What is a carbon flux system

A carbon flux system is an integrated system of components used to measure, record, and process the carbon dioxide (CO2) or methane (CH4) exchange between the biosphere and atmosphere in the surface layer of the atmosphere. These systems are used over different land types (such as forests, grasslands, crops, and shrubs) across the globe to provide information needed for management and research purposes. Carbon flux systems that offer a rugged design, low power requirement, low maintenance, and network monitoring software can operate remotely for long periods without the need for expensive site visits.

How does a carbon flux system work?

The carbon flux systems offered by Campbell Scientific measure the carbon exchange between the biosphere and atmosphere using the eddy-covariance technique. This technique relies on a high-resolution, fast-response 3-D sonic anemometer, as well as a fast-response gas analyzer, data logger, and flux processing program.

In some ecosystems, profile measurements of carbon dioxide, water vapor, and temperature are measured, and a carbon storage flux term is identified.

What are the different components of a carbon flux system?

A typical carbon flux system, using the eddy-covariance technique, comprises a variety of components, including the following:

• • High-resolution, fast-response 3-D sonic anemometer
  • Fast-response gas analyzer
  • Storage flux system
  • Data logger
  • Flux processing program
  • Surface energy balance sensors
  • Ancillary biometeorology sensors
  • Mounting structures
  • Remote communications

3-D sonic anemometer

A 3-D sonic anemometer (such as Campbell Scientific’s CSAT3B, CSAT3A, or IRGASON®) measures three orthogonal wind components and the speed of sound by determining the time of flight of sound between three pairs of transducers. In a carbon flux system, the turbulent fluctuations of vertical wind measured by the 3-D sonic anemometer are used in conjunction with an infrared gas analyzer to estimate the magnitude and direction of CO2 exchange. This instrument must have adequate frequency response to capture small eddies in the atmosphere. All of Campbell Scientific’s sonic anemometers have adequate frequency response to be used in carbon flux systems.

Gas analyzer

A gas analyzer in a carbon flux system measures the scalar component of the flux. Campbell Scientific’s gas analyzers for measuring CO₂ exchange (IRGASON^®, EC150 and EC155) are non-dispersive mid-infrared (NDIR) absorption analyzers. A source of infrared radiation is passed along an optical path to a detector. Interference filters are used to filter light wavelengths that correspond to CO₂ absorption and CO₂ transmittance. Thus, the dual wavelength nature of the analyzers provides both a reference and sample reading without the need for a separate reference cell and detector.

Rather than using NDIR, a tunable diode laser is used (TDL) to measure CH₄ exchange. Lasers used in Campbell Scientific’s TGA200A analyzer are interband cascade lasers (ICL) and can be chosen for target wavelengths between 3 to 6 µm. This mid-infrared range captures where many important trace gases have their peak absorption (CO₂, N₂O, and CH₄).

Gas analyzers used for measuring carbon flux with the eddy-covariance method must be aerodynamic and sample fast enough to resolve small eddies.

• • At Campbell Scientific, we have designed our open-path gas analyzers to be aerodynamic and minimize flow distortion compared to other designs that place a large bluff-bodied gas analyzer adjacent to the sampling volume of the sonic anemometer.
  • The closed-path design also minimizes flow distortion by minimizing the size of the sample intake and inlet.

Gas analyzers must also be able to respond rapidly to small eddies in the atmosphere. The IRGASON^®, EC150, EC155, and TGA200A all have adequate frequency response to be used in carbon flux systems.

Storage flux system

During periods of little to no turbulence, especially during the night, there can be a significant contribution of the flux that cannot be captured by the gas analyzer and sonic anemometer listed above. The lack of turbulence causes a gradient buildup of CO2 that is not transported by eddies into the sensing volume of the instruments. This gradient remains until the sun rises and turbulence is restored. The result of the above cycle results in apparent periods of little to no flux followed by a large contribution to the flux. With a profile system, such as Campbell Scientific’s AP200, a gradient of CO2 flux can be measured with a slower response gas analyzer, such as the LI-840A (LI-COR Biosciences, Lincoln NE) and several intakes. This gradient can then be integrated over time, with a storage flux term calculated and added to the daily flux total. The result is a more accurate CO­2 flux and better estimation of net ecosystem exchange (NEE).

Data Logger

In almost all applications, Campbell Scientific carbon flux systems operate in conjunction with a data logger (that is, the CR3000, CR6, CR1000, or CR1000X).

• For the open-path carbon flux systems and closed-path CH₄ flux system, the data logger is used to do the following:
  • Store the raw data.
  • Process the raw data and store fluxes.
  • Allow for remote communications to the station.
  • Provide diagnostic information about the system.
• For the closed-path CO₂ flux systems, in addition to those functions listed above, the data logger also provides the control for the pump flow and valve switching (for zero/span calibration).
• For the storage flux system, the data logger provides all the same functions listed above for the open-path system and additionally controls the valve switching and timing for the profile gradient measurements.

Flux processing program

The data loggers used with Campbell Scientific carbon flux systems require a program for data storage and system operation. For the open- and closed-path carbon flux system, this program is provided as part of our EasyFlux^® DL offering. EasyFlux^® DL is a CRBasic program that enables the data logger to report fully corrected carbon fluxes processed from raw high frequency time series data by applying commonly used corrections found in scientific literature. Additionally, this program stores diagnostic and calibration information.

For the storage flux system, a program is provided that offers system control, diagnostic and calibration information storage, and profile site concentration averages that can be used to calculate a daily storage value.

Surface energy balance sensors

Carbon flux systems are often equipped with additional sensors to measure the surface energy balance. The surface energy balance consists of the net radiation (Rn), soil heat flux (G), latent heat flux (LE), and sensible heat flux (H). A CNR4-L, NR01-L, or NR-LITE2-L (from Campbell Scientific) is used to measure the net radiation. In a Campbell Scientific carbon flux system, HFP01-L or HFP01SC-L heat flux plates are used with CS616 or CS65(x) water content reflectometers and TCAV-L averaging thermocouples to calculate the ground heat flux. The gas analyzer and sonic anemometer provide carbon flux measurements for the latent and sensible heat flux. When an examination of the surface energy balance is performed, it checks for a conservation of energy and can provide estimates of flux reliability or bias in the measurements.

Biometeorology sensors

In conjunction with the surface energy balance sensors discussed in the previous section, additional biometeorology sensors are often used with the carbon flux system. In a Campbell Scientific carbon flux system, these include temperature and relative humidity probes (such as the HMP155A-L or EE181-L), rain gages (such as the TE525-L), and infrared leaf radiometers (such as the SI-111). Data from these sensors are important for gap filling carbon flux data and for understanding environmental conditions.

Mounting structures

Campbell Scientific offers tripods and towers for mounting carbon flux systems ranging in height from 2 to 9 m. These offerings include the CM106B, CM110, CM120, UT10, UT20, and UT30.

Remote communications

Cellular, radio, or satellite telemetry can be used for remote communications to carbon flux systems. This can allow for data monitoring as well as data transfer of fully processed fluxes. Moreover, with the addition of EasyFlux^® Web, it is easier to monitor institution networks of remote carbon flux systems from anywhere in the world.