Characterizing the Vertical Flux of CO2 within the Nocturnal Boundary Layer near a Tall Tower
Abstract
Understanding the vertical dispersion of carbon dioxide respired from plants at night is crucial to distinguishing local- to regional-scale transport and continental-scale transport in global carbon budgets. When atmospheric conditions are stable, CO2 will be weakly mixed, and nearby detectors above the surface layer will instead sample CO2 carried from large distances. These conditions often prevail during the nighttime, making that period ideal for continental-scale sampling. On the other hand, during periods of moderate or intermittent nocturnal turbulence, locally-respired CO2 will be transported through the surface layer and produce a signal at the detector. In August 2008, a 329m tall TV tower (33.4058N, 81.834W) in Aiken, South Carolina (the "South Carolina Tower" http://www.esrl.noaa.gov/gmd/ccgg/towers/#sct) was incorporated into the NOAA-Global Monitoring Division's Tall Tower network. This site is located within a region that varies from agricultural, broken forests, suburban, urban and industrial. Emissions from several cities (most notably Augusta, GA) and industrial sites are within 50km of the tower and may contribute disproportionately to the nighttime tower readings. To distinguish local and regional sources, it is necessary to characterize vertical turbulent transport at this site. There are several ways to do this, and we focus on three. First, a mesoscale model was run at high-resolution to recreate the winds and temperature observed during a May 2009 nocturnal tracer release field project conducted in the region surrounding the site. The model data then served as input to a Lagrangian transport model. This was done for two eight-hour periods on successive but different nights: one slightly stable, and the other more stable. The coupled mesoscale/transport model was then validated against the tracer data, and was used to calculate the dispersion properties of the tracer and provide a 3-dimensional picture of the plume. For comparison, we apply two other methods to calculate eddy diffusivity. We calculate it directly using sonic anemometer and fast-response CO2 flux and concentration data from the tall tower. The fast response data (10Hz) allows for the explicit calculation of the turbulent transport and, along with the vertical gradient, provide an estimate of the diffusivity. As a third method, the eddy diffusivity can also be calculated by an empirical method that uses as input the turbulent properties measured at the tower. We select one such method and compare the results to the other two estimates.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFM.A13D0252W
- Keywords:
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- 0426 BIOGEOSCIENCES / Biosphere/atmosphere interactions;
- 3307 ATMOSPHERIC PROCESSES / Boundary layer processes;
- 3322 ATMOSPHERIC PROCESSES / Land/atmosphere interactions;
- 3323 ATMOSPHERIC PROCESSES / Large eddy simulation