A differential mobility and optical particle size spectrometer (DMOPSS) was developed to measure ambient size distributions based on geometric particle diameter in the size range of 0.1 to 1.0 μm diameter. The DMOPSS consists of a high-flow differential mobility analyzer (HF-DMA) followed by an optical particle counter (OPC) and condensation nucleus counter (CNC) operating in parallel. The OPC and CNC sample monodisperse aerosol of known geometric diameter from the HF-DMA output or, alternatively, polydisperse aerosol with known dilution directly from the ambient air. The monodisperse samples are used to create time-dependent calibrations of the OPC, providing optical response versus geometric size for the ambient aerosol under study. The direct ambient measurements are then reduced, using this ambient-based calibration. A field test of the DMOPSS system was performed in the summer of 1992 at Meadview, Arizona, where more than 12,000 size spectra were collected; they consisted of roughly one-third direct ambient samples and two-thirds HF-DMA sized samples. Measured aerosol volumes and calculated particle scattering coefficients were strongly correlated with nephelometer measurements, with a mean scattering-to-volume ratio of 5 m2/cm3. With the ambient aerosol calibration, the measured aerosol volumes were 47% larger, and volume geometric mean diameters were 12% larger, than would have been obtained using a polystyrene latex calibration.