Parameters are accessed with C4DAtom::GetParameter()/ C4DAtom::SetParameter(). See Parameter IDs.
Parameter: Position
IN_PART_POSITIONParameter: Velocity
IN_PART_VELOCITYParameter: Life
IN_PART_LIFEParameter: Age
IN_PART_AGEParameter: Size
IN_PART_SIZEParameter: Scale
IN_PART_SCALEParameter: Mass
IN_PART_MASSParameter: Alignment
IN_PART_ALIGNMENTParameter: DT Factor
IN_PART_DTFACParameter: Random Seed
IN_PART_RANDOMSEEDParameter: On
IN_PART_ONParameter: Color
IN_PART_COLOR