Compressibility of air effects on eddy accumulation flux measurements

2020 ◽  
Author(s):  
Anas Emad ◽  
Lukas Siebicke
Keyword(s):  
Wind Velocity ◽  
Flow Rate ◽  
Vertical Wind ◽  
Design Stage ◽  
Old Growth Forest ◽  
Flux Measurements ◽  
System Configurations ◽  
The Impact ◽  
Compressibility Effects ◽  
Vertical Wind Velocity

<p>Eddy accumulation is a direct flux measurement technique for trace gas exchange between the land surface and the atmosphere. Eddy accumulation complements the now common eddy covariance method in its ability to measure even small fluxes accurately with slow response gas analyzers and being power efficient. However, the physically most direct way of eddy accumulation, also known as true eddy accumulation (TEA), requires the sampling of air at a rate proportional to the vertical wind velocity at a fast rate of typically 10 Hz or more. Lack of suitable methods for high-speed air sampling has been a primary limitation for the practical application of eddy accumulation in the past. The compressibility of air causes a variation of pressure inside the sampling system, which affects the ability to control the sample flow rate accurately, potentially compromising the derived flux measurements. It is therefore essential to quantify the effect of compressibility on fluxes and understand the parameters which allow for mitigating the effect at the design stage.<br>In this study, we present successful true eddy accumulation measurements over the old-growth forest at the Fluxnet site Hainich (DE-Hai) and quantify the compressibility effects on fluxes. Performing simulations on high-frequency data of CO<sub>2</sub> and vertical wind velocity for a range of system configurations, we are able to quantify the impact of compressibility on fluxes and explain why our measurements were successful. We find that different system configurations lead to flux changes over a representative range of 1 to 25 percent of the flux. Key controlling parameters are the size and arrangement of internal buffer volumes and the appropriate control of the inlet flow rate sampling device as a function of internal and external pressure states. This knowledge allows to mitigate compressibility effects and design accurate true eddy accumulation flux measurements for a range of atmospheric constituents.</p>

scholarly journals Measurements of Rainfall Velocity and Raindrop Size Distribution Using Coherent Doppler Lidar

2016 ◽  
Vol 33 (9) ◽  
pp. 1949-1966 ◽  
Author(s):  
Makoto Aoki ◽  
Hironori Iwai ◽  
Katsuhiro Nakagawa ◽  
Shoken Ishii ◽  
Kohei Mizutani
Keyword(s):  
Size Distribution ◽  
Wind Velocity ◽  
Vertical Wind ◽  
Doppler Spectrum ◽  
Doppler Lidar ◽  
Stratiform Rain ◽  
Raindrop Size Distribution ◽  
Raindrop Size ◽  
Rain Events ◽  
Vertical Wind Velocity

AbstractRainfall velocity, raindrop size distribution (DSD), and vertical wind velocity were simultaneously observed with 2.05- and 1.54-μm coherent Doppler lidars during convective and stratiform rain events. A retrieval method is based on identifying two separate spectra from the convolution of the aerosol and precipitation Doppler lidar spectra. The vertical wind velocity was retrieved from the aerosol spectrum peak and then the terminal rainfall velocity corrected by the vertical air motion from the precipitation spectrum peak was obtained. The DSD was derived from the precipitation spectrum using the relationship between the raindrop size and the terminal rainfall velocity. A comparison of the 1-min-averaged rainfall velocity from Doppler lidar measurements at a minimum range and that from a collocated ground-based optical disdrometer revealed high correlation coefficients of over 0.89 for both convective and stratiform rain events. The 1-min-averaged DSDs retrieved from the Doppler lidar spectrum using parametric and nonparametric methods are also in good agreement with those measured with the optical disdrometer with a correlation coefficient of over 0.80 for all rain events. To retrieve the DSD, the parametric method assumes a mathematical function for the DSD and the nonparametric method computes the direct deconvolution of the measured Doppler lidar spectrum without assuming a DSD function. It is confirmed that the Doppler lidar can retrieve the rainfall velocity and DSD during relatively heavy rain, whereas the ratio of valid data significantly decreases in light rain events because it is extremely difficult to separate the overlapping rain and aerosol peaks in the Doppler spectrum.

scholarly journals A relaxed eddy accumulation system for measuring vertical fluxes of nitrous acid

2011 ◽  
Vol 4 (10) ◽  
pp. 2093-2103 ◽  
Author(s):  
X. Ren ◽  
J. E. Sanders ◽  
A. Rajendran ◽  
R. J. Weber ◽  
A. H. Goldstein ◽  
...  
Keyword(s):  
Wind Velocity ◽  
Nitrous Acid ◽  
Sonic Anemometer ◽  
Field Studies ◽  
Fast Response ◽  
Vertical Wind ◽  
Measured Temperature ◽  
Relaxed Eddy Accumulation ◽  
Vertical Fluxes ◽  
Vertical Wind Velocity

Abstract. A relaxed eddy accumulation (REA) system combined with a nitrous acid (HONO) analyzer was developed to measure atmospheric HONO vertical fluxes. The system consists of three major components: (1) a fast-response sonic anemometer measuring both vertical wind velocity and air temperature, (2) a fast-response controlling unit separating air motions into updraft and downdraft samplers by the sign of vertical wind velocity, and (3) a highly sensitive HONO analyzer based on aqueous long path absorption photometry that measures HONO concentrations in the updrafts and downdrafts. A dynamic velocity threshold (±0.5σw, where σw is a standard deviation of the vertical wind velocity) was used for valve switching determined by the running means and standard deviations of the vertical wind velocity. Using measured temperature as a tracer and the average values from two field deployments, the flux proportionality coefficient, β, was determined to be 0.42 ± 0.02, in good agreement with the theoretical estimation. The REA system was deployed in two ground-based field studies. In the California Research at the Nexus of Air Quality and Climate Change (CalNex) study in Bakersfield, California in summer 2010, measured HONO fluxes appeared to be upward during the day and were close to zero at night. The upward HONO flux was highly correlated to the product of NO2 and solar radiation. During the Biosphere Effects on Aerosols and Photochemistry Experiment (BEARPEX 2009) at Blodgett Forest, California in July 2009, the overall HONO fluxes were small in magnitude and were close to zero. Causes for the different HONO fluxes in the two different environments are briefly discussed.

scholarly journals A relaxed eddy accumulation system for measuring vertical fluxes of nitrous acid

2011 ◽  
Vol 4 (3) ◽  
pp. 4105-4130 ◽  
Author(s):  
X. Ren ◽  
J. E. Sanders ◽  
A. Rajendran ◽  
R. J. Weber ◽  
A. H. Goldstein ◽  
...  
Keyword(s):  
Wind Velocity ◽  
Nitrous Acid ◽  
Sonic Anemometer ◽  
Field Studies ◽  
Fast Response ◽  
Vertical Wind ◽  
Measured Temperature ◽  
Relaxed Eddy Accumulation ◽  
Vertical Fluxes ◽  
Vertical Wind Velocity

Abstract. A relaxed eddy accumulation (REA) system combined with a nitrous acid (HONO) analyzer was developed to measure atmosperhic HONO vertical fluxes. The system consists of three major components: (1) a fast-response sonic anemometer measuring vertical wind velocity and air temperature, (2) a fast-response controlling unit separating air motions into updraft and downdraft samplers by the sign of vertical wind velocity, and (3) a highly sensitive HONO analyzer based on aqueous long path absorption photometry measuring HONO concentations in these updrafts and downdrafts. A dynamic velocity threshold (±0.5σw, where σw is a standard deviation of the vertical wind velocity) was used for valve switching determined by the running means and standard deviations of the vertical wind velocity. Using measured temperature as a tracer and the average values from two field deployments, the flux proportionality coefficient, β, was determined to be 0.42 ± 0.02, in good agreement with the theoretical estimation. The REA system was deployed in two ground-based field studies. In the California Research at the Nexus of Air Quality and Climate Change (CalNex) study in Bakersfield, California in summer 2010, measured HONO fluxes appeared to be upward during the day and were close to zero at night. The upward HONO flux was highly correlated to the product of NO2 and solar radiation. During the Biosphere Effects on Aerosols and Photochemistry Experiment (BEARPEX 2009) at Blodgett Forest, California in July 2009, the overall HONO fluxes were small in magnitude and were close to zero. Causes for the differences in HONO fluxes in the two different environments are briefly discussed.

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