We present observations of local enhancements in carbon dioxide (CO₂) from local emissions sources over three eastern US regions during four deployments of the Atmospheric Carbon Transport-America (ACT-America) campaign between summer 2016 and spring 2018. Local CO₂ emissions were characterized by carbon monoxide (CO) to CO₂ enhancement ratios (i.e. ΔCO/ΔCO₂) in airmass mixing observed during aircraft transects within the atmospheric boundary layer. By analyzing regional-scale variability of CO₂ enhancements as a function of ΔCO/ΔCO₂ enhancement ratios, observed relative contributions to CO₂ emissions were contrasted between different combustion regimes across regions and seasons. Ninety percent of observed summer combustion in all regions was attributed to high efficiency fossil fuel (FF) combustion (ΔCO/ΔCO₂ < 0.5%). In other seasons, regional contributions increased from less efficient forms of FF combustion (ΔCO/ΔCO₂ 0.5-2%) to as much as 60% of observed combustion. CO₂ emission contributions attributed to biomass burning (BB) (ΔCO/ΔCO₂ > 4%) were negligible during summer and fall in all regions, but climbed to 10-12% of observed combustion in the South during winter and spring. Vulcan v3 CO₂ 2015 emission analysis showed increases in residential and commercial sectors seasonally matching increases in less efficient FF combustion, but could not explain regional trends. WRF-Chem modeling, driven by CarbonTracker CO₂ fire emissions, matched observed winter and spring BB contributions, but conflictingly predicted similar levels of BB during fall. Satellite fire data from MODIS and VIIRS suggested higher spatial resolution fire data might improve modeled BB emissions.