| 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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The manuscript “Portable system for characterizing greenhouse gas analyzers for sensitivity to water vapor” by Liu et al. claims to describe a “lightweight, low-cost, and portable system” for performing water vapor corrections of greenhouse gas analyzers.
The basic premise of their system is forming a “wet” stream of calibration gas using a Nafion-based humidity exchanger, then combining it with a “dry” stream at set ratios using mass flow controllers and valves. This is a description of common laboratory practice that has been used for decades in air pollution / air quality and VOC laboratories and in instrument development and testing, in commercially available instruments (e.g. 2B Technologies DewLine), and presumably also used already in the greenhouse gas community.
The claim to novelty, then, comes from the weight, cost, portability, and apparent stability of their system. They provide dimensions (30.5 cm x 30.5 cm x 20.3 cm, 5 kg) but it is not clear whether that includes all parts of the system (including the water and a battery) or not. They claim “low power requirements” but do not specifically test or report the power draw or make any claims for how long a typical battery-operated system could run in the field. They also provide no head-to-head comparisons of their system to existing, commercially available systems to provide an equivalent comparison.
The authors make a claim of “high stability” (3 ppmv H2O). However, in the manuscript they appear to only take the average of the last 60 seconds of a 3.5 minute step. It would be much more convincing if they reported a response time or equilibration time to humidity change and provided the long-term (e.g. 8 hour, 24 hour, etc.) stability of the system instead of 60 second transients. A 60-second transient does not show “high stability”. Depending upon the type of tubing used (polymer, stainless steel) and specific instruments used, the system response time to water vapor could be on the order of 60 seconds or longer. The apparent high stability could be a system-induced broadening kernel. The authors do not state what materials, tube diameters and lengths, etc. they used, nor do they provide any residence time distribution data to prove that their 60-second stability is actually the stability of their humidity generation and not a system artifact.
The authors claim “stable and repeatable humidity control…” but provide no statistics or demonstration of long-term stability and either short-term or long-term repeatability. As far as I can tell, their experiments were purely done in a climate-controlled laboratory environment. This performance might not transfer to field environments. In particular, the stability of the humidity generation will be highly sensitive to the temperature of the system, so ambient temperature fluctuations will adversely impact the stability of the system. It is not clear that the authors have thoroughly evaluated and addressed these impacts.
The authors do not use any independent humidity or dewpoint sensors (such as a chilled mirror or capacitive sensor) to confirm the accuracy or stability of their humidity generation. Instead, they rely on instrument-reported water vapor lines. However, they provide no calibration or traceability for these measurements. As a humidification system or humidity generator it has no inherent humidity measurement capability.
As mentioned earlier, much of the system (split flows, Nafion humidification, mass flow controllers to adjust humidity, etc.) are already common practice in the field. If the authors want to make a novel claim about the superiority of their system they need to provide additional details about the other factors (mechanical designs, CAD files, Python code, parts lists, etc.) to make it reproducible and independently testable. If the authors can address some of these concerns and provide more convincing data about the value of their system I think it would be a useful contribution to the field, especially if the technology is made available (public domain or commercially).
I do not address the Aeris and Picarro testing here, as those aspects are already well-covered in the scientific literature and well known to be dependent upon instrument, firmware / software version, etc. and not necessarily transferrable (although the calibration process and applied correction models could be).