Disjunct eddy sampling approach for eddy accumulation and eddy covariance measurements

Janne Rinne

NCAR/ Finnish Weather Service

Traditional trace gas eddy flux measurement techniques require either fast sensors or fast valve switching and flow-controlling abilities. It is, however, also possible to calculate the fluxes using a small subset of the continuous time series. In disjunct eddy sampling, grab samples are taken very quickly with relatively long time intervals between them. Although not continuous, the resulting subset is statistically equivalent to the continuous result and relaxes some of the technical requirements of flux measurements

The disjunct eddy sampling approach has been applied to the true eddy accumulation and the eddy covariance method. In both systems, air is sampled in 0.2 seconds into intermediate storage reservoirs periodically at 20-30 second intervals. The sample is then processed either by accumulating a part of it into a solid adsorbent cartridge proportionally to the vertical wind velocity, or by analyzing it on-line with an analyzer with response time between 1 and 30 seconds. Design and operation of these systems will be described and the advantages and limitations of the disjunct eddy sampling methods will be discussed. Some flux data collected using the instruments will be presented.