Using Molecular Sieve to Zero Infrared Gas Analyzers for Eddy Covariance or Atmospheric Profile Measurements

Infrared gas analyzers (IRGAs) used to measure carbon dioxide (CO2) or water vapor for eddy covariance (EC) flux or atmospheric profile applications can drift over time. It is important to regularly monitor and set their zero (offset) and span (gain). The span of an IRGA is generally more stable than its zero, but even an error in the zero can lead to errors in EC flux and storage flux estimates. Most commonly the zero and CO2 span are checked using cylinders of compressed gasses, and water vapor span is checked with a dewpoint generator. Cylinders of compressed gases are difficult to transport to a field site because of their bulk, weight, and safety issues, and dewpoint generators are generally not designed for field use. Chemicals such as magnesium perchlorate and sodium hydroxide can be used to scrub water vapor and carbon dioxide from ambient air to zero an IRGA, but these chemicals are difficult to transport, use, and dispose of safely. Many IRGAs used in the field are not checked regularly because of these difficulties. Molecular sieve can provide a safer, more convenient alternative for zeroing an IRGA in situ. Three example configurations are discussed: a field-rugged scrub module that includes a 2 L min-1 pump for automated on-line zeroing of a closed-path EC system, a field-rugged scrub bottle with no pump for automated on-line zeroing of an atmospheric profile system, and a light-weight (1.16 kg), hand-held zero air generator with a battery-operated, 0.2 L min-1 pump for manually zeroing an open-path IRGA. The molecular sieve provided air with CO2 and water vapor concentrations equal to or lower than ultra-zero grade air, and required very little maintenance.