Comparison between Eddy‐Covariance and Flux‐Gradient size‐resolved dust fluxes during wind erosion events

Abstract. Our understanding of biosphere–atmosphere exchange has been considerably enhanced by eddy covariance measurements. However, there remain many trace gases, such as molecular hydrogen (H2), that lack suitable analytical methods to measure their fluxes by eddy covariance. In such cases, flux-gradient methods can be used to calculate ecosystem-scale fluxes from vertical concentration gradients. The budget of atmospheric H2 is poorly constrained by the limited available observations, and thus the ability to quantify and characterize the sources and sinks of H2 by flux-gradient methods in various ecosystems is important. We developed an approach to make nonintrusive, automated measurements of ecosystem-scale H2 fluxes both above and below the forest canopy at the Harvard Forest in Petersham, Massachusetts, for over a year. We used three flux-gradient methods to calculate the fluxes: two similarity methods that do not rely on a micrometeorological determination of the eddy diffusivity, K, based on (1) trace gases or (2) sensible heat, and one flux-gradient method that (3) parameterizes K. We quantitatively assessed the flux-gradient methods using CO2 and H2O by comparison to their simultaneous independent flux measurements via eddy covariance and soil chambers. All three flux-gradient methods performed well in certain locations, seasons, and times of day, and the best methods were trace gas similarity for above the canopy and K parameterization below it. Sensible heat similarity required several independent measurements, and the results were more variable, in part because those data were only available in the winter, when heat fluxes and temperature gradients were small and difficult to measure. Biases were often observed between flux-gradient methods and the independent flux measurements, and there was at least a 26% difference in nocturnal eddy-derived net ecosystem exchange (NEE) and chamber measurements. H2 fluxes calculated in a summer period agreed within their uncertainty and pointed to soil uptake as the main driver of H2 exchange at Harvard Forest, with H2 deposition velocities ranging from 0.04 to 0.10 cm s−1.

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Ecosystem fluxes of hydrogen: a comparison of flux-gradient methods

Atmospheric Measurement Techniques Discussions ◽

10.5194/amtd-7-2879-2014 ◽

2014 ◽

Vol 7 (3) ◽

pp. 2879-2928 ◽

Cited By ~ 1

Author(s):

L. K. Meredith ◽

R. Commane ◽

J. W. Munger ◽

A. Dunn ◽

J. Tang ◽

...

Keyword(s):

Eddy Covariance ◽

Forest Canopy ◽

Trace Gases ◽

Gradient Methods ◽

Net Ecosystem Exchange ◽

Heat Fluxes ◽

Sensible Heat ◽

Flux Gradient ◽

Harvard Forest ◽

Flux Measurements

Abstract. Our understanding of biosphere-atmosphere exchange has been considerably enhanced by eddy-covariance measurements, however there remain many trace gases, such as molecular hydrogen (H2), for which there are no suitable analytical methods to measure their fluxes by eddy covariance. In such cases, flux-gradient methods can be used to calculate ecosystem-scale fluxes from vertical concentration gradients. The budget of atmospheric H2 is poorly constrained by the limited available observations, thus the ability to quantify and characterize the sources and sinks of H2 by flux-gradient methods in various ecosystems is important. We developed an approach to make nonintrusive, automated measurements of ecosystem-scale H2 fluxes both above and below the forest canopy at the Harvard Forest in Petersham, MA for over a year. We used three flux-gradient methods to calculate the fluxes: two similarity methods that do not rely on a micrometeorological determination of the eddy diffusivity, K, based on (1) trace gases or (2) sensible heat and one flux-gradient method that (3) parameterizes K. We quantitatively assessed the flux-gradient methods on CO2 and H2O by comparison to their simultaneous independent flux measurements via eddy covariance and chambers. All three flux-gradient methods performed well in certain locations, seasons, and times of day, and the best methods were trace gas similarity above and K parameterization below the canopy. Sensible heat similarity required several independent measurements and the results were more variable, in part because those data were only available in the winter when heat fluxes and temperature gradients were small and difficult to measure. Biases were often observed between flux-gradient methods and the independent flux measurements, including at least a 26% difference in nocturnal eddy-derived Net Ecosystem Exchange (NEE) and soil chamber measurements. All flux-gradient methods used to calculate above and below canopy H2 fluxes pointed to soil uptake as the main driver of H2 exchange at Harvard Forest. H2 fluxes calculated in a summer period agreed within their uncertainty and indicated that H2 deposition velocities ranged from 0.04 to 0.1 cm s−1.

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An evaluation of the flux-gradient and the eddy covariance method to measure CH4, CO2, and H2O fluxes from small ponds

Agricultural and Forest Meteorology ◽

10.1016/j.agrformet.2019.05.032 ◽

2019 ◽

Vol 275 ◽

pp. 255-264 ◽

Cited By ~ 1

Author(s):

Jiayu Zhao ◽

Mi Zhang ◽

Wei Xiao ◽

Wei Wang ◽

Zhen Zhang ◽

...

Keyword(s):

Eddy Covariance ◽

Eddy Covariance Method ◽

Flux Gradient

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Size-segregated fluxes of mineral dust from a desert area of northern China by eddy covariance

Atmospheric Chemistry and Physics ◽

10.5194/acp-7-2839-2007 ◽

2007 ◽

Vol 7 (11) ◽

pp. 2839-2854 ◽

Cited By ~ 43

Author(s):

G. Fratini ◽

P. Ciccioli ◽

A. Febo ◽

A. Forgione ◽

R. Valentini

Keyword(s):

Eddy Covariance ◽

Wind Erosion ◽

Mineral Dust ◽

Northern China ◽

Particle Mass ◽

Dust Particles ◽

Storm Event ◽

Mass Fluxes ◽

Intense Storm ◽

Desert Areas

Abstract. Mineral dust emission accounts for a substantial portion of particles present in the troposphere. It is emitted mostly from desert areas, mainly through intense storm episodes. The aim of this work was to quantify size-segregated fluxes of mineral dust particles emitted during storm events occurring in desert areas of northern China (Alashan desert, Inner Mongolia), known to act as one of the strongest sources of mineral dust particles in the Asian continent. Long-range transport of mineral dust emitted in this area is responsible for the high particle concentrations reached in densely populated areas, including the city of Beijing. Based on a theoretical analysis, an eddy covariance system was built to get size-segregated fluxes of mineral dust particles with optical diameters ranging between 0.26 and 7.00 µm. The system was optimised to measure fluxes under intense storm event conditions. It was tested in two sites located in the Chinese portion of the Gobi desert. During the field campaign, an intense wind erosion event, classified as a "weak dust storm", was recorded in one of them. Data obtained during this event indicate that particle number fluxes were dominated by the finer fraction, whereas in terms of mass, coarser particle accounted for the largest portion. It was found that during the storm event, ratios of size-segregated particle mass fluxes remained substantially constant and a simple parameterization of particle emission from total mass fluxes was possible. A strong correlation was also found between particle mass fluxes and the friction velocity. This relationship is extremely useful to investigate mechanisms of particle formation by wind erosion.

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Biogenic volatile organic compound emissions during BEARPEX 2009 measured by eddy covariance and flux-gradient similarity methods

Atmospheric Chemistry and Physics Discussions ◽

10.5194/acpd-12-25081-2012 ◽

2012 ◽

Vol 12 (9) ◽

pp. 25081-25120 ◽

Cited By ~ 1

Author(s):

J.-H. Park ◽

S. Fares ◽

R. Weber ◽

A. H. Goldstein

Keyword(s):

Eddy Covariance ◽

Sierra Nevada ◽

Ponderosa Pine ◽

Regional Scale ◽

Experimental Conditions ◽

Eddy Covariance Method ◽

Flux Gradient ◽

Volatile Organic ◽

Ion Species ◽

Similarity Method

Abstract. The Biosphere Effects on AeRosols and Photochemistry EXperiment (BEARPEX) took place in Blodgett Forest, a Ponderosa pine forest in the Sierra Nevada Mountains of California, during summer 2009. We deployed a Proton Transfer Reaction – Mass Spectrometer (PTR-MS) to measure fluxes and concentrations of biogenic volatile organic compounds (BVOCs). Eighteen ion species including the major BVOC expected at the site were measured sequentially at 5 heights to observe their vertical gradient from the forest floor to above the canopy. Fluxes of the 3 dominant BVOCs methanol, 2-Methyl-3-butene-2-ol (MBO), and monoterpenes, were measured above the canopy by the eddy covariance method. Canopy scale fluxes were also determined by the flux-gradient similarity method (K-theory). A universal K (Kuniv) was determined as the mean of individual K's calculated from the measured fluxes divided by vertical gradients for methanol, MBO, and monoterpenes. This Kuniv was then multiplied by the gradients of each observed ion species to compute their fluxes. The flux-gradient similarity method showed very good agreement with the Eddy Covariance method. Fluxes are presented for all measured species and compared to historical measurements from the same site, and used to test emission algorithms used to model fluxes at the regional scale. MBO was the dominant emission observed followed by methanol, monoterpenes, acetone, and acetaldehyde. The flux-gradient similarity method is shown to be a useful, and we recommend its use especially in experimental conditions when fast measurement of BVOC species is not available.

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Calculating Canopy Stomatal Conductance from Eddy Covariance Measurements, in Light of the Energy Budget Closure Problem

10.5194/bg-2020-154 ◽

2020 ◽

Author(s):

Richard Wehr ◽

Scott R. Saleska

Keyword(s):

Heat Flux ◽

Stomatal Conductance ◽

Eddy Covariance ◽

Energy Budget ◽

Sensible Heat Flux ◽

Closure Problem ◽

Spatial And Temporal Patterns ◽

Flux Gradient ◽

Canopy Stomatal Conductance ◽

The Impact

Abstract. Canopy stomatal conductance (gsV) is commonly estimated from eddy covariance (EC) measurements of latent heat flux (LE) by inverting the Penman-Monteith (PM) equation. That method implicitly represents the sensible heat flux (H) as the residual of all other terms in the site energy budget – even though H is measured at least as accurately as LE at every EC site while the rest of the energy budget almost never is. We argue that gsV should instead be calculated from EC measurements of both H and LE, using the flux-gradient formulation that defines conductance and underlies the PM equation. The flux-gradient formulation dispenses with unnecessary assumptions, is conceptually simpler, and provides more accurate values of gsV for all plausible scenarios in which the measured energy budget fails to close, as is common at EC sites. The PM equation, on the other hand, contributes biases and erroneous spatial and temporal patterns to gsV, skewing its relationships with drivers such as light and vapor pressure deficit. To minimize the impact of the energy budget closure problem on the PM equation, it was previously proposed that the eddy fluxes should be corrected to close the long-term energy budget while preserving the Bowen ratio (B = H/LE). We show that such a flux correction does not fully remedy the PM equation but should produce accurate values of gsV when combined with the flux-gradient formulation.

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Biogenic volatile organic compound emissions during BEARPEX 2009 measured by eddy covariance and flux–gradient similarity methods

Atmospheric Chemistry and Physics ◽

10.5194/acp-14-231-2014 ◽

2014 ◽

Vol 14 (1) ◽

pp. 231-244 ◽

Cited By ~ 20

Author(s):

J.-H. Park ◽

S. Fares ◽

R. Weber ◽

A. H. Goldstein

Keyword(s):

Eddy Covariance ◽

Sierra Nevada ◽

Ponderosa Pine ◽

Regional Scale ◽

Vertical Gradient ◽

Experimental Conditions ◽

Flux Gradient ◽

Volatile Organic ◽

Ion Species ◽

Similarity Method

Abstract. The Biosphere Effects on AeRosols and Photochemistry EXperiment (BEARPEX) took place in Blodgett Forest, a Ponderosa pine forest in the Sierra Nevada of California, USA, during summer 2009. We deployed a proton transfer reaction–quadrupole mass spectrometer (PTR-QMS) to measure fluxes and concentrations of biogenic volatile organic compounds (BVOCs). Eighteen ion species, including the major BVOC expected at the site, were measured sequentially at 5 heights to observe their vertical gradient from the forest floor to above the canopy. Fluxes of the 3 dominant BVOCs methanol, 2-Methyl-3-butene-2-ol (MBO), and monoterpenes were measured above the canopy by the disjunct eddy covariance (EC) method. Canopy-scale fluxes were also determined by the flux–gradient similarity method (K-theory). A universal K (Kuniv) was determined as the mean of individual K's calculated from the measured fluxes divided by vertical gradients for methanol, MBO, and monoterpenes. This Kuniv was then multiplied by the gradients of each observed ion species to compute their fluxes. The flux–gradient similarity method showed very good agreement with the disjunct EC method. Fluxes are presented for all measured species and compared to historical measurements from the same site, and used to test emission algorithms used to model fluxes at the regional scale. MBO was the dominant emission observed, followed by methanol, monoterpenes, acetone, and acetaldehyde. The flux–gradient similarity method is shown to be tenable, and we recommend its use, especially in experimental conditions when fast measurement of BVOC species is not available.

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Turbulent flux estimation in the atmospheric surface layer through flux-gradient similarity during the ALEX17 field campaign

10.5194/egusphere-egu2020-9145 ◽

2020 ◽

Author(s):

Belén Martí ◽

Daniel Martínez-Villagrasa ◽

Joan Cuxart

Keyword(s):

Heat Flux ◽

Surface Layer ◽

Eddy Covariance ◽

Friction Velocity ◽

Turbulent Flux ◽

Sensible Heat Flux ◽

Sensible Heat ◽

Flux Gradient ◽

Eddy Covariance System ◽

Flux Estimation

Turbulent flux measurements require high frequency sampling in order to characterize appropriately all the variability scales of the atmosphere. A 3D sonic anemometer coupled with a gas detector allows for applying the eddy-covariance method which has become the standard. However, the high cost of this system often implies to look for alternative methods, specially when multiple stations are required. Turbulent fluxes can also be estimated through the flux-gradient similarity theory, requiring observations of mean quantities of (at least) air temperature and humidity at two levels and wind at one height. This approach is more sensitive to the disturbing influence of heterogeneous and complex surfaces and a comparison between methodologies is required under these conditions. The data used in this study is part of the ALaiz EXperiment 2017-2018 (ALEX17). This campaign was the last within the New European Altas project. It had a duration of over a year with measurements in complex terrain. The location of the experiment is a valley bounded by two mountain ranges that rise 150 m north and over 600 m south. A central site in the centre of the valley was instrumented with a sodar-RASS, an 80-m tower, a surface energy balance (SEB) station with an eddy-covariance system and a surface-layer station (SLS) with the necessary measurements to estimate the turbulent fluxes. In addition, eight supplementary SLS were deployed along the longitudinal and transverse valley axes to characterize the surface layer variability within the valley. This communication will present a comparison of the friction velocity and sensible heat flux obtained from both the eddy-covariance system and the flux-gradient method at the central site for a time series of 8 months. Friction velocity is highly comparable between methodologies with a correlation of 0.92 and a standard deviation of 0.05. The performance of the sensible heat flux estimation differs between stable and unstable cases, with a correlation of 0.70 and 0.89, respectively, after applying a quality control procedure. The poorer results obtained under stable conditions points out the need for alternative estimations of the sensible heat flux for these cases.

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Calculating canopy stomatal conductance from eddy covariance measurements, in light of the energy budget closure problem

Biogeosciences ◽

10.5194/bg-18-13-2021 ◽

2021 ◽

Vol 18 (1) ◽

pp. 13-24

Author(s):

Richard Wehr ◽

Scott R. Saleska

Keyword(s):

Heat Flux ◽

Stomatal Conductance ◽

Eddy Covariance ◽

Energy Budget ◽

Sensible Heat Flux ◽

Flux Gradient ◽

Site Energy ◽

Eddy Covariance Measurements ◽

Canopy Stomatal Conductance ◽

Monteith Equation

Abstract. Canopy stomatal conductance is commonly estimated from eddy covariance measurements of the latent heat flux (LE) by inverting the Penman–Monteith equation. That method ignores eddy covariance measurements of the sensible heat flux (H) and instead calculates H implicitly as the residual of all other terms in the site energy budget. Here we show that canopy stomatal conductance is more accurately calculated from eddy covariance (EC) measurements of both H and LE using the flux–gradient equations that define conductance and underlie the Penman–Monteith equation, especially when the site energy budget fails to close due to pervasive biases in the eddy fluxes and/or the available energy. The flux–gradient formulation dispenses with unnecessary assumptions, is conceptually simpler, and is as or more accurate in all plausible scenarios. The inverted Penman–Monteith equation, on the other hand, contributes substantial biases and erroneous spatial and temporal patterns to canopy stomatal conductance, skewing its relationships with drivers such as light and vapor pressure deficit.

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