Urban emissions remain an underexamined part of the methane budget. Here we present and interpret aircraft observations of six old and leak‐prone major cities along the East Coast of the United States. We use direct observations of methane (CH₄), carbon dioxide (CO₂), carbon monoxide (CO), ethane (C₂H₆), and their correlations to quantify CH₄ emissions and attribute to natural gas. We find the five largest cities emit 0.85 (0.63, 1.12) Tg CH₄/year, of which 0.75 (0.49, 1.10) Tg CH₄/year is attributed to natural gas. Our estimates, which include all thermogenic methane sources including end use, are more than twice that reported in the most recent gridded EPA inventory, which does not include end‐use emissions. These results highlight that current urban inventory estimates of natural gas emissions are substantially low, either due to underestimates of leakage, lack of inclusion of end‐use emissions, or some combination thereof.

Carbon monoxide (CO) is an ozone precursor, oxidant sink, and widely used pollution tracer. The importance of anthropogenic versus other CO sources in the US is uncertain. Here, we interpret extensive airborne measurements with an atmospheric model to constrain US fossil and nonfossil CO sources. Measurements reveal a low bias in the simulated CO background and a 30% overestimate of US fossil CO emissions in the 2016 National Emissions Inventory. After optimization we apply the model for source partitioning. During summer, regional fossil sources account for just 9%–16% of the sampled boundary layer CO, and 32%–38% of the North American enhancement—complicating use of CO as a fossil fuel tracer. The remainder predominantly reflects biogenic hydrocarbon oxidation plus fires. Fossil sources account for less domain‐wide spatial variability at this time than nonfossil and background contributions. The regional fossil contribution rises in other seasons, and drives ambient variability downwind of urban areas.

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.

We present the development and assessment of a new flight system that uses a commercially available continuous-wave, tunable infrared laser direct absorption spectrometer to measure N₂O, CO₂, CO, and H₂O. When the commercial system is operated in an off-the-shelf manner, we find a clear cabin pressure–altitude dependency for N₂O, CO₂, and CO. The characteristics of this artifact make it difficult to reconcile with conventional calibration methods. We present a novel procedure that extends upon traditional calibration approaches in a high-flow system with high-frequency, short-duration sampling of a known calibration gas of near-ambient concentration. This approach corrects for cabin pressure dependency as well as other sources of drift in the analyzer while maintaining a ∼90 % duty cycle for 1 Hz sampling. Assessment and validation of the flight system with both extensive in-flight calibrations and comparisons with other flight-proven sensors demonstrate the validity of this method. In-flight 1σ precision is estimated at 0.05 ppb, 0.10 ppm, 1.00 ppb, and 10 ppm for N₂O, CO₂, CO, and H₂O respectively, and traceability to World Meteorological Organization (WMO) standards (1σ) is 0.28 ppb, 0.33 ppm, and 1.92 ppb for N₂O, CO₂, and CO. We show the system is capable of precise, accurate 1 Hz airborne observations of N₂O, CO₂, CO, and H₂O and highlight flight data, illustrating the value of this analyzer for studying N₂O emissions on ∼100 km spatial scales.