Errors in differential infrared carbon dioxide analysis resulting from water vapor

Area of Science:

• Environmental Science
• Analytical Chemistry
• Atmospheric Science

Background:

• Differential infrared analyzers are commonly used for measuring carbon dioxide (CO2) concentrations.
• Accurate CO2 measurements are crucial for environmental monitoring and climate studies.
• Water vapor can interfere with CO2 measurements in infrared gas analyzers.

Purpose of the Study:

• To investigate the impact of water vapor on the accuracy of differential infrared carbon dioxide analyzers.
• To quantify potential errors in CO2 differential measurements caused by water vapor.
• To determine how CO2 concentration, water vapor content, and dewpoint affect measurement accuracy.

Main Methods:

• Differential infrared carbon dioxide analyzers from three different manufacturers were tested.
• Water vapor was introduced into the gas streams entering the analyzer cells.
• Analyzer responses were compared to theoretical predictions based on gas dilution.
• Tests were conducted across various CO2 concentrations (0, 365, 730 cm³ m⁻³) and dewpoints (15, 20, 25°C).

Main Results:

• None of the tested analyzers consistently responded as predicted by simple dilution models.
• Significant errors, on the order of a few cm³ m⁻³, were observed in CO2 differential estimates.
• The magnitude of these errors varied depending on the CO2 concentration range, the difference in water content between cells, and sometimes the dewpoint range.

Conclusions:

• The theoretical correction for water vapor dilution is insufficient for accurate CO2 differential measurements with these analyzers.
• Water vapor introduces systematic errors in CO2 measurements that are dependent on operating conditions.
• Careful calibration and consideration of water vapor effects are necessary for reliable CO2 analysis in varying atmospheric conditions.