Response Characteristics of the Particle Measuring Systems Active Scattering Aerosol Spectrometer Probes

Abstract

Predictions of the size response of various light-scattering aerosol counters manufactured by Particle Measuring Systems are reported. Models that exploit the high intensity of light available within the cavity of a He-Ne gas laser (generically referred to by the manufacturer as ''active scattering aerosol spectrometer probes'') are considered. The new response function properly averages over particle trajectories through nodes, antinodes, and intermediate regions of the intracavity laser beam. Our studies address probes having two basic scattering geometries: those that collect light scattered over a relatively narrow solid angle (subtending angles between 4° and 22° from the laser beam axis, as in the model ASASP-300 and ASASP-300X probes) and those that collect light over a rather large solid angle (between 35° and 120° , as in the ASASP-X, ASASP-100X, LAS-250X, LAS-X, and HS-LAS probes). The theoretical response predictions for both narrow-angle and wide-angle probes are compared to previous measurements of monodisperse test aerosols of polystyrene latex, dyoctylphthalate, nigrosin dye, and carbon black. The new response function predicts smoother dependence on particle size than the previous response function of Pinnick and Auvermann (1979) and is in better agreement with measurement. Response calculations for common atmospheric aerosol (water, sulfuric acid, ammonium sulfate, and black carbon) reveal the considerable sensitivity of the response to particle dielectric properties. Response functions for internal mixtures (black carbon inclusions in water droplets, quartz in sulfuric acid, carbon in ammonium sulfate, and metal in sulfuric acid) are somewhat different than those for homogeneous particles. Comparison of response calculations with the manufacturer's calibration reveal conditions for which the manufacturer's calibration is most appropriate and the potential for errors (as much as a factor of two in sizing) when it is blindly applied. Finally, response functions for multiline laser operation, as the manufacturer suggests might be appropriate for the HS-LAS and LAS-X probes, are nearly the same as for single-line lasing. These results should help the user of these instruments to more realistically interpret size distribution measurements.