Submitted to: Agronomy Journal
Publication Type: Peer reviewed journal
Publication Acceptance Date: 2/26/1996
Publication Date: N/A
Citation: Interpretive Summary: Carbon dioxide is a critical determinant of crop growth and it is also one of the key factors in global climate change, because it absorbs longwave radiation and its atmospheric concentration in increasing. Consequently there are many research areas in which carbon dioxide is a key variable, but there are only a few ways to measure it and they are generally expensive. One inexpensive alternative has been to equilibrate air with water, then measure the electrical conductivity of the water, which increases with increasing carbon dioxide. An impediment to use of this method has been a lack of understanding of it, and the apparent need for frequent empirical calibration against more expensive methods. We have developed the fundamental theory that makes the measurement possible, and have shown that it is independent of any empiricism. Hence there is no need to calibrate against other methods. In the process, we have developed a system that is accurate, and that can be automated and multiplexed, so that measurements can be made automatically from a number of different sampling points. The system uses a time domain reflectometer (TDR) to make the measurement. TDR is also useful for measuring soil moisture and soil salinity, so a single instrument can now make an array of environmental measurements. These results should be useful to many scientists studying topics related to carbon dioxide.
Technical Abstract: Atmospheric carbon dioxide is critical to plant growth, and it also plays a key role in the global energy balance. These are active research areas because mean atmosphere concentration is rising. Thus there is a need for reliable methods for measuring CO2 concentrations ([CO2]). One approach is the conductimetric method, where sampled gas is bubbled through deionized water. Some CO2 dissolves and ionizes, causing an increase in solution electrical conductivity. The method is inexpensive relative to other techniques, but usage has been limited, possibly due to its apparently empirical basis and suggestions that it must be frequently recalibrated due to temperature and other effects. We derive the fundamental basis for the method, and show that [CO2] can be directly determined from measurements of solution electrical conductivity and temperature, with no empirical calibration. The required equilibrium constants and ionic conductivities are all temperature dependent, but those dependencies are well known and easily computed. The approach was tested with a system in which the conductivity measurement was made with a time domain reflectometer (TDR), using a coaxial cell through which the aerated water was circulated. The system was compared against an infrared gas analyzer (IRGA) over a range of [CO2] at three different temperatures. Regression of [CO2] calculated directly from the conductivity measurements against the IRGA measurements produced a slope of 1.00, with r2 of 0.996 and a standard error of estimate of 19.4 umol mol**-1. The approach that is described should work with any suitably accurate device for measuring solution conductivity.