Agriculture and waste are thought to account for half or more of the U.S. anthropogenic methane source. However, current bottom-up inventories contain inherent uncertainties from extrapolating limited in situ measurements to larger scales. Here, we employ new airborne methane measurements over the U.S. Corn Belt and Upper Midwest, among the most intensive agricultural regions in the world, to quantify emissions from an array of key agriculture and waste point sources. Nine of the largest concentrated animal feeding operations in the region and two sugar processing plants were measured, with multiple revisits during summer (August 2017), winter (January 2018), and spring (May–June 2018). We compare the top-down fluxes with state-of-science bottom-up estimates informed by U.S. Environmental Protection Agency methodology and site-level animal population and management practices. Top-down point source emissions are consistent with bottom-up estimates for beef concentrated animal feeding operations but moderately lower for dairies (by 37% on average) and significantly lower for sugar plants (by 80% on average). Swine facility results are more variable. The assumed bottom-up seasonality for manure methane emissions is not apparent in the aircraft measurements, which may be due to on-site management factors that are difficult to capture accurately in national-scale inventories. If not properly accounted for, such seasonal disparities could lead to source misattribution in top-down assessments of methane fluxes.
Bibliographical noteFunding Information:
We thank Jason Rosenthal, Justin Pifer, and Ian Locko for their excellent flying; Julian Deventer, Xiang Li, and Matt Erickson for assistance with instrument calibration; and Kurt Spokas, Lisa Scheirer, and David Weinand for the insightful suggestions. The GEM project is supported by NASA's Interdisciplinary Research in Earth Science program (IDS Grant NNX17AK18G). X. Y. acknowledges support from a NASA Earth and Space Science Fellowship (Grant 80NSSC18K1393). J. D. W. acknowledges support from the U.S. Department of Energy (DOE), Office of Science, Office of Biological and Environmental Research Program, through Oak Ridge National Laboratory's (ORNL) Terrestrial Ecosystem Science?Science Focus Area; ORNL is managed by UT-Battelle, LLC, for DOE under Contract DE-AC05-00OR22725. Data presented in this paper are publicly available online (https://doi.org/10.13020/f50r-zh70).
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