Top-down Constraints on Regional Nitrous Oxide and Methane Emissions within the U.S. Corn Belt

Abstract

Nitrous oxide (N2O) and methane (CH4) are important long-lived greenhouse gases. However, evidence has been found that CH4 and N2O emissions from the US Corn Belt are much larger than budgeted for in the most up-to-date emission inventories, implying that one or more sources in bottom-up approaches are underestimated, resulting in poorly constrained source partitioning. Therefore, this dissertation sought to: 1) quantify importance of direct versus indirect N2O emissions, and explore their seasonality and regional budgets; 2) assess the retrospective and future N2O emissions at fine spatiotemporal scales to propose mitigation priorities for the Corn Belt; 3) partition methane emissions into natural (e.g. wetlands) and anthropogenic (e.g. livestock, waste, and natural gas) sources, and explore their temporal variability. We compared in-situ measurements of N2O from the University of Minnesota tall tower site with a time-inverted transport model and a scale factor Bayesian inverse (SFBI) method. Our analysis suggested an upward adjustment of the emission factor of indirect sources by 1.9 to 4.6 times relative to the IPCC inventory. Further, indirect emissions were predicted to exhibit a trend of 0.36 nmol N2O m-2 s-1 per year in an Eularian modeling study, using the Weather Research and Forecasting Chemistry (WRF-Chem) model with implementation of the CLM45-BGC-CROP land surface scheme under RCP8.5 scenario. With respect to CH4, our analysis revealed that the anthropogenic source (7.8 ±1.6 Tg CH4 yr-1) was 1.5 times greater than accounted for in the EPA inventory. Most prominently livestock and oil/gas sources were underestimated by 1.8- and 1.3-fold, respectively. In contrast, the temporal variability of total CH4 emissions was dominated by wetlands with peak emissions occurring in August. Our findings suggest that indirect N2O and anthropogenic CH4 emissions within the Corn Belt Region need to be upward-adjusted in inventory updates. Further, our model predictions indicate large N2O mitigation potential in the lower Midwest region.