We have performed more than 300 atmospheric simulations of the 1991 Pinatubo eruption using the AER 2-D sulfate aerosol model to optimize the initial sulfur mass injection as a function of altitude, which in previous modeling studies has often been chosen in an ad hoc manner (e.g., by applying a rectangular-shaped emission profile). Our simulations are generated by varying a four-parameter vertical mass distribution, which is determined by a total injection mass and a skew-normal distribution function. Our results suggest that (a) the initial mass loading of the Pinatubo eruption is approximately 14 Mt of SO2; (b) the injection vertical distribution is strongly skewed towards the lower stratosphere, leading to a peak mass sulfur injection at 18-21 km; (c) the injection magnitude and height affect early southward transport of the volcanic clouds as observed by SAGE II.