The attenuation of fluctuations in scalar concentrations through sampling tubes

Abstract

We present results of a laboratory investigation of the attenuation of humidity fluctuations in tubing. The purpose is to obtain simple formulations for use in turbulent flux measurement applications, where the measurement of a trace species may be remote from the sampling point so that the sample must be transported through a tube to the sensor. The laboratory data are compared to the attenuation predicted from Taylor's (1953 and 1954) formulations for the virtual longitudinal eddy diffusivity of a constituent introduced into a pipe, for both laminar and turbulent flow, by plotting the normalized half‐power frequency against Reynolds number. Results were first obtained for fully developed equilibrium flow (both laminar and turbulent) through a straight tube. The following modifications were then studied: an intake section, a 90° elbow bend, and curved tubes (bending through 90°) with tube‐radius to radius‐of‐curvature ratios of a/R = 0.034 and 0.081. The effect, if any, of these modifications is to increase the half‐power frequency over that of the equivalent length of straight tubing. Applying this analysis to a situation where a sample is collected on a tower and transported to an instrument at the base of the tower, we find that as long as the flow in the tube is turbulent, the attenuation in the tubing should not be a significant factor for introducing errors in flux measurements in the unstably stratified atmospheric surface layer.

Carbon budgets for an irrigated intensively grazed dairy pasture and an unirrigated winter-grazed pasture

“…For each site, the fetch was homogeneous for at least 130 m in all directions, ensuring that a large fraction of the flux footprint was representative of the target paddock. Care was taken to minimise high-frequency attenuation (Lenschow and Raupach, 1991;Moore, 1986) with the use of a small measurement cell, short tube length (1.0 m) and high flow rate (12 L min −1 ). To minimise flux loss due to sensor separation, the intake tube was attached underneath the sensing volume of the sonic anemometer (Kristensen et al, 1997).…”

Seasonal variation of ozone deposition to a tropical rain forest in southwest Amazonia

“…The short inlet system of the O 3 analyzers partly damped the temperature fluctuations (Ammann, 1999) which had to be adequately considered. High frequency flux loss caused by the physical properties of sensors, the setup, and data acquisition was corrected for according to Moore (1986), Zeller et al (1989), Lenschow and Raupach (1991), and Horst (2000), using the semi-empirical spectral formulations of Kaimal et al (1972). Flux loss at low frequencies was estimated with a transfer function after Kristensen (1998) (see Aubinet et al, 2000) to be about 6% during the daytime, with lower values during the night.…”

Application of thermal-dissociation laser induced fluorescence (TD-LIF) to measurement of HNO<sub>3</sub>, Σalkyl nitrates, Σperoxy nitrates, and NO<sub>2</sub> fluxes using eddy covariance

“…When fluids pass through long sample tubes, the high frequency fluctuations are dampened by smearing due to the different speeds traveled by fluctuations being carried at different frequencies, or spectral attenuation (Lenschow and Raupach, 1991). We calculate the half-power fluctuation damping frequency of the sample tubing (1/8"i.d.)…”

“…We calculate the half-power fluctuation damping frequency of the sample tubing (1/8"i.d.) according to Lenschow and Raupach (1991); this is the frequency at which half the spectral power is lost due to attenuation. The halfpower damping frequency is ∼6 Hz for our system.…”