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Relaxed Eddy Accumulation

Relaxed Eddy Accumulation

Relaxed eddy accumulation (REA) is a conditional sampling technique used to measure the exchange of chemical species between the atmosphere and terrestrial sources or sinks -- the vertical flux.

Because air tends to roll as the wind blows across the ground, there is a rising and falling component to its motion. These eddy currents transport chemical species between things near the ground and the atmosphere. An eddy can pick up chemicals close to the ground and carry them aloft as well as carry chemicals toward the ground where they are sequestered. For most chemical species, their atmospheric concentrations are reduced by both dilution and chemical reactions.

When the transfer is into the atmosphere, the flux is said to be positive, and negative when the atmosphere is the source. An example of a positive flux would be the oxygen expired by living plants while their carbon dioxide uptake is a negative flux.

Eddy accumulation systems collect two segregated samples, one extracted from the rising air, the other from the falling currents. The flux is deduced, in part, from the difference in concentration between the two samples.

Early eddy accumulation systems collect samples at a rate proportional to the instant vertical velocity of the air. A higher velocity means a greater flow into the sample accumulator. Because of the proportional flow requirement these systems have been hard to implement.

In 1990, Joost A. Businger and Steven Oncley proposed an approach that relaxed this proportional flow requirement. They concluded that since a small range of eddy size is responsible for most of the mass-transfer, sample gas collection flow rates can be constant and need not vary with vertical velocity.

Their Relaxed Eddy Accumulator allows the use of simple on/off valves and constant flow pumps. When the upward velocity exceeds the "up" threshold,the "up" accumulator is filled at a fixed rate. Conversely,if the downward velocity exceeds the "down" threshold, the "down" accumulator is filled at the same fixed rate. When the vertical velocity is in the "midband" between the two thresholds, the sample is usually discarded. Businger was issued US Patent 5,067,356 for this technique.

Flux calculations using REA systems follow the basic formula:

F = SD_WindSpeed x (C_up - C_down) x b.

Where:

F = Flux

SD_WindSpeed = standard deviation of the vertical wind velocity

C_up = average concentration of the updraft sample

C_down = average concentration of the down-draft sample

b = a constant ( often around .6)

[See: Businger & Oncley, J. Atmos. & Oceanic Tech., 349-352, 1990; also, Businger, 1991, U.S. Patent 5,067,356]

In 1991, while at the National Center for Atmospheric Research (NCAR), Gary Hampton began the development of practical REA systems. In 1992, Hampton tested his first REA as part of a flux measurement inter-comparison study conducted at the National Oceanic and Atmospheric Administration’s (NOAA) Oak Ridge, Tennessee research site. Scientists from NCAR, Washington State University (WSU), NOAA, and the U.S. Environmental Protection Agency (EPA) measured isoprene fluxes using five methods with fairly consistent results. Additional efficacy verification was done by Oncley, Delany, Horst & Tans while at: Atmospheric Technology Division of NCAR [see, Verification of flux measurement using relaxed eddy accumulation, Atmos. Environ., 27A, 2417-2426, 1993].

In 1994 Hampton formed a new company to continue system development. They have advanced REA technology by adding Digital Eddy Capture Control, High Volume Flow Control and Internet Connected Supervision. His patented Constant Flow Control (US Patent 5,269,659) is being upgraded using new technology. Current REA Systems are easily adaptable for a wide range of atmospheric species and can be tailored to accommodate sample collection techniques that include inert fluoro-plastic bags, absorbent cartridges or filters.

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