SIZE-DEPENDENT 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 labora tory experiments. Objectives of this work included measurement of size -dependent counting efficiencies and of angular scattering patterns fo r spherical particles of known size and composition. Counting efficien cies for polystyrene latex (PSL) spheres of nine sizes in the 0.14-0.9 7-mum diameter range were obtained as a function of the trigger thresh old level. Counting efficiencies were found to increase with increasin g particle size and decreasing trigger threshold level. Maximum observ ed counting efficiencies were in the range of 50% to 60%, indicating t hat 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 distri bution of pulse heights was observed for particles of a given size, re flecting the variability of the illumination intensity. Angular scatte ring patterns of PSL, dioctyl sebacate (DOS), and methylene blue for n ine different sizes in the 0.14-0.97-mum size range were obtained; mea surements were also done with 0.55-mum (at 7% relative humidity) sulfu ric acid droplets exposed to eight different relative humidities in th e 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 in ferred imaginary component of refractive index was zero as expected, w hile 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 mum. Small signal-to -noise led to poor agreement between theory and measurement for 0.14-m um particles. For methylene blue, which has a nonzero imaginary compon ent (expected value = 0.82 + 0.40i), there were large uncertainties in the inferred refractive index values due to problems in generating sp herical, homogeneous particles.