We apply airborne measurements across three seasons (summer, winter and spring 2017-2018) in a multi-inversion framework to quantify methane emissions from the US Corn Belt and Upper Midwest, a key agricultural and wetland source region. Combing our seasonal results with prior fall values we find that wetlands are the largest regional methane source (32 %, 20 [16-23] Gg/d), while livestock (enteric/manure; 25 %, 15 [14-17] Gg/d) are the largest anthropogenic source. Natural gas/petroleum, waste/landfills, and coal mines collectively make up the remainder. Optimized fluxes improve model agreement with independent datasets within and beyond the study timeframe. Inversions reveal coherent and seasonally dependent spatial errors in the WetCHARTs ensemble mean wetland emissions, with an underestimate for the Prairie Pothole region but an overestimate for Great Lakes coastal wetlands. Wetland extent and emission temperature dependence have the largest influence on prediction accuracy; better representation of coupled soil temperature-hydrology effects is therefore needed. Our optimized regional livestock emissions agree well with the Gridded EPA estimates during spring (to within 7 %) but are g≈ 25 % higher during summer and winter. Spatial analysis further shows good top-down and bottom-up agreement for beef facilities (with mainly enteric emissions) but larger (g≈ 30 %) seasonal discrepancies for dairies and hog farms (with > 40 % manure emissions). Findings thus support bottom-up enteric emission estimates but suggest errors for manure; we propose that the latter reflects inadequate treatment of management factors including field application. Overall, our results confirm the importance of intensive animal agriculture for regional methane emissions, implying substantial mitigation opportunities through improved management.
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Acknowledgements. We acknowledge the entire GEM science team for their contributions. We thank Jason Rosenthal, Justin Pifer, and Ian Locko for their excellent flying; Stephen A. Conley, Mackenzie L. Smith, and Alexander Gvakharia for their contributions to the GEM flights; Joshua P. DiGangi and Zachary R. Barkley for their work on the ACT-America project; Arlyn E. Andrews, Jonathan E. Thom, and Jonathan Kofler for their work on the LEF tower measurements; and Natasha L. Miles for her work on the WSD tower measurements. The GEM project is supported by NASA’s Interdisciplinary Research in Earth Science program (IDS grant no. NNX17AK18G) and by the Minnesota Supercomputing Institute. Xueying Yu acknowledges support from a NASA Earth and Space Science Fellowship (grant no. 80NSSC18K1393). Eric A. Kort and Genevieve Plant acknowledge support from NSF (grant no. 1650682). Ankur R. Desai acknowledges support from the DOE Ameriflux Network Management Project, the NOAA ESRL Tall Tower Greenhouse Gas Monitoring Program, and NSF (grant no. 0845166). Contributions from A. Anthony Bloom were carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with NASA. The Atmospheric Carbon and Transport (ACT) – America project is a NASA Earth Venture Suborbital 2 project funded by NASA’s Earth Science Division (grant no. NNX15AG76G to Penn State). ATom methane measurements are supported by NASA grant no. NNX16AL92A.
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