Abstract
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 interlaboratory 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. Setups that dry sample air using Nafion membranes may be affected as well if there are differences in residual water vapor levels between sample and calibration air. The biases 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. In our experiments with the commonly used droplet method, this requirement was not fulfilled. Correcting CO2 measurements proved challenging, presumably because of our humidification method. Open questions pertain to differences among analyzers and variability over time. In our experiments, the biases amounted to considerable fractions of the WMO interlaboratory compatibility goals. Since measurements of dry air mole fractions of CO2 and CH4 are also subject to other uncertainties, correcting the cavity pressure-related biases helps keep the overall accuracy of measurements obtained with Picarro GHG analyzers in humid and potentially in Nafion-dried air within the WMO goals.
Highlights
Measurements of atmospheric greenhouse gas (GHG) mole fractions are integral data for quantifying their sources and sinks using inverse models of atmospheric transport (e.g., Kirschke et al, 2013; McGuire et al, 2012)
To ensure the high quality of GHG observations required for inverse models of atmospheric transport, the World Meteorological Organization (WMO) has set compatibility goals for atmospheric CO2 and CH4 measurements to ±0.1 ppm for CO2 (±0.05 ppm in the Southern Hemisphere) and ±2 ppb for CH4 (WMO, 2016) among laboratories
We first demonstrate the relevance of cavity pressure for CO2 and CH4 measurements performed with Picarro GHG analyzers and establish the sensitivities of the independent pressure monitoring methods to changes in cavity pressure (Sect. 3.1)
Summary
Measurements of atmospheric greenhouse gas (GHG) mole fractions are integral data for quantifying their sources and sinks using inverse models of atmospheric transport (e.g., Kirschke et al, 2013; McGuire et al, 2012). To ensure the high quality of GHG observations required for inverse models of atmospheric transport, the World Meteorological Organization (WMO) has set compatibility goals for atmospheric CO2 and CH4 measurements to ±0.1 ppm for CO2 (±0.05 ppm in the Southern Hemisphere) and ±2 ppb for CH4 (WMO, 2016) among laboratories. This compatibility is ensured if individual laboratories keep errors of measurements with respect to a common calibration scale below half of these goals, which corresponds to the so-called internal reproducibility goals (WMO, 2016). Water vapor is excluded because its variability would mask signals in the GHGs
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