Granted if PSYS_PART_EMISSIVE_MASK is not provided. Set interpolation with start/end like you do with scale or color.
Otherwise, if PSYS_PART_EMISSIVE_MASK is given, it should ignore the interpolation input.
Ex:
[PSYS_PART_FLAGS, PSYS_PART_INTERP_EMISSIVE_MASK, PSYS_PART_START_EMISSIVE, 0.0, PSYS_PART_END_EMISSIVE, 1.0]