Surface Flux Monitoring

The eddy covariance technique has become a popular tool for evaluating net CO₂ exchange from terrestrial ecosystems to the atmosphere, and in recent years, it has been tested for its potential ability to detect CO₂ releases from underground storage reservoirs. Instruments mounted on towers above the land surface are used to measure CO₂ gas concentration, vertical wind speed, relative humidity, and temperature. Carbon dioxide flux is then calculated from these field measurements based on the covariance of CO₂ concentration and instantaneous vertical wind velocity above or below their mean values. Depending on the height of the towers, the resulting CO₂ flux estimates provide a spatial average for an area of up to several square kilometers. Data can be integrated over the time period of interest, which may be several days to a year or more.¹

Surface Flux Tool Summary

Description: Flux measurement technique used to measure atmospheric CO₂ concentrations at a specified height above the ground surface.
Benefits: Can provide continuous data, averaged over both time and space, over a large area (hundreds of meters to several kilometers).
Challenges: Specialized equipment and robust data processing are required. Natural spatial and temporal variability in CO₂ flux may mask release signal.²

U.S. Geological Survey scientists service an eddy covariance station located in a tidal freshwater marsh in Louisiana. These stations allow agency scientists to assess the net exchange of CO₂ and CH₄ from wetlands over large spatial scales, and help determine the carbon sequestration capacity of managed marshes. These techniques can also be adapted to monitor carbon storage sites.³

An eddy covariance system situated atop a tower, consisting of an ultrasonic anemometer (upper left) to measure three-dimensional air flow and infrared gas analyzer (lower right) to measure CO₂.⁴

Application

The eddy covariance method is a micrometeorological technique for direct measurements of gas transport between the surface and the atmosphere. It is widely used for quantifying CO₂ emission rates from natural, urban and agricultural ecosystems, including areas of agricultural carbon sequestration. In geological carbon storage projects, the method can be an important monitoring tool in large areas where CO₂ may escape from the subsurface to detect and quantify CO₂ leakage.⁵

Eddy covariance stations allow direct automated measurements of the gas emissions from a specific territory reported in weight or volume of gas per unit area per unit time. Industrial applications, such as a CO₂ storage project, often involve large territories with complex surfaces, so more than one station may be required for confident measurements of fluxes from these large areas.⁶

An example of a novel use of the eddy covariance method for monitoring is at the Archer Daniels Midland Company’s carbon storage project in Decatur, Illinois. An eddy covariance system has been in use since June 2009. Baseline data from eddy covariance monitoring was used to characterize pre-injection conditions, and was then used to look for any changes in net exchange CO₂ fluxes that could be the result of CO₂ leakage into the near-surface environment during or following injection.⁷ No anomalies related to CO₂ injection were identified as part of eddy covariance and related monitoring at the carbon storage project in Decatur.

Eddy covariance monitoring station at the Archer Daniels Midland Company’s carbon storage project in Decatur, Illinois (left)⁸ and map (right) showing location of the eddy covariance tower (red triangle) and CO2 injection well (blue square).⁹

Image Credits

surface flux image 1: Ken W. Krauss, U.S. Geological Survey
Surface-flux-monitoring-Eddy-covariance-Decatur-EDIT: National Energy Technology Laboratory