Calculations are made of the radiation flux of gamma rays that have originated from a point isotropic source and have been singly scattered in the air lying beyond a plane interface. Calculations are made in the limit that the source-detector separation distance is small compared to a mean-free-path in air. These results are interpreted in terms of an image source. The results, combined with earlier calculations of the radiation flux reflected from a condensed medium, such as ground, predict the effect of the ground-air interface on radiation fluxes in air near the interface. The results are extrapolated to source-detector separation of the order of a mean-free-path by using infinite-medium buildup factors. Comparisons with experiment show that the model produces results that are in qualitative agreement with experiment.