Joseph Berry

Understanding the Global Carbon Budget

The year-to-year increase in the CO₂ concentration of the atmosphere is largely driven by combustion of fossil fuels and deforestation. However, natural processes such as exchange of CO₂ with the ocean, growth of plants (driven by photosynthesis), respiration of living organisms, and decomposition also play a key role. These processes, which constitute the Earth's carbon cycle, account for about 95% of the total exchange of CO₂ with the atmosphere. In preindustrial times, the global carbon cycle was balanced and the CO₂ concentration of the atmosphere was stable. At present, approximately two-thirds of the CO₂ added to the atmosphere by human activities is sequestered by these natural processes -- presumably as bicarbonate in the ocean, wood in the forests, and organic matter in the soil. The remaining one-third accumulates in the atmosphere, perturbing the radiative properties of the atmosphere and possibly altering the climate. CO₂ sequestration by the global carbon cycle is, therefore, a major factor mitigating the potential impact of human activity on climate. The future course of anthropogenic climate change depends in no small part on how these natural components of the Earth's carbon cycle respond in the future. A great deal of effort is now focused on understanding where the carbon is going and in establishing a system that will permit us to monitor the dynamics of carbon cycling by the oceans and terrestrial ecosystems of the world.

The most powerful approach developed to date is based on a "top-down" analysis of the dynamics of CO₂ and other tracers in the atmosphere. Flask samples of air are collected at frequent intervals at stations located around the globe and sent to central laboratories operated by NOAA, Scripps Institution of Oceanography, and CSIRO. Analyses of these samples, for O₂ and CO₂ concentrations and other trace gases, provide important constraints on inversion calculations that attempt to determine the net uptake or release of CO₂ from surface reservoirs and to locate these sources and sinks. One of the key measurements is the ¹³C and ¹⁸O isotopes of CO₂. Great care is taken in the measurement protocols used to analyze the flasks so that the isotopic measurements are of the highest possible accuracy and to provide standards so that measurements conducted at different times or in different labs are comparable. However, these methods require large samples of air (typically 2 liters), and the measurements are labor intensive.

It is widely recognized that these global measurements need to be supplemented by process-level studies of isotopic effects at the regional, ecosystem and plant scales. Such data is needed to calibrate and test the models of isotopic fractionation used to interpret the global observations. Work at these scales needs to be conducted with a precision similar to that of the global-scale networks; even a modest level of experimentation could easily generate as many samples as are now processed by the global sampling programs. While the managers of these global programs are sympathetic to the need for local-scale measurements, they simply do not have the capacity to fill this need. Ecosystem scientists have been reluctant to make the large commitments of laboratory space, equipment, and personnel to duplicate these facilities.

This report describes a new technology for measurements of CO₂ concentration and isotopic composition of CO₂ in air samples. This technology is specifically intended for ecosystem and plant or leaf-scale measurements. Significantly, the technology uses very small samples of air (less than 5 ml for a complete analysis) while achieving levels of accuracy and cross-calibration that approach those of the flask networks. Most important, the procedure can be automated. In this new method, the batch inlet system of a conventional isotope ratio mass spectrometer is replaced with a continuous flowing helium inlet system. Samples are introduced as plugs in the flow of helium, and the isotope ratio is calculated from measurement of the "peaks" at m/e 44, 45, and 46 that are recorded when a sample of CO₂ passes through the mass spectrometer source. Typically, the quantity of the sample required for a measurement with the flowing helium inlet is about 10⁵ that required for a measurement with a batch inlet system. The system described here functions to isolate the CO₂ from a given quantity of air and to release that CO₂ into a stream of helium that carries the sample into the mass spectrometer. Additionally, by measuring the amount of air used and the size of the CO₂ peak, one can calculate the CO₂ concentration in a sample.

A schematic diagram of the system is shown in Fig. 1. Air containing CO₂ is withdrawn from a flask and flows into an evacuated volume through a capillary tube immersed in liquid nitrogen. CO₂ is quantitatively trapped from the air, and it is rapidly released when the trap is withdrawn from the liquid nitrogen.

Fig. 1. A two-position-six port Valco valve (1) is used to switch the connections between a "trapping state," with the capillary connecting the sample or standard flask to the vacuum chamber, and a "CO₂ release state," with the helium flow directed through the trapping capillary and on the mass spectrometer. An air-actuated plunger (5) is used to submerge or withdraw the trapping capillary in a liquid nitrogen dewar.

With the six-port valve in the trapping state, air is permitted to bleed through the trap and into the vacuum chamber. During this time the valve diverts the flow of helium directly to the mass spectrometer. Pneumatically activated microvalves (2) provide on/off control on the flow of air from either the sample or the standard flasks. The quantity of air sampled is measured manometrically with a high-precision capacitance manometer (0-100 mbar). Once a preselected quantity of air is admitted, the inlet valve is closed, the pressure is measured, and the residual air in the system is pumped away. The six-port valve is then switched to the release state. The trap is withdrawn from the dewar, and a microswitch is tripped, permitting a current to flow through the stainless steel capillary. This current heats the capillary to about 80°C, quickly volatilizing all of the CO₂ and water that had been trapped. A nafion drier (7) downstream of the trap removes the water and a capillary gas chromatography column separates the CO₂ from the N₂O before the gas reaches the mass spectrometer. By alternately sampling air from the sample flask and one containing a standard, both the CO₂ concentration and the isotope ratio of the sample can be referenced to other measurements. The CO₂ concentration is determined to a precision of 0.5 ppm and the isotope ratios are determined to a precision of 0.05 parts per mille.

All of the valves can be computer controlled, permitting the process to be automated. Each cycle of trapping and release typically takes about four minutes. A typical measurement sequence consists of 10 cycles, 5 each of the sample and a flask containing a standard air. A multiposition valve (not shown) can be used to permit analysis of several sample flasks overnight.