Evaluating Inventory Emissions of Methane and Nitrous Oxide within an Agricultural Landscape with Tall Tower Measurements and a Trajectory Model

In order to improve the inventory emissions estimates of methane (CH₄) and nitrous oxide (N₂O) within an agricultural landscape, we conducted an experiment on a 244 m communications tower (44°41'19''N, 93°04'22''W) in Rosemount, Minnesota. The concentrations of carbon dioxide (CO₂), CH₄, and N₂O were measured at 3m and 200m continuously from August to September, 2009. In addition, CO₂, CH₄, and N₂O fluxes of soybean and corn plants were measured with a steady-state flow-through chamber throughout the growing season. The net ecosystem N₂O flux was measured with the flux-gradient method in a soybean-corn rotation field. In this paper, we used the tall tower concentration data to constrain the inventory emissions estimates in the region surrounding the tower.

We deployed the Stochastic Time-inverted Lagrangian Transport Model (STILT), with the model driven by the meteorological fields produced by the Eta Data Assimilation System. Assuming a constant and homogeneous emission rate of CH₄ at the land surface, the simulated concentration matched well the diurnal variation of the CH₄ concentration at 3 m, with a linear correlation coefficient of r = 0.49. However, the model failed to simulate the observed variations at the 200 m height (r=0.01), implying that the land CH₄ flux was heterogeneous on a larger scale. Research is underway to analyze the footprint of the tower observation, determine its source region, and deploy an optimization method to constrain the regional fluxes of CH₄ and N₂O.