also a propagated uncertainty arising from the uncertainties of the data used to compute the mean. The law of propagation depends on the type of uncertainty component (random or systematic). In situ hourly means are compared with simultaneous and collocated NOAA flask samples. The uncertainty of the differences is computed and used to determine whether the differences are significant. For 2009–2011, only 24.5% of the differences are significant, and 68% of the differences are between 2.39 and 2.5 nmol mol 1. Total and annual mean differences are computed using conventional expressions but also expressions with weights based on the minimum variance method. The annual mean differences for 2009–2011 are well within the ±2 nmol mol 1 compatibility goal of GAW.