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Abstract

Measurements of dry air mole fractions of atmospheric greenhouse gases are used in inverse models of atmospheric tracer transport to quantify their sources and sinks. The measurements have to be calibrated to a common scale to avoid bias in the inferred fluxes. For this purpose, the World Meteorological Organization (WMO) has set requirements for the inter-laboratory compatibility of atmospheric greenhouse gas (GHG) measurements. A widely used series of devices for these measurements are the GHG analyzers manufactured by Picarro, Inc. These are often operated in humid air, and the effects of water vapor are corrected for in post-processing. Here, we report on rarely detected and previously unexplained biases of the water correction method for CO2 and CH4 in the literature. They are largest at water vapor mole fractions below 0.5% H2O, which were undersampled in previous studies, and can therefore affect measurements obtained in humid air and in air dried with a Nafion membrane. The biases, which can amount to considerable fractions of the WMO goals, are caused by a sensitivity of the pressure in the measurement cavity to water vapor. We correct these biases by modifying the water correction method from the literature. Our method relies on experiments that maintain stable water vapor levels to allow equilibration of cavity pressure. The commonly used droplet method does not fulfill this requirement. Correcting CO2 measurements proved challenging, presumably because of our humidification method. Open questions pertain to differences between analyzers and variability over time. Correcting the cavity pressure-related biases helps keeping the overall accuracy of measurements of dry air mole fractions of CO2 and CH4 obtained with Picarro GHG analyzers in humid and Nafion-dried air within the WMO goals.