Atmospheric size distributions measured by differential mobility optical particle size spectrometry

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 mu m diameter. The DMOPSS consists of a high-how 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 m(2)/cm(3). 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. (C) 1998 American Association for Aerosol Research.