AbstractFrontal boundaries have been shown to cause large changes in CO₂ mole‐fractions, but clouds and the complex vertical structure of fronts make these gradients difficult to observe. It remains unclear how the column average CO₂ dry air mole‐fraction (XCO₂) changes spatially across fronts, and how well airborne lidar observations, data assimilation systems, and numerical models without assimilation capture XCO₂ frontal contrasts (ΔXCO_2, i.e., warm minus cold sector average of XCO₂). We demonstrated the potential of airborne Multifunctional Fiber Laser Lidar (MFLL) measurements in heterogeneous weather conditions (i.e., frontal environment) to investigate the ΔXCO₂ during four seasonal field campaigns of the Atmospheric Carbon and Transport‐America (ACT‐America) mission. Most frontal cases in summer (winter) reveal higher (lower) XCO₂ in the warm (cold) sector than in the cold (warm) sector. During the transitional seasons (spring and fall), no clear signal in ΔXCO₂ was observed. Intercomparison among the MFLL, assimilated fields from NASA's Global Modeling and Assimilation Office (GMAO), and simulations from the Weather Research and Forecasting‐—Chemistry (WRF‐Chem) showed that (a) all products had a similar sign of ΔXCO₂ though with different levels of agreement in ΔXCO₂ magnitudes among seasons; (b) ΔXCO₂ in summer decreases with altitude; and (c) significant challenges remain in observing and simulating XCO₂ frontal contrasts. A linear regression analyses between ΔXCO₂ for MFLL versus GMAO, and MFLL versus WRF‐Chem for summer‐2016 cases yielded a correlation coefficient of 0.95 and 0.88, respectively. The reported ΔXCO₂ variability among four seasons provide guidance to the spatial structures of XCO₂ transport errors in models and satellite measurements of XCO₂ in synoptically‐active weather systems.