Accurate measurements of CO2 mole fraction in the atmospheric surface layer by an affordable instrumentation

We aimed to assess the feasibility of an affordable instrumentation, based on a non-dispersive infrared analyser, to obtain atmospheric CO2 mole fraction data for background CO2 measurements from a flux tower site in southern Finland. The measurement period was November 2006 to December 2011. We describe the instrumentation, calibration, measurements and data processing and a comparison between two analysers, inter-comparisons with a flask sampling system and with reference gas cylinders and a comparison with an independent inversion model. The obtained accuracy was better than 0.5 ppm. The inter-comparisons showed discrepancies ranging from -0.3 ppm to 0.06 ppm between the measured and reference data. The comparison between the analyzers showed a 0.1 +/- 0.4 ppm difference. The trend and phase of the measured and simulated data agreed generally well and the bias of the simulation was 0.2 +/- 3.3 ppm. The study highlighted the importance of quantifying all sources of measurement uncertainty