This is a study to examine the impact of modeling photochemistry from aircraft emissions in an expanding plume versus at the grid scale in an atmospheric model. Differences in model treatments for a single flight occurred at all altitudes during takeoff, cruise, and landing. After 10 h, the plume treatment decreased grid-scale ozone production by 33%, methane destruction by 30%, and carbon monoxide destruction by 32% at cruise altitude compared with the grid-scale treatment. The plume treatment changed the odd nitrogen partitioning by ~10%. For multiple overlapping flights at cruise altitude, final ozone, methane, and carbon monoxide perturbations decreased by 77, 68, and 74%, respectively, compared with the grid-scale treatment. Enhanced mixing with ambient air reduced the plume-scale and grid-scale differences. The persistent differences in photochemical activity indicate that individual plume treatment should be incorporated into 3-D modeling studies.