U.S. Department of Energy Office of Biological and Environmental Research

PI-Submitted Research Highlights for
Terrestrial Ecosystem Science Program

Vertical Transport of Greenhouse Gases through the Nocturnal Atmospheric Boundary Layer

David Werth
Savannah River National Laboratory

Highlight

The Augusta Tower, LLC television transmitter tower near Beech Island, SC.  SRNL's meteorological sensors and ice booms can be seen at 34 and 68m. 

17 October 2016
The application of the results from a tracer release study to estimate a tower footprint

The Science  
At night, carbon dioxide sensors placed at high levels (~300 m) are assumed to be sampling air transported from great distances, as the air released from nearby locations is subject to languid vertical mixing.   The vertical transport of carbon dioxide through the nighttime boundary layer is not well understood, however, because of the weak, intermittent nature of nocturnal turbulence.   Can an upper-level sensor be strongly influenced by the local environment, giving the sensor a small nocturnal ‘footprint’ and reducing our confidence in its ability to provide information on continental-scale surface fluxes?   The degree to which a gas released at night is confined to a shallow layer near the surface was studied with a field project in South Carolina comprising 1) the release of five perfluorocarbons (inert airborne ‘tracer’ gases) from multiple surface locations and 2) the downwind detection of the tracers at four elevations on a tall television transmitter tower.  The experiment was conducted over a 15 km x 30 km domain on two nights.

The Impact
A small but significant concentration of perfluorocarbon was detected at the 329 m level from a release point 25 km upwind.     A subsequent simulation of the tracer release reproduced the motion of tracer from its source to the detector, but also indicated that the highest sampling height, 329 m, was mainly sampling air from far beyond 25 km, with a minor contribution from areas within that range.   The low-level nocturnal jet (located at 100 m to 1000 m above ground, and at 8-20 m/sec speed) was an important reason for the dominant role of distant upwind sources, and also contributed to turbulent mixing aloft.

Summary
On two nights characterized by moderate to strong vertical stability, tracer gases were released at the surface from locations upwind of a South Carolina tower equipped with sensors at 34 m, 68 m and 329 m.   The uppermost sensor was able to detect the tracer gas released from the ground at a distance of about 25 km - evidence for some vertical transport despite the weak vertical mixing on the nights it was released.   Simulations of the experiment, validated against the field project data, were then done to estimate the tower ‘footprint’ - the total area from which tracer released at the surface will be detected by the 329 m sensor.   These indicate that most of the air reaching the highest tower level came from surface locations much more distant than the domain of the tracer release, with the sensor footprint extending well beyond 25 km.   Therefore, for nocturnal conditions, we can be more confident the tower is ‘sampling’ air from over a regional-scale area (25 - 150 km), and is only weakly influenced by nearby emissions.          

Contacts
(BER PM)

Dan Stover
SC-23.1
daniel.stover@science.doe.gov (301-903-0289)

(PI Contact)
David Werth
Savannah River National Laboratory
David.Werth@srnl.doe.gov (803-725-3717)

Funding
Funding was provided by the DOE Office of Science Terrestrial Carbon Processes program under Mike Kuperberg and Dan Stover.   This work was performed by the Savannah River National Laboratory/Savannah River Nuclear Solutions, LLC under contract no. DE-AC09-08SR22470.

Publications
D. Werth, R. Buckley, G. Zhang, R. Kurzeja, M. Leclerc, H. Duarte, M. Parker, and T. Watson, “Quantifying the local influence at a tall tower site in nocturnal conditions.”, 2015: Theoretical and Applied Climatology, DOI:10.1007/s00704-015-1648-y, (Reference link)

Collaborators

Savannah River National Laboratory

Laboratory for Atmospheric and Environmental Physics, University
of Georgia

Tracer Technology Group, Brookhaven National Laboratory

Funding

Funding was provided by the DOE Office of Science – Terrestrial Carbon Processes program under Mike Kuperberg and Dan Stover.  This work was performed by the Savannah River National Laboratory/Savannah River Nuclear Solutions, LLC under contract no. DE-AC09-08SR22470.

Software

We applied the Regional Atmospheric Modeling System, developed at Colorado State University, and the HYbrid Single-Particle Lagrangian Integrated Trajectory
(HYSPLIT) dispersion model, developed at NOAA's Air Resources Laboratory

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