We simulate the record of sulfur mass-independent fractionation (MIF) in sulfates found in the Antarctic ice cores from the 1991 eruption of Mt. Pinatubo. Discovery of MIF in sulfur isotopes in Archean/Early Proterozoic sediments and lack of MIF in sulfur in younger rocks placed a strict upper limit on the amount of oxidants and oxygen in the ancient Earth's atmosphere prior to 2.3 Gyr ago. However, recent measurements in ice cores suggest that some non-zero MIF in sulfate can be delivered to Antarctica following large volcanic eruptions. Current photochemical experiments do not identify the specific reaction responsible for sulfur MIF production. Here we show that a time-dependent non-zero sulfur MIF can be delivered to the ground in the present atmosphere if the MIF--producing process is SO3 photolysis. Fractionation from such photolysis has not been considered before, but is consistent with the available experimental data.