Location: Soil and Water Management ResearchTitle: Estimating regional greenhouse gas fluxes: An uncertainty analysis of planetary boundary layer techniques and bottom-up inventories Author
Submitted to: Atmospheric Chemistry and Physics
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/27/2014
Publication Date: 9/15/2014
Publication URL: http://handle.nal.usda.gov/10113/60456
Citation: Zhang, X., Lee, X., Griffis, T.J., Baker, J.M., Xiao, W. 2014. Estimating regional greenhouse gas fluxes: An uncertainty analysis of planetary boundary layer techniques and bottom-up inventories. Atmospheric Chemistry and Physics. 14:10705-10719. Interpretive Summary: It is now generally accepted that changes in atmospheric composition have influenced global climate andthere is growing sentiment that this problem must be addressed. The first step in developing regional mitigation strategies is to determine what the current regional contributions of the major greenhouse gases (CO2, CH4, N2O) are. This can be done from a "top-down" approach, in which atmospheric measurements are used to infer the net exchange of gases between the surface and the atmosphere, or it can be done from the "borrom-up" standpoint, in which ground-based data are used to estimate emissions. We used three different top-down approaches and one bottom-up approach to estimate the greenhouse gas budget for the region surrounding a tall (244 m) radio tower in Minnesota on which a sensitive gas exchange system has been making measurements for a number of years. We found that all three methods produce similar seasonal patterns of greenhouse gas exchange, but that the simplest of the top-down methods substantially underestimated summer (July) exchange of CO2. For CH4 and N2O, all top-down methods produced much higher annial totals that the bottom-up, inventory-based approach. Overall, for this region the results indicate that the global warming potential of CH4 and N2O emission are roughly equal to the cooling potential of net annual CO2 uptake.
Technical Abstract: Quantification of regional greenhouse gas (GHG) fluxes is essential for establishing mitigation strategies and evaluating their effectiveness. Here, we used multiple top-down approaches and multiple trace gas observations at a tall tower to estimate GHG regional fluxes and evaluate the GHG fluxes derived from bottom-up approaches. We first applied the eddy covariance, equilibrium, inverse modeling (Carbon Tracker), and flux aggregation methods using three years of carbon dioxide (CO2) measurements on a 244-meter tall tower in the Upper Midwest, USA. We then applied the equilibrium method for estimating CH4 and N2O fluxes with one-month high-frequency CH4 and N2O gradient measurements on the tall tower and one-year concentration measurements on a nearby tall tower, and evaluated the uncertainties of this application. The results indicate that: 1) The flux aggregation, eddy covariance, the equilibrium method, and the Carbon Tracker product all gave similar seasonal patterns of the regional CO2 flux (105-106 km2), but that the equilibrium method underestimated the July CO2 flux by 52-69%. 2) The annual budget varied among these methods from -54 to -131 g C-CO2 m-2 yr-1, indicating a large uncertainty in the annual CO2 flux estimation. 3) The regional CH4 and N2O emissions according to a top-down method were at least six and two times higher than the emissions from a bottom-up inventory (Emission Database for Global Atmospheric Research), respectively. 4) The global warming potentials of the CH4 and N2O emissions were equal in magnitude to the cooling benefit of the regional CO2 uptake. The regional GHG budget, including both biological and anthropogenic origins, is estimated at 7 ± 160 g CO2 eq m-2 yr-1.