Correcting atmospheric CO<sub>2</sub> and CH<sub>4</sub> mole fractions obtained with Picarro analyzers for sensitivity of cavity pressure to water vapor

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.

Accurate measurements of atmospheric carbon dioxide and methane mole fractions at the Siberian coastal site Ambarchik

“…For the case studies shown here, 15 d back trajectories were calculated for the period from August 2014 to December 2015. Atmospheric transport was modeled using STILT (Lin et al, 2003) driven by WRF (Skamarock et al, 2008), for which boundary and initial conditions were taken from MERRA reanalysis fields (Rienecker et al, 2011). The resolution of the transport model in our domain was mostly 10 km horizontally with 41 vertical levels.…”

Evaluation and optimization of ICOS atmospheric station data as part of the labeling process

“…Indeed after about six hours of dry air injection, the cavity is extremely dry compared to the usual injections after wet ambient air. This effect seen only on Cavity Ring-Down Spectroscopy (CRDS) analyzers (which however make the majority of the CO 2 /CH 4 /CO instruments in ICOS Atmosphere) is thought to be due to residual water on the pressure sensor in the instrument cavity (Reum et al, 2019). The extent of the effect is depending on the analyzer, and thus this effect is really important to assess as it allows to improve the bias estimate based on the target.…”

Recent advances in measurement techniques for atmospheric carbon monoxide and nitrous oxide observations

“…as described by Reum et al (2019) or as implemented by the ICOS Metrology Lab, which is using a Bronkhorst Vapour Delivery Module (VDM) able to humidify a gas stream from a tank, might give better results. In addition to improvements of the droplet method, alternative ways to compensate for the water vapour dependent CO bias need to be explored.…”