A measurement system for measuring an electrical mobility aerosol size distribution of a polydisperse aerosol, the measurement system comprising: an optical particle counter for measuring optical diameters of aerosol particles, the optical particle counter comprising: an electromagnetic radiation source operatively coupled with beam shaping optics for generation of a beam of electromagnetic radiation; an inlet nozzle for passage of an aerosol flow stream there through, said aerosol flow stream containing particles and intersecting said beam of electromagnetic radiation to define an interrogation volume, said particles scattering said electromagnetic radiation from said interrogation volume; a radiation collector for collection of a portion of said electromagnetic radiation scattered from the particles in said interrogation volume; and a detector for detection of said portion of said electromagnetic radiation collected by said radiation collector; the measurement system further comprising a calibration system including an electrical mobility device for generating one or more sets of calibration data from polydisperse aerosol particles measured by the optical particle counter, said calibration system operatively coupled to said detector, wherein the electrical mobility device is adapted to generate one or more monodisperse aerosol particles from said polydisperse aerosol particles, and wherein a set of calibration data comprises a mobility diameter of said monodisperse aerosol particles generated by the electrical mobility device and an optical diameter of said monodisperse aerosol particles measured by the optical particle counter, the measurement system further comprising digital processing means operatively coupled to said calibration system for computing a mobility diameter from an optical diameter, said digital processing means configured to generate a Mie light scattering model to determine an optimal refractive index of the polydisperse aerosol particles using the one or more sets of calibration data and then generate a new calibration curve using the resulting optimal refractive index and convert the optical diameters of said polydisperse aerosol particles to mobility diameters using the new calibration curve. an optical particle counter for measuring optical diameters of aerosol particles, the optical particle counter comprising: an electromagnetic radiation source operatively coupled with beam shaping optics for generation of a beam of electromagnetic radiation; an inlet nozzle for passage of an aerosol flow stream there through, said aerosol flow stream containing particles and intersecting said beam of electromagnetic radiation to define an interrogation volume, said particles scattering said electromagnetic radiation from said interrogation volume; a radiation collector for collection of a portion of said electromagnetic radiation scattered from the particles in said interrogation volume; and a detector for detection of said portion of said electromagnetic radiation collected by said radiation collector; an electromagnetic radiation source operatively coupled with beam shaping optics for generation of a beam of electromagnetic radiation; an inlet nozzle for passage of an aerosol flow stream there through, said aerosol flow stream containing particles and intersecting said beam of electromagnetic radiation to define an interrogation volume, said particles scattering said electromagnetic radiation from said interrogation volume; a radiation collector for collection of a portion of said electromagnetic radiation scattered from the particles in said interrogation volume; and a detector for detection of said portion of said electromagnetic radiation collected by said radiation collector; the measurement system further comprising a calibration system including an electrical mobility device for generating one or more sets of calibration data from polydisperse aerosol particles measured by the optical particle counter, said calibration system operatively coupled to said detector, wherein the electrical mobility device is adapted to generate one or more monodisperse aerosol particles from said polydisperse aerosol particles, and wherein a set of calibration data comprises a mobility diameter of said monodisperse aerosol particles generated by the electrical mobility device and an optical diameter of said monodisperse aerosol particles measured by the optical particle counter, the measurement system further comprising digital processing means operatively coupled to said calibration system for computing a mobility diameter from an optical diameter, said digital processing means configured to generate a Mie light scattering model to determine an optimal refractive index of the polydisperse aerosol particles using the one or more sets of calibration data and then generate a new calibration curve using the resulting optimal refractive index and convert the optical diameters of said polydisperse aerosol particles to mobility diameters using the new calibration curve.