The performance of water-based fire suppression systems is governed by the dispersion of the droplets in the spray. Characterization of the spray is essential for predicting and evaluating the performance of these suppression systems. The accuracy of the spray characterization is quite sensitive to the initial spray specification when using particle tracking method to model spray dispersion. An atomization model based on first principles has been developed for predicting the distributed properties for the initial spray. Inputs to this model include injector geometry, operating conditions, and suppressant fluid properties. This modeling approach has also been integrated with drop dispersion models in FDS 4.0 to characterize spray dispersion behavior. The effect of initial spray specification on spray dispersion behavior in a quiescent environment has also been addressed. The drop size predictions using the proposed atomization model have demonstrated favorable agreement with actual sprinkler spray measurements over a range of operating conditions.