| 14 Jul 2026
Portable system for characterizing greenhouse gas analyzers for sensitivity to water vapor
Abstract. Water vapor introduces dilution, spectral interference and pressure-broadening effects in greenhouse gas (GHG) measurements using infrared (IR) techniques. Accurate water vapor corrections are therefore required for measurements of ambient air samples that are undried or incompletely dried as is the case for commonly used permeation water exchangers. In this study, we developed a lightweight, low-cost, and portable system for performing water vapor corrections of GHG analyzers in both laboratory and field environments. The system uses a permeable membrane-based moisture exchanger (Permapure, Inc., BE series) to humidify gas from a compressed cylinder with known analyte mole fractions. This humidified stream is mixed with a parallel dry stream from the same cylinder to produce air with controlled water vapor levels. Water-vapor (H2O) concentrations are held constant for a defined interval and then stepped across a range of user-defined levels. Gas flow rates are regulated using mass flow controllers, and a Raspberry Pi-based Linux system automates the sequence of water vapor steps and records measurement data for subsequent analysis. Compared to previously available dew point generators, the system provides substantially improved water vapor stability (approximately 3 ppm H2O). The system was used to characterize water vapor corrections for Aeris MIRA Ultra and Picarro cavity ring-down spectroscopy (CRDS) analyzers. The two tested Aeris MIRA Ultra analyzers exhibited quadratic relationships for methane (CH4), with distinct behaviors observed between low- and high-H2O regimes, with errors of up to 30 ppb CH4 at 3 % H2O if uncorrected. However, these two tested Aeris MIRA Ultra analyzers did not show clear dependence on water vapor for ethane (C2H6). Across the investigated humidity range, all three tested Picarro analyzers showed a nearly linear dependence for carbon dioxide (CO2), and both analyzers tested for CH4 exhibited a similar near-linear response. For carbon monoxide (CO), however, two of the three tested analyzers displayed a quadratic humidity dependence, and one did not show any dependence on water vapor. For the Picarro analyzers, the errors if uncorrected for water vapor response were as large as 0.25 ppm CO2, 2.0 ppb CH4, and 12 ppb CO, all at 1.7 % H2O. After applying the derived analyzer-specific water vapor correction functions to all datasets, the corrected CH4, CO2, and CO measurements from Aeris MIRA Ultra and Picarro analyzers were well within the WMO/GAW inter-laboratory compatibility goals. These results demonstrate that the system provides a practical and reliable approach for conducting water vapor corrections for GHG analyzers in laboratory and field applications.
Competing interests: The authors declare that a provisional patent application related to the P-WAVES system described in this manuscript has been filed by the Pennsylvania State University.
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