Size- and Composition-Dependent Response of the DAWN-A Multiangle Single-Particle Optical Detector

The performance of the DAWN-A differential light-scattering detector (Wyatt et al., 1988. Appl. Opt. 27:217–221) was characterized in laboratory experiments. Objectives of this work included measurement of size-dependent counting efficiencies and of angular scattering patterns for spherical particles of known size and composition.

Counting efficiencies for polystyrene latex (PSL) spheres of nine sizes in the 0.14–0.97-μm diameter range were obtained as a function of the trigger threshold level. Counting efficiencies were found to increase with increasing particle size and decreasing trigger threshold level. Maximum observed counting efficiencies were in the range of 50% to 60%, indicating that the half-width of the laser beam was about a factor of 2 narrower than the width of the particle beam in the scattering volume. A distribution of pulse heights was observed for particles of a given size, reflecting the variability of the illumination intensity.

Angular scattering patterns of PSL, dioctyl sebacate (DOS), and methylene blue for nine different sizes in the 0.14–0.97-μm size range were obtained; measurements were also done with 0.55-μm (at 7% relative humidity) sulfuric acid droplets exposed to eight different relative humidities in the 7% to 81% range. The PSL data were used to calibrate the detectors. For the other materials, Lorenz-Mie theory was used to determine the “best” value of the complex refractive index to match measurements to theory for each particle size investigated. For sulfuric acid, the inferred imaginary component of refractive index was zero as expected, while the real component was within 2% of the literature value over the range of relative humidities investigated. For DOS (expected value = 1.46 + 0.00i), the inferred real component of refractive index was, on average, 4% greater than the expected value, and the average inferred imaginary component was 0.02 for particles 0.32 μm. Small signal-to-noise led to poor agreement between theory and measurement for 0.14-μm particles. For methylene blue, which has a nonzero imaginary component (expected value = 0.82 + 0.40i), there were large uncertainties in the inferred refractive index values due to problems in generating spherical, homogeneous particles.