Genetic algorithm inversion of dual polarization polar nephelometer data to determine aerosol refractive index

A polar nephelometer that measures the light scattered into a two dimensional plane by a stream of aerosols that intersect a 350 milliwatt diode laser beam with a wavelength of 670 nm is described. A half wave plate is used to orient the laser light both parallel and perpendicular to the measurement plane. This scattering information combined with simultaneously measured size distribution information is used to determine the real (m(r)) refractive index of the aerosols using a Genetic Algorithm search method. Errors in the retrieved mr based on noise in the scattering measurement as well as uncertainties in the size distribution measurement are examined. The m(r) of polystyrene latex spheres and ammonium sulfate droplets are determined and match expectations within experimental uncertainties using Mie-Lorenz theory. The angular scattering properties of spherical secondary organic aerosols generated by oxidizing alpha-pinene do not follow this theory and calls into question the inferred m(r).