Comparison of measured and computed light scattering in the Baltic

Abstract

Mie theory was used to calculate the average scattering functions of suspended particles from the surface layer of the Baltic in summer and winter. Good agreement with data has been achieved in the angle range of ~ 15° to ~ 165°. Corresponding average particle size distributions measured using a Coulter counter in the diameter range of 2.5 to 20 μm were used in the calculations. The size distribution of smaller particles and the refractive indices of particles in the entire optically important diameter range were determined using the trial and error method. A refractive index of 1.1 was obtained for both summer and winter particles in the diameter range of 0.1 to 2 μm. The size distributions of these particles, also determined from light scattering, were hyperbolic with a slope of 4.1. The concentration of particles with diameters between 0.1 and 2 μm in summer was about twice that in winter. Refractive indices: 1.05- 0.005i and 1.03-0,01i were obtained for summer particles with diameters between 2 and ~ 10 pm and over ~ 10 pm respectively. A refractive index of 1.1 was obtained for winter particles larger than 2 μm. Only particles with diameters in the range of 0.1 to 10 μm contributed significantly to the volume scattering function measured. Particles smaller than ~ 2 μm dominated light scattering at angles > 10° and larger particles at smaller angles. The calculated volume scattering function at angles smaller than ~ 15° for the summer particles agreed with the experimental data. Values of the scattering function in this angular range for the winter particles were about half of those measured. This is explained as a consequence of an underestimation of the projected areas of particles when using Coulter counter data in the computation of light scattering. It can be compensated for in the case of summer particles, with a small refractive index and slope of the size distribution, by selecting a higher than actual refractive index of the particles. Such a compensation is not possible in winter for mostly mineral particles whose refractive index and the slope of the size distribution are already high. DOI: 10.1111/j.1600-0889.1986.tb00096.x