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Atmospheric Measurement Techniques An interactive open-access journal of the European Geosciences Union
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© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 18 Mar 2019

Research article | 18 Mar 2019

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This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Measurement Techniques (AMT).

True eddy accumulation trace gas flux measurements: proof-of-concept

Lukas Siebicke and Anas Emad Lukas Siebicke and Anas Emad
  • Bioclimatology, Georg-August-University of Goettingen, Goettingen, Germany

Abstract. Micrometeorological methods to quantify fluxes of atmospheric constituents are key to understanding and managing the impact of land surface sources and sinks on air quality and atmospheric composition.

Important greenhouse gases are water vapor, carbon dioxide, methane, and nitrous oxide. Further important atmospheric constituents are aerosols which impact air quality and cloud formation, and volatile organic compounds. Many atmospheric constituents therefore critically affect the health of ecosystems, and humans as well as climate.

The micrometeorological eddy covariance (EC) method has evolved as the method-of-choice for CO2 and water vapor flux measurements using fast-response gas analyzers. While the EC method has also been used to measure other atmospheric constituents including methane, nitrous oxide, and ozone, the often relatively small fluxes of these constituents over ecosystems are much more challenging to measure by eddy covariance than CO2 and water vapor fluxes. For many further atmospheric constituents, eddy covariance is not an option due to the lack of sufficiently accurate and fast-response gas analyzers.

Therefore, alternative flux measurement methods are required for the observation of atmospheric constituent fluxes for which no fast-response gas analyzers exist or which require more accurate measurements. True eddy accumulation (TEA) is a direct flux measurement technique capable of using slow-response gas analyzers. Unlike its more frequently used derivative, known as the relaxed eddy accumulation (REA) method, TEA does not require the use of proxies and is therefore superior to the indirect REA method.

The true eddy accumulation method is by design ideally suited for measuring a wide range of trace gases and other conserved constituents transported with the air. This is because TEA obtains whole air samples and is, in combination with constituent-specific fast or slow analyzers, a universal method for conserved scalars.

Despite the recognized value of the method, true eddy accumulation flux measurements remained very challenging to perform as they require fast and dynamic modulation of the air sampling mass flow rate proportional to the magnitude of the instantaneous vertical wind velocity. Appropriate techniques for dynamic mass flow control have long been unavailable, preventing the unlocking of the TEA method's potential for more than 40 years.

Recently, a new dynamic and accurate mass flow controller which can resolve turbulence at a frequency of 10 Hz and higher has been developed by the author. This study presents the proof-of-concept that practical true eddy accumulation trace gas flux measurements are possible today using dynamic mass flow control, advanced real-time processing of wind measurements, and fully automatic gas handling.

We describe setup and methods of the TEA and EC reference flux measurements. The experiment was conducted over grassland and comprised seven days of continuous flux measurements at 30-min flux integration intervals. The results show that fluxes obtained by TEA compared favourably to EC reference flux measurements with coefficients of determination of up to 86 % and a slope of 0.98.

We present a quantitative analysis of uncertainties of the mass flow control system, the gas analyzer and gas handling system and their impact on trace gas flux uncertainty, the impact of different approaches to coordinate rotation and uncertainties of vertical wind velocity measurements.

Challenges of TEA are highlighted and solutions presented. The current results are put into context of previous works. Finally, based on the current successful proof-of-concept, we suggest specific improvements towards long-term and reliable true eddy accumulation flux measurements.

Lukas Siebicke and Anas Emad
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Status: open (until 13 May 2019)
Status: open (until 13 May 2019)
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Lukas Siebicke and Anas Emad
Lukas Siebicke and Anas Emad
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Publications Copernicus
Short summary
We present the proof-of-concept for an innovative flux measurement method able to quantify the land-atmosphere exchange of a large number of greenhouse gases and trace gases, which are important for air quality and atmospheric composition. Key to the success was an innovative high-speed air sampling system. The new flux measurement method performed very well compared to the established alternative. Finally, we suggest new features and applications of the technique.
We present the proof-of-concept for an innovative flux measurement method able to quantify the...