Results from a tracer release study were applied to estimate a tower footprint.
At night, can an upper-level carbon dioxide sensor be overly influenced by gas released from nearby vegetation, reducing researchers’ confidence in its ability to provide information on continental-scale surface fluxes? The vertical dispersion of a gas released at night 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) downwind detection of the tracers at four elevations on a tall television transmitter tower.
A simulation of the tracer release reproduced the motion of tracer from its source to the detectors, but also indicated that the uppermost detector (at 329 m above ground) was mainly sampling air from far beyond 25 km, with a minor contribution from areas within that range. Therefore, for nocturnal conditions, the researchers are confident that the tower is sampling air from over a regional-scale area (25 km to 150 km), and is only weakly influenced by nearby emissions.
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 conducted to estimate the tower “footprint,” or total area from which tracer released at the surface will be detected by the 329-m sensor. These simulations 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. The low-level nocturnal jet (located at 100 m to 1000 m above ground, and at 8 to 20 m per sec speed) was an important reason for the dominant role of distant upwind sources.
BER Program Manager
Savannah River National Laboratory
Funding was provided by the Terrestrial Ecosystem Science program of the Office of Biological and Environmental Research, within the U.S. Department of Energy Office of Science. This work was performed by the Savannah River National Laboratory/Savannah River Nuclear Solutions, LLC, under Contract No. DE-AC09-08SR22470.
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.” Theoretical and Applied Climatology 127, 627–642 (2017). [DOI:10.1007/s00704-015-1648-y]
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