Biases of CO2 Storage in Eddy Flux Measurements pertinent to Vertical Configurations of a Profile System and CO2 Density Averaging
- ORNL
- University of Missouri
- NOAA ATDD
- ATDD, NOAA
CO2 storage in a 30-minute period in a tall forest canopy often makes significant contributions to net ecosystem exchange (NEE) in the early morning and at night. When CO2 storage is properly measured and taken into account, underestimations of NEE on calm nights can be greatly reduced. Using CO2 data from a 12-level profile, we demonstrate that the lower canopy layer (below the thermal inversion) is a disproportional contributor to the total CO2 storage. This is because time derivative of CO2 density ( c/ t) generally shows increasing magnitude of mean and standard deviation with decreasing heights at night and from sunrise to 1000 hr in both growing and dormant seasons. Effects of resolution and configuration in a profiling system on the accuracy of CO2 storage estimation are evaluated by comparing subset profiles to the 12-level benchmark profile. It is demonstrated that the effectiveness of a profiling system in estimating CO2 storage is not only determined by its number of sampling levels but, more importantly, by its vertical configuration. To optimize a profile, one needs to balance the influence of two factors, c/ t and layer thickness, among all vertical sections within a forest. As a key contributor to the total CO2 storage, the lower canopy (with relatively large means and standard deviations of c/ t) requires a higher resolution in a profile system than the layers above. However, if the upper canopy is over-sparsely sampled relative to the lower canopy, the performance of a profile system might be degraded since, in such a situation, the influence of layer thickness dominates over that of c/ t. We also find that, because of different level of complexity in canopy structure, more sampling levels are necessary at our site in order to achieve the same level of accuracy as at a boreal aspen site. These results suggest that, in order to achieve an adequate accuracy in CO2 storage measurements, the number of sampling levels in a profile and its design should be subject to the site properties, e.g., canopy architecture and the resulted thermodynamic and flow structures. If CO2 density from a single profile is averaged in time and then used in assessing CO2 storage to make this measurement more spatially representative, biases associated with this averaging procedure become inevitable. Generally, larger window sizes used in averaging CO2 density generate poorer estimates of CO2 storage. If absolute errors are concerned, it appears that the more significant the CO2 storage is during a period (nighttime and early morning hours versus late morning and afternoon, peak growing season versus early growing season), the larger effects the averaging procedure has.
- Research Organization:
- Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States)
- Sponsoring Organization:
- USDOE Office of Science (SC)
- DOE Contract Number:
- DE-AC05-00OR22725
- OSTI ID:
- 932008
- Journal Information:
- Journal of Geophysical Research, Vol. 112, Issue D20123; ISSN 0747-7309
- Country of Publication:
- United States
- Language:
- English
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