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Abstract. We apply airborne measurements across three seasons(summer, winter and spring 2017–2018) in a multi-inversion framework toquantify methane emissions from the US Corn Belt and Upper Midwest, a keyagricultural and wetland source region. Combing our seasonal results withprior fall values we find that wetlands are the largest regional methanesource (32 %, 20 [16–23] Gg/d), while livestock (enteric/manure; 25 %,15 [14–17] Gg/d) are the largest anthropogenic source. Naturalgas/petroleum, waste/landfills, and coal mines collectively make up theremainder. Optimized fluxes improve model agreement with independentdatasets within and beyond the study timeframe. Inversions reveal coherentand seasonally dependent spatial errors in the WetCHARTs ensemble meanwetland emissions, with an underestimate for the Prairie Pothole region butan overestimate for Great Lakes coastal wetlands. Wetland extent andemission temperature dependence have the largest influence on predictionaccuracy; better representation of coupled soil temperature–hydrologyeffects is therefore needed. Our optimized regional livestock emissionsagree well with the Gridded EPA estimates during spring (to within 7 %) butare ∼ 25 % higher during summer and winter. Spatial analysisfurther shows good top-down and bottom-up agreement for beef facilities (withmainly enteric emissions) but larger (∼ 30 %) seasonaldiscrepancies for dairies and hog farms (with > 40 % manureemissions). Findings thus support bottom-up enteric emission estimates butsuggest errors for manure; we propose that the latter reflects inadequatetreatment of management factors including field application. Overall, ourresults confirm the importance of intensive animal agriculture for regionalmethane emissions, implying substantial mitigation opportunities throughimproved management. 

Abstract. We present the development and assessment of a new flight system that uses acommercially available continuous-wave, tunable infrared laser directabsorption spectrometer to measure N₂O, CO₂, CO, andH₂O. When the commercial system is operated in an off-the-shelfmanner, we find a clear cabin pressure–altitude dependency forN₂O, CO₂, and CO. The characteristics of this artifactmake it difficult to reconcile with conventional calibration methods. Wepresent a novel procedure that extends upon traditional calibrationapproaches in a high-flow system with high-frequency, short-duration samplingof a known calibration gas of near-ambient concentration. This approachcorrects for cabin pressure dependency as well as other sources of drift inthe analyzer while maintaining a ∼90% duty cycle for 1Hz sampling.Assessment and validation of the flight system with both extensive in-flightcalibrations and comparisons with other flight-proven sensors demonstrate thevalidity of this method. In-flight 1σ precision is estimated at0.05ppb, 0.10ppm, 1.00ppb, and 10ppm for N₂O,CO₂, CO, and H₂O respectively, and traceability to WorldMeteorological Organization (WMO) standards (1σ) is 0.28ppb,0.33ppm, and 1.92ppb for N₂O, CO₂, and CO. We showthe system is capable of precise, accurate 1Hz airborne observations ofN₂O, CO₂, CO, and H₂O and highlight flightdata, illustrating the value of this analyzer for studying N₂Oemissions on ∼100km spatial scales.