Tutorial
Measuring CO2
Atmospheric carbon dioxide can be accurately measured using the right equipment. Here we present a more technical description of the methods employed for measurement. Below you will find the basics, the instrument, and the system. These sections describe the properties and background neccessary to fully understand how CO2 is measured, the instrument that employes the basics, and the system neccessary for a precise and accurate CO2 measurement, respectively.
The Basics. Most all measurements of carbon dioxide take advantage of this molecules absorption of infrared radiation or heat. Heat is a form of enegy with a specific wavelength.
electromagnetic spectrum
co2 molecule
co2 molecule
co2 molecule
co2 molecule
When infrared energy is applied to a carbon dioxide molecule, some of that energy is used when the molecule moves as these images illustrate.

While you could make a CO2 molecule listen to music (radio waves), or show it a photo (visible radiation), or take its x-ray, the only energy that will elicit this response is infrared or heat.

The Instrument.
The instrument used for carbon dioxide measurement is typically an infrared gas analyzer. These come in many shapes and sizes.
instrument
instrument
instrument
The premise to most infrared gas analyzers is the same. A beam of infrared radiation is shown along a path through which the air sample of interest is passed, while an infrared radiation detector (opposing the beam) measures the quantity of infrared radiation. The greater the quantity of carbon dioxide within the path, the less infrared radiation is detected. Below is a schematic of a simple infrared gas analyzer.

irga schematic

The System. Measuring CO2 as accurately and precisely as possible requires many additional pieces. An infrared gas analyzer is influenced by temperature and pressure of the path. If the path contains air with 380 parts per million (ppm) CO2, and then the temperature or pressure increase, the apparent concentration will increase even though the concentration of CO2 has not changed. For this reason, one must periodically flow air through the analyzer of known concentration. It helps to have the analyzer wired to a data logging device and other control electronics to allow for automation. Below is a schematic of our UofU setup. The compressed air cylinders have a known concentration of CO2 and are used to purge the path of the analyzer periodically. The analyzer is wired to a datalogger. Other features such as meteorological sensors, allow for interpretation of any observed variation in atmospheric CO2. system schematic