Top‐Down Constraints on Methane Point Source Emissions From Animal Agriculture and Waste Based on New Airborne Measurements in the U.S. Upper Midwest

Abstract

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. Plain Language SummaryKey agricultural methane sources are quantified using new airborne measurements in the U.S. Corn Belt and Upper Midwest. Measurements spanned multiple seasons and targeted nine of the largest concentrated animal feeding operations in the region along with two sugar processing plants. Compared with bottom-up estimates informed by U.S. Environmental Protection Agency methodology and site-level animal and management data, top-down fluxes agree well with bottom-up estimates for beef but are lower for dairies and sugar plants and suggest a possible mismatch in the timing of emissions.Key Points: • We used aircraft measurements to quantify methane emissions from key agricultural point sources in the Upper Midwest during three seasons • Top-down methane fluxes are consistent with bottom-up values for beef facilities but reveal a mismatch for dairies and sugar plants • These discrepancies point to potential spatial and temporal misattribution of emissions used for atmospheric inverse modeling Supporting Information: • Supporting Information S1

Aircraft-based inversions quantify the importance of wetlands and livestock for Upper Midwest methane emissions

“…For anthropogenic emissions, we employ a scale-dependent uncertainty (encompassing magnitude and displacement uncertainties) following Maasakkers et al (2016); the resulting error standard deviation averages 40 %-105 % across sectors over our study region. For other sources we assume a prior error standard deviation of 50 % following earlier studies (Maasakkers et al, 2019;Turner et al, 2015;Wecht et al, 2014;Zhang et al, 2018;Sheng et al, 2018b). For inversions optimizing the total methane flux across sectors, the above terms are combined in quadrature as the diagonal elements of the prior error covariance matrix.…”

Aircraft-based inversions quantify the importance of wetlands and livestock for Upper Midwest methane emissions

“…This agrees with our findings here of a +15% adjustment for beef facilities (dominated by enteric emissions) and an approximate +30% summer and winter adjustment for dairies and hog facilities (with a larger role for manure emissions). In our previous work, we applied aircraft-based mass balance to quantify facilitylevel emissions for concentrated animal feeding operations in the Upper Midwest and found (as here) good topdown/bottom-up agreement for enteric emissions but discrepancies for manure (Yu et al, 2020). A recent site-level study at a large Wisconsin dairy farm observed low manure emissions (~30% of the enteric flux) owing to frequent field application throughout the year (Wiesner et al, 2020), further supporting our characterization of manure management as a key uncertainty in current large-scale bottom-up inventories.…”

“…Along with mixed-layer surveying, each flight included 1-2 vertical profiles to characterize the atmosphere's vertical structure from the surface to lower free troposphere. The GEM flights also included extensive point source characterization as described by Yu et al (2020).…”

“…We use 1-minute averaged observations to constrain regional fluxes. Additional onboard observations included: nitrous oxide (N2O), carbon monoxide (CO), H2O, and CO2 mole fractions by continuous-wave tunable infrared laser absorption spectrometry (0.5 Hz, Aerodyne Research Inc., USA) as described by Gvakharia et al (2018); ozone (O3) mole fractions (0.2 Hz; dual-beam ultraviolet spectrometer, model 205, 2B Technologies Inc., USA); temperature and relative humidity (1 Hz; model HMP60, Vaisala Corp., Finland); plus GPS location, wind speed and direction, ambient pressure, and other relevant flight parameters as described by Yu et al (2020).…”

“…Dry systems make up the remainder. As a result, enteric emissions are thought to account for more than > 95% of the methane flux from beef facilities but only 60% for dairies (they are minor for hog facilities) (Yu et al, 2020).…”