Spray breakup processes of small-scale simplex atomizers have been characterized. With tangential entry of fuel, the cases of no air flow and concentric co-swirl air flow are considered. Ten simplex atomizers with different values of dimensionless tangential port area K have been employed. The breakup mode changes from film breakup to jet breakup, with increase in K value. For small K values, small port area causes high liquid swirl, which aids in the formation of an air core and a hollow cone spray with large cone angle. At high K values, jet breakup gives rise to a full cone spray with a small cone angle. The Sauter Mean Diameter (SMD) for the spray exhibits bimodal variation in the radial direction close to the atomizer. Based on a large volume of data for kerosene and water sprays, accurate correlations (covering wide operating conditions) have been developed for the spray cone angle, Cd, and axial variation of SMD for small-scale simplex atomizers. Swirl air interaction with the liquid film causes a transition from closed tulip shape to an open spray at a critical air flow rate. The critical air flow rate for transition is initial condition-dependent and it exhibits hysteresis.