The problem of shear dispersion in the atmospheric boundary layer is revisited. The aim is to understand the difference in behaviour between state-of-the-art `random flight' Lagrangian particle dispersion models, which include a realistic stochastic model of turbulent velocity fluctuations, and simpler `random displacement'  or eddy diffusion models. Both the effective horizontal diffusivity of a tracer in the ABL,  and its large-deviation statistics, are calculated for each model. The importance of three-dimensional effects, such as an Ekman spiral in the mean wind, are also considered. Two separate regimes, in which  plume widths are controlled by direct horizontal diffusion, or by secondary shear dispersion effects, are identified.