New constraints on regional methane emissions in the US Upper Midwest based on GEM aircraft measurements and the GEOS-Chem model

Methane is the most important anthropogenic greenhouse gas after carbon dioxide and a precursor of ozone. The US Corn Belt and Upper Midwest region is central to the US methane budget, including >40% crop land cover, >700 million livestock (~28 million cattle and a majority of national swine feeding operations), several industrial centers and extensive natural wetlands. In this study, we apply new aircraft measurements from the Greenhouse Emissions in the Midwest (GEM) study with a high-resolution nested version of the GEOS-Chem chemical transport model (CTM) to test current understanding of the above key sources and their importance for regional and national greenhouse gas budgets. The GEM flights were conducted during summer (August 2017), winter (January 2018) and spring (May-June 2018), spanning three seasons, and included 23 flight days and 150 flight hours. Based on these aircraft measurements, we evaluate multiple optimization frameworks in terms of their ability to reconcile the model predictions with the observations. First, the GEOS-Chem adjoint model is used to spatially optimize regional methane emissions at the 0.25° × 0.3125° resolution of the model grid. Second, analytical inversions are performed for specific source sectors, with the prior spatial distribution of each taken as a hard constraint in the optimization. Third, a Gaussian Mixture Model (GMM) is used to spatially cluster emitting grid cells based on source contribution and model-measurement mismatch, with subsequent analytical optimization. This presentation will compare results from the above inversions in terms of their implications for understanding the factors driving methane emissions in this region.