to measure biosphere-atmosphere exchange of isoprene and other biological trace gases

The micrometeorological flux measurement technique known as relaxed eddy accumulation (REA) holds promise as a powerful new tool for ecologists. The more popular eddy covariance (eddy correlation) tech- nique requires the use of sensors that can respond at fast rates (10 Hz), and these are unavailable for many eco- logically relevant compounds. In contrast, the use of REA allows flux measurement with sensors that have much slower response time, such as gas chromatography and mass spectrometry. In this review, relevant micro- meteorological details underlying REA are presented, and critical analytical and system design details are discussed, with the goal of introducing the technique and its potential applications to ecologists. The validity of REA for measuring fluxes of isoprene, a photochemi- cally reactive hydrocarbon emitted by several plant species, was tested with measurements over an oak- hickory forest in the Walker Branch Watershed in eastern Tennessee. Concurrent eddy covariance mea- surements of isoprene flux were made using a newly available chemiluminesence instrument. Excellent agreement was obtained between the two techniques (r 2 a 0.974, n a 62), providing the first direct com- parison between REA and eddy covariance for mea- suring the flux rate of a reactive compound. The influ- ence of a bias in vertical wind velocity on the accuracy of REA was examined. This bias has been thought to be a source of significant error in the past. Measurements of normalized bias (w=rw) alone would lead us to think that a large potential error exists at this site. However, with our isoprene data and through simulations of REA with fast-response H2O and CO2 data, we conclude that accurate REA flux measurements can be made even in the presence of a bias in w.