A system to measure a sampled particle-laden airstream , the system comprising: an optical particle spectrometer configured to measure a concentration of particulate matter in the sampled particle-laden airstream; a particle diverter in fluid communication with the optical particle spectrometer, the particle diverter configured to divert at least a portion of the particle-laden airstream at predetermined intervals; a mass sensor configured to measure a mass of a fraction of particles within the diverted particle-laden airstream received from the particle diverter; a calibration communication loop configured to provide the measured mass of the particles to the optical particle spectrometer; and a processor configured to calculate a calibration factor, C f , from mass concentration readings provided from the mass sensor via the calibration communication loop or mass concentration values provided from the optical particle spectrometer via the calibration communication loop, wherein the system further comprises either: a) a mass filter coupled upstream of the mass sensor so as to receive the portion of the particle-laden airstream and being configured to filter a fraction of the particles within the particle-laden airstream that are above a predetermined particle size, or b) a mass filter coupled upstream of the optical particle spectrometer so as to receive the sampled particle-laden airstream and being configured to filter a fraction of the particles within the particle-laden airstream that are above a predetermined particle size. an optical particle spectrometer configured to measure a concentration of particulate matter in the sampled particle-laden airstream; a particle diverter in fluid communication with the optical particle spectrometer, the particle diverter configured to divert at least a portion of the particle-laden airstream at predetermined intervals; a mass sensor configured to measure a mass of a fraction of particles within the diverted particle-laden airstream received from the particle diverter; a calibration communication loop configured to provide the measured mass of the particles to the optical particle spectrometer; and a processor configured to calculate a calibration factor, C f , from mass concentration readings provided from the mass sensor via the calibration communication loop or mass concentration values provided from the optical particle spectrometer via the calibration communication loop, wherein the system further comprises either: a) a mass filter coupled upstream of the mass sensor so as to receive the portion of the particle-laden airstream and being configured to filter a fraction of the particles within the particle-laden airstream that are above a predetermined particle size, or b) a mass filter coupled upstream of the optical particle spectrometer so as to receive the sampled particle-laden airstream and being configured to filter a fraction of the particles within the particle-laden airstream that are above a predetermined particle size. a) a mass filter coupled upstream of the mass sensor so as to receive the portion of the particle-laden airstream and being configured to filter a fraction of the particles within the particle-laden airstream that are above a predetermined particle size, or b) a mass filter coupled upstream of the optical particle spectrometer so as to receive the sampled particle-laden airstream and being configured to filter a fraction of the particles within the particle-laden airstream that are above a predetermined particle size. <EOS>