How do we measure CO2 in water and the atmosphere?
Earth breathes carbon dioxide in and out over many different time scales; there are daily inhales, seasonal exhales, as well as deep breaths over much larger time scales. Therefore, to get accurate measurements of carbon dioxide concentrations on Earth, there has to be long term periodic sampling and data collection in both the ocean and the atmosphere. As part of the National Oceanic and Atmospheric Administration (NOAA) World Climate Research Programme’s Climate Variability and Predictability project, research vessels have been patrolling the oceans for the past 30 years (see figure 1 below), in an effort to collect data on CO2 concentrations.
One of the most frequently used instruments that scientists use on these long expeditions is called a CTD rosette, which stands for conductivity, temperature and depth (see figure 2). The rosette is a large metal frame that usually holds between 30 and 40 sample bottles that can be lowered into the ocean, filled with samples of sea water at varying depths and brought back to the surface to be collected and analyzed. The samples can be analyzed for indicators of chemical or biological processes including chlorophyll fluorescence, particulate organic carbon and dissolved oxygen concentrations. The sampling process is repeated sometimes as frequently as every 30 nautical miles and by the time a research cruise has been completed, the scientists will have collected an average of about 50,000 samples.
Carbon dioxide is a very different entity in the ocean than it is in the atmosphere, therefore the methods for measuring it in both environments has to be different as well. At the Mauna Loa Observatory in 1957, Dave Keeling was the first to make accurate measurements of CO2 in the atmosphere and measurements have been taken there ever since (see figure 3). A CO2 analyzer is used in the observatory’s lab by measuring infrared absorption using the Non Dispersive Infrared (NDI) detection method. This process starts with light passing through two sample cells; one containing a reference gas like nitrogen and the other containing the gas to be analyzed (see figure 4). Once the light passes through both cells it hits a detector where the amount of light is used as an indicator of how much carbon dioxide was present in the sample; more CO2 in a sample will cause more absorption which leads to less light hitting the detector.
Figure 1. NOAA’s research cruise paths for the past 30 years as part of the World Climate Research Programme’s Climate Variability and Predictability project.
Figure 2. Crew members aboard the R/V Roger Revelle retrieve a CTD rosette from the frigid waters of the Southern Ocean. As the device is lowered into the ocean, electronic instruments measure salinity, temperature, and depth. Each of the white bottles collects seawater at different depths for detailed analysis.
Figure 3. The results from atmospheric analysis of CO2 concentrations at the Mauna Loa Observatory from 1958 to present. https://scripps.ucsd.edu/programs/keelingcurve/
Figure 4. Non Dispersive Infrared (NDI) detection process starts with infrared light passing through two cells; one contains a reference gas like nitrogen and the other contains the gas to be analyzed.